Electrical Plan Review
Objectives
By reviewing this brochure, the Electrical Inspector,
Electrical Contractor, Plan Examiner, Consulting
Engineer and others will be able to . . .
■ Understand the importance of overcurrent protection.
■ Understand the meaning and importance of electrical
terms commonly used relating to overcurrent protection.
■ Understand and discuss the KEY National Electrical
Code® requirements regarding overcurrent protection.
■ Calculate short-circuit currents using the simple
POINT-TO-POINT method.
■ Check electrical plans to determine conformance to
National Electrical Code® sections that cover shortcircuit currents, interrupting ratings, interrupting
capacities, short-circuit ratings, ground faults,
grounding electrode conductors, equipment
grounding conductors, etc.
■ Verify that circuit, feeder, service, grounding electrode conductors, equipment grounding conductors,
and bonding conductors have adequate capacity to
conduct safely ANY fault current likely to be
imposed on them.
■ Determine let-through current values (peak & RMS)
when current-limiting overcurrent devices are used.
■ Apply current-limiting data to protect downstream
electrical components that have withstand ratings
LESS than the available fault-current at any given
point on the system.
■ Realize that whenever overcurrent protection is
discussed, the two most important issues are:
— HOW MUCH CURRENT WILL FLOW?
— HOW LONG WILL THE CURRENT FLOW?
■ Adopt a Form Letter and a Data Required Form that
can be used to “log-in” the necessary data relating
to available fault currents, interrupting ratings, and
let-through currents.
■ Understand that the major sources of short-circuit
currents are motors and generators.
■ Understand that transformers are NOT a source of
short-circuit current.
■ Know how to ask the right questions.
Copyrighted April, 1999 by Cooper Bussmann, Inc., Printed in U.S.A.
1
Electrical Plan Review
®
Critical National Electrical Code Sections
250-96(a) General. Metal raceways, cable trays, cable
armor, cable sheath, enclosures, frames, fittings,
and other metal noncurrent-carrying parts that are
to serve as grounding conductors, with or without
the use of supplementary equipment grounding
conductors, shall be effectively bonded where
necessary to ensure electrical continuity and the
capacity to conduct safely any fault current likely
to be imposed on them. Any nonconductive paint,
enamel, or similar coating shall be removed at
threads, contact points, and contact surfaces or
be connected by means of fittings designed so
as to make such removal unnecessary.
110-3(b) Installation and Use. Listed or labeled equipment
shall be installed and used in accordance with
any instructions included in the listing or labeling.
110-9
Interrupting Rating. Equipment intended to
interrupt current at fault levels shall have an
interrupting rating sufficient for the nominal
circuit voltage and the current that is available
at the line terminals of the equipment.
Equipment intended to interrupt current at other
than fault levels shall have an interrupting rating
at nominal circuit voltage sufficient for the current
that must be interrupted.
110-10
Circuit Impedance and Other Characteristics.
The overcurrent protective devices, the total
impedance, the component short-circuit current
ratings, and other characteristics of the circuit to
be protected shall be selected and coordinated to
permit the circuit-protective devices used to clear
a fault to do so without extensive damage to the
electrical components of the circuit. This fault shall
be assumed to be either between two or more of
the circuit conductors, or between any circuit
conductor and the grounding conductor or
enclosing metal raceway. Listed products
applied in accordance with their listing shall be
considered to meet the requirements of this
section.
240-1
240-11
Table
250-122 Minimum Size Equipment Grounding
Conductors for Grounding Raceway and
Equipment.
250-90
Copper
Aluminum or
Copper-Clad
Aluminum
15
20
30
40
60
14
12
10
10
10
12
10
8
8
8
100
200
300
400
500
8
6
4
3
2
6
4
2
1
1/0
600
800
1000
1200
1600
1
1/0
2/0
3/0
4/0
2/0
3/0
4/0
250
350
2000
2500
3000
4000
5000
6000
250
350
400
500
700
800
400
600
600
800
1200
1200
Scope (FPN). Overcurrent protection for conductors and equipment is provided to open the
circuit if the current reaches a value that will
cause an excessive or dangerous temperature
in conductors or conductor insulation. See also
Sections 110-9 and 110-10 for requirements for
interrupting ratings and protection against fault
currents.
Definition of Current-Limiting Overcurrent
Protection Device. A current-limiting overcurrent
protective device is a device that, when interrupting
currents in its current-limiting range, will reduce the
current flowing in the faulted circuit to a magnitude
substantially less than that obtainable in the same
circuit if the device were replaced with a solid
conductor having comparable impedance.
250-2(d) Performance of Fault Current Path. The fault
current path shall be permanent and electrically
continuous, shall be capable of safely carrying
the maximum fault likely to be imposed on it, and
shall have sufficiently low impedance to facilitate
the operation of overcurrent devices under fault
conditions.
Size
(AWG or kcmil)
Rating or setting of
Automatic Overcurrent
Device in Circuit Ahead
of Equipment, Conduit,
etc., Not Exceeding
(Amperes)
NOTE: Where necessary to comply with Section 250-2(d), the equipment
grounding conductor shall be sized larger than this table.
620-62
General. Bonding shall be provided where necessary to ensure electrical continuity and the
capacity to conduct safely any fault current likely
to be imposed.
Selective Coordination. Where more than one
driving machine disconnecting means is supplied
by a single feeder, the overcurrent protective
devices in each disconnecting means shall be
selectively coordinated with any other supply side
overcurrent protective devices.
NOTE: Short-circuit currents may be calculated many ways. A simple method
is the Point-To-Point method as presented in Bussmann’s Electrical
Protection Handbook, Bulletin SPD. That data is included in this bulletin for
ease of use.
2
Electrical Plan Review
The Meaning of Short-Circuit Interrupting Rating
For a better understanding of interrupting rating, consider the following series of analogies.
Normal Current Operation
FLOOD GATES
ANALOGOUS TO
OVERCURRENT
PROTECTIVE
DEVICE
RESERVOIR CAPACITY
ANALOGOUS TO
AVAILABLE FAULT
CURRENT
OVERCURRENT
PROTECTIVE
DEVICE
AVAILABLE FAULT
CURRENT (e.g., 50,000 AMPS)
LOAD
CURRENT
(100 GALLONS
PER MINUTE)
Short-Circuit Operation with
Inadequate Interrupting Rating
FLOOD GATES ARE
DESTROYED BECAUSE
OF INADEQUATE
INTERRUPTING RATING
INADEQUATE INTERRUPTING
RATING. THEREFORE,
OVERCURRENT PROTECTIVE
DEVICE IS DESTROYED
AVAILABLE FAULT
CURRENT (e.g., 50,000 AMPS)
SHORT CIRCUIT
CURRENT
(50,000 GALLONS
PER MINUTE)
Downstream components may
not be able to withstand the
amount of let-through current.
Short-Circuit Operation with
Adequate Interrupting Rating
ADEQUATELY RATED
OVERCURRENT PROTECTIVE
DEVICE IS UNDAMAGED
AVAILABLE FAULT
CURRENT (e.g., 50,000 AMPS)
Downstream components
capable of withstanding
let-through current.
SHORT CIRCUIT
CURRENT SAFELY
CLEARED
FLOOD GATES HAVE
ADEQUATE INTERRUPTING
RATING. FAULT CURRENT
SAFELY INTERRUPTED
3
Electrical Plan Review
Ratings of Overcurrent Protective Devices
Most overcurrent protective devices are labeled with two current ratings.
The Fuse
(2) INTERRUPTING RATING
(1) NORMAL
CURRENT
RATING
The Circuit Breaker
(1) NORMAL CURRENT RATING
(2) INTERRUPTING RATINGS
4
Electrical Plan Review
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5
Electrical Plan Review
“Log In” Data Required Form
DEPARTMENT OF ELECTRICAL INSPECTION
CITY OF
Date
Permit
Electrical Contractor
Street Address
City
State
Zip
The following information is requested to determine that the electrical equipment to be
installed at:
Name of occupant or owner
is in compliance with the National Electrical Code® as it relates to available short-circuit
currents and interrupting ratings. See Sections 110-3(b), 110-9, 110-10, 240-1, 240-11,
250-2(d), 250-90, 250-96(a), and Table 250-122 Note. This form is to be completed
and returned to the Department of Electrical Inspection for approval prior to installation. THE FOLLOWING INFORMATION IS TO BE SUPPLIED BY THE ELECTRICAL CONTRACTOR OR OTHER RESPONSIBLE PARTY:
TRANSFORMER KVA
PHASE
IMPEDANCE
3 OR 4 WIRE
%
SECONDARY VOLTAGE
LENGTH OF SERVICE CONDUCTORS
SIZE & NUMBER OF SERVICE CONDUCTORS PER PHASE
TYPE OF CONDUCTORS: COPPER ■
ALUMINUM ■
CONDUIT SIZE
STEEL ■
NON-MAGNETIC ■
TYPE, SIZE, AND INTERRUPTING RATING OF OVERCURRENT DEVICES IN SERVICE DISCONNECT
(MAIN DISTRIBUTION PANEL)
SIZE OF GROUNDING ELECTRODE CONDUCTOR
BRACING OF SERVICE EQUIPMENT
(page 1 of 2)
6
Electrical Plan Review
“Log In” Data Required Form
I
T
E
M
Location
Of
Short-Circuit Current
1
AT TRANSFORMER
SECONDARY TERMINALS
(INFINITE PRIMARY)
2
ON LINE SIDE OF MAIN
SERVICE EQUIPMENT
3
AT PANEL
4
AT PANEL
5
AT PANEL
6
AT PANEL
7
AT PANEL
8
AT PANEL
9
AT PANEL
10
AT PANEL
11
AT PANEL
12
AT PANEL
ShortCircuit
Current
Overcurrent Device
Ampere
Rating
Interrupting
Rating
Apparent RMS
Let-Through
Current
Use back of form or attach separate sheet for data on additional panels.
Use back of form or attach separate sheet to show one-line diagram of service, feeders, and all related panels.
Attach let-through curves if current-limiting devices are used.
All current values in RMS line-to-line unless otherwise noted.
The undersigned accepts full responsibility for the values given herein.
SIGNED
DATE
PHONE WHERE YOU CAN BE REACHED
Page 2 of 2
7
Peak
Let-Through
Current
Electrical Plan Review
Point-To-Point Method Of Short-Circuit Calculation
Calculation Of Short-Circuit Currents —
Point-To-Point Method.
Example Of Short-Circuit Calculation
Fault #1
Adequate interrupting rating and protection of electrical components
are two essential aspects required by the National Electrical Code in
Sections 110-9, 110-10, 240-1, 250-2(d), 250-90, 250-96(a) and Table
250-122 Note. The first step to ensure that system protective devices
have the proper interrupting rating and provide component protection
is to determine the available short-circuit currents. The application of
the Point-To-Point method permits the determination of available shortcircuit currents with a reasonable degree of accuracy at various points
for either 3o or 1o electrical distribution systems. This method
assumes unlimited primary short-circuit current (infinite bus).
MAIN SERVICE
PANEL
300 KVA, 2%Z
Fault #2
Infinite
Primary
Available
20', 500 MCM CU
Steel conduit
20', #2 CU
Steel conduit
BRANCH CIRCUIT PANEL
Basic Short-Circuit Calculation Procedure.
Procedure
Step 1 Determine transf.
full-load amperes
from either:
a) Name plate
b) Tables 3A & 3B
c) Formula
Formula
Step 2 Find transf.
multiplier
Step 1
IFLA =
KVA x 1000
EL-L
Step 2
Multiplier =
3o transf.
IFLA
1o transf.
IFLA =
—
Multiplier =
Step 3 Determine transf.
let-through shortcircuit current
(Table 5 or
formula).
Step 4 Calculate
“f” factor.
FAULT #1
KVA x 1000
=
EL-L x 1.73
†I SCA
f=
100
Transf. % Z
1.73 x L x lL-L-L
C x EL-L
1o line-to-line
(L-L) faults on
1o, center-tapped
transformers
f = 2 x L x l L-L
C x EL-L
1o line-to-neutral
(L-N) faults on
1o, center-tapped
transformers
f = 2 x L x l L-N*
C x EL-N
Step 6 Compute the
available shortcircuit current
(symmetrical) at
the fault.
M= 1
1+f
= 833A
100
100
= 55.55
=
.9x Transf. % Z
Step 3
** I SCA = 833 X 55.55 = 46,273
Step 4
f=
Step 5
M=
Step 6
At Transformer Secondary
1.73 x L x lL-L-L
1.73 x 20 x 46,273
=
= .347
C x EL-L
22,185 x 208
1
1
= 1 + .347 = .742 (See Table 4)
1+f
I SCA = 46,273 x .742 = 34,343A
Fault #1
1.73 x 20 x 34,343
= .968
5,906 x 208
Step 4
f=
Step 5
M=
Step 6
I SCA = 34,343 x .508 = 17,447A
1
1
= 1 + .968 = .508 (See Table 4)
1+f
Fault # 2
** For simplicity, the motor contribution and voltage variance was not included.
See Notes 1 and 4.
†† Transformer
% Z is multiplied by .9 to establish a worst case condition.
See Note 3.
Note 3: The marked impedance values on transformers may vary ±10% from
the actual values determined by ANSI / IEEE test. See U.L. Standard 1561.
Therefore, multiply transformer % Z by .9.
I SCA = I SCA x M
at
fault
300 x 1000
FAULT #2 (Use I SCA @ Fault #1 to calculate)
L = length (feet) of conduit to the fault.
C = constant from Tables 1, 2. For
parallel runs, multiply C values by
the number of conductors per phase.
I = available short-circuit current in
amperes at beginning of circuit.
Step 5 Calculate “M”
(multiplier) or take
from Table 4.
=
††
= Transf.FLA x multiplier
3o faults
KVA x 1000
at
beginning of circuit.
† Note 1. Motor short-circuit contribution, if significant, may be added to the transformer secondary short-circuit current value as determined in Step 3. Proceed with
this adjusted figure through Steps 4, 5, and 6. A practical estimate of motor short-circuit contribution is to multiply the total load current in amperes by 4.
* Note 2. The L-N fault current is higher than the L-L fault current at the secondary terminals of a single-phase center-tapped transformer. The short-circuit current available
(I) for this case in Step 4 should be adjusted at the transformer terminals as follows:
At L-N center tapped transformer terminals
I L-N = 1.5 x I L-L at Transformer Terminals
At some distance from the terminals, depending upon wire size, the L-N fault current is
lower than the L-L fault current. The 1.5 multiplier is an approximation and will theoretically vary from 1.33 to 1.67. These figures are based on change in turns ratio between
primary and secondary, infinite source available, zero feet from terminals of transformer,
and 1.2 x %X and 1.5 x %R for L-N vs. L-L resistance and reactance values. Begin
L-N calculations at transformer secondary terminals, then proceed point-to-point.
8
Note 4. Utility voltages may vary ±10% for power, and ±5.8% for 120-volt lighting services. Therefore, for worst case conditions, multiply values as calculated
in Step 3 by 1.1 and/or 1.058 respectively.
Note 5: Bolted fault approximations:
L-L-L
100% of Step 6.
L-L
87% of Step 6.
L-G, L-N 25-125% of Step 6. (Use 50% as typical)
Note 6: Arcing fault approximation for sustained arcs (percentages of L-L-L
bolted fault values)
480 Volts
208 Volts
L-L-L
89%
12%
L-L
74%
2%
L-G
38%
—
L-L-L with one primary open
80%
—
Electrical Plan Review
Point-To-Point Method Of Short-Circuit Calculation
Calculation Of Short-Circuit Currents
At Second Transformer In System.
Procedure For Second Transformer in System
Procedure
Formula
Use the following procedure to calculate the
level of fault current at the secondary of a second, downstream transformer in a system
when the level of fault current at the transformer primary is known.
Step 1
3o transformer (I P(SCA) and
I S(SCA) are 3o fault values).
KNOWN
FAULT
CURRENT
Calculate “f”
(IP(SCA), known).
H.V. UTILITY
CONNECTION
Step 2
Calculate “M”
(multiplier) or take
from Table 4.
Calculate short-circuit
current at secondary
of transformer.
(See Note 1 under
“Basic Procedure”)
1
1+f
IS(SCA) =
—
VP
x M x IP(SCA)
VS
KVATRANS = KVA rating of transformer.
%Z = Percent impedance of transformer.
Note: To calculate fault level at the end
of a conductor run, follow Steps 4, 5, and 6
of Basic Procedure.
Table 3A. Three-Phase Transformer—Full-Load
Current Rating (In Amperes).
Ampacity
Busway
Plug-In
Feeder
High Impedance
Copper
Aluminum
Copper
Aluminum
Copper
225
28700
23000
18700
12000
—
400
38900
34700
23900
21300
—
600
41000
38300
36500
31300
—
800
46100
57500
49300
44100
—
1000
69400
89300
62900
56200
15600
1200
94300
97100
76900
69900
16100
1350
119000
104200
90100
84000
17500
1600
129900
120500
101000
90900
19200
2000
142900
135100
134200
125000
20400
2500
143800
156300
180500
166700
21700
3000
144900
175400
204100
188700
23800
4000
—
—
277800
256400
—
Note: These values are equal to one over the impedance per foot for
impedance in a survey of industry.
Copper
AWG
Three Single Conductors
or
Conduit
kcmil Steel
600V
5KV
15KV
14
389
389
389
12
617
617
617
10
981
981
981
8
1557
1551
1557
6
2425
2406
2389
4
3806
3750
3695
3
4760
4760
4760
2
5906
5736
5574
1
7292
7029
6758
1/0
8924
8543
7973
2/0
10755
10061
9389
3/0
12843
11804
11021
4/0
15082
13605
12542
250
16483
14924
13643
300
18176
16292
14768
350
19703
17385
15678
400
20565
18235
16365
500
22185
19172
17492
600
22965
20567
17962
750
24136
21386
18888
1000 25278
22539
19923
M=
—
IP(SCA) = Available fault current at transformer primary.
IS(SCA) = Available fault current at transformer secondary.
VP = Primary voltage L-L.
VS = Secondary voltage L-L.
Table 1. “C” Values for Busway.
Table 2. “C” Values for Conductors
IP(SCA) x VP x 1.73 (%Z)
100,000 x KVATRANS.
1o transformer (I P(SCA) and
I
x VP x (%Z)
I S(SCA) are 1o fault values; f= P(SCA)
100,000 x KVATRANS.
I S(SCA is L-L.)
Step 3
KNOWN
FAULT
CURRENT
f=
Voltage
(Line-toLine)
208
220
240
440
460
480
600
Transformer KVA Rating
150
167
225
300
417
464
625
834
394
439
592
788
362
402
542
722
197
219
296
394
189
209
284
378
181
201
271
361
144
161
216
289
500
1388
1315
1203
657
630
601
481
750
2080
1970
1804
985
945
902
722
1000
2776
2630
2406
1315
1260
1203
962
1500
4164
3940
3609
1970
1890
1804
1444
Table 3B. Single-Phase Transformer—Full-Load
Current Rating (In Amperes).
Voltage
115/230
120/240
230/460
240/480
Transformer KVA Rating
25
50
75
100
109 217 326 435
104 208 313 416
54
109 163 217
52
104 156 208
150
652
625
326
313
167
726
696
363
348
200
870
833
435
416
250
1087
1042
544
521
333
1448
1388
724
694
(Note: These values are equal to one over the impedance per foot for impedances found in IEEE Std. 241-1990,
IEEE Recommended Practice for Commercial Building Power Systems.)
Nonmagnetic
600V
5KV
389
389
617
617
981
981
1558
1555
2430
2417
3825
3789
4802
4802
6044
5926
7493
7306
9317
9033
11423
10877
13923
13048
16673
15351
18593
17120
20867
18975
22736
20526
24296
21786
26706
23277
28033
25203
28303
25430
31490
28083
2000
5552
5260
4812
2630
2520
2406
1924
Three-Conductor Cable
Conduit
Steel
600V
5KV
389
389
617
617
981
981
1559
1557
2431
2424
3830
3811
4760
4790
5989
5929
7454
7364
9209
9086
11244
11045
13656
13333
16391
15890
18310
17850
20617
20051
22646
21914
24253
23371
26980
25449
28752
27974
31050
30024
33864
32688
15KV
389
617
981
1558
2406
3752
4802
5809
7108
8590
10318
12360
14347
15865
17408
18672
19731
21329
22097
22690
24887
9
15KV
389
617
981
1559
2414
3778
4760
5827
7188
8707
10500
12613
14813
16465
18318
19821
21042
23125
24896
26932
29320
Nonmagnetic
600V
5KV
389
389
617
617
981
981
1559
1558
2433
2428
3837
3823
4802
4802
6087
6022
7579
7507
9472
9372
11703
11528
14410
14118
17482
17019
19779
19352
22524
21938
24904
24126
26915
26044
30028
28712
32236
31258
32404
31338
37197
35748
15KV
389
617
981
1559
2420
3798
4802
5957
7364
9052
11052
13461
16012
18001
20163
21982
23517
25916
27766
28303
31959
500
2174
2083
1087
1042
Electrical Plan Review
Point-To-Point Method Of Short-Circuit Calculation
Table 4. “M” (Multiplier).*
f
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.20
*M=
M
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.93
0.92
0.91
0.87
0.83
0.80
0.77
0.74
0.71
0.67
0.63
0.59
0.55
0.53
0.50
0.45
Table 5. Short-Circuit Currents Available from Various Size Transformers
f
1.50
1.75
2.00
2.50
3.00
3.50
4.00
5.00
6.00
7.00
8.00
9.00
10.00
15.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
M
0.40
0.36
0.33
0.29
0.25
0.22
0.20
0.17
0.14
0.13
0.11
0.10
0.09
0.06
0.05
0.03
0.02
0.02
0.02
0.01
0.01
0.01
0.01
1
1+f
Voltage
And
Phase
KVA
Full
%
Short
Load
Impedance††
Circuit
Amps
(Nameplate)
Amps†
25
104
1.58
11,574
371/ 2
156
1.56
17,351
120/240
50
209
1.54
23,122
1 ph.*
75
313
1.6
32,637
100
417
1.6
42,478
167
695
1.8
60,255
25
69
1.6
4,791
50
139
1.6
9,652
75
208
1.11
20,821
100
278
1.11
27,828
150
416
1.07
43,198
120/208
225
625
1.12
62,004
3 ph.**
300
833
1.11
83,383
500
1388
1.24
124,373
750
2082
3.5
66,095
1000
2776
3.5
88,167
1500
4164
3.5
132,190
2000
5552
5.0
123,377
2500
6950
5.0
154,444
1121/ 2
135
1.0
15,000
150
181
1.2
16,759
225
271
1.2
25,082
300
361
1.2
33,426
277/480
500
601
1.3
51,362
3 ph.**
750
902
3.5
28,410
1000
1203
3.5
38,180
1500
1804
3.5
57,261
2000
2406
5.0
53,461
2500
3007
5.0
66,822
* Single phase values are L-N values at transformer terminals. These figures are
based on change in turns ratio between primary and secondary, 100,000 KVA
primary, zero feet from terminals of transformer, 1.2 (%X) and 1.5 (%R)
multipliers for L-N vs. L-L reactance and resistance values and transformer
X/R ratio = 3.
** Three-phase short-circuit currents based on “infinite” primary.
†† U.L. listed transformers 25 KVA or greater have a ±10% impedance toler-
ance. Short-circuit amps reflect a “worst case” condition.
† Fluctuations in system voltage will affect the available short-circuit current.
For example, a 10% increase in system voltage will result in a 10% increase
in the available short-circuit currents shown in the table.
Aluminum
AWG
Three Single Conductors
or
Conduit
kcmil Steel
600V
5KV
15KV
14
236
236
236
12
375
375
375
10
598
598
598
8
951
950
951
6
1480
1476
1472
4
2345
2332
2319
3
2948
2948
2948
2
3713
3669
3626
1
4645
4574
4497
1/0
5777
5669
5493
2/0
7186
6968
6733
3/0
8826
8466
8163
4/0
10740
10167
9700
250
12122
11460
10848
300
13909
13009
12192
350
15484
14280
13288
400
16670
15355
14188
500
18755
16827
15657
600
20093
18427
16484
750
21766
19685
17686
1000 23477
21235
19005
Nonmagnetic
600V
5KV
236
236
375
375
598
598
951
950
1481
1478
2350
2341
2958
2958
3729
3701
4678
4631
5838
5766
7301
7152
9110
8851
11174
10749
12862
12343
14922
14182
16812
15857
18505
17321
21390
19503
23451
21718
25976
23701
28778
26109
Three-Conductor Cable
Conduit
Steel
600V
5KV
236
236
375
375
598
598
951
951
1481
1480
2351
2347
2948
2956
3733
3719
4686
4663
5852
5820
7327
7271
9077
8980
11184
11021
12796
12636
14916
14698
15413
16490
18461
18063
21394
20606
23633
23195
26431
25789
29864
29049
15KV
236
375
598
951
1476
2333
2958
3672
4580
5645
6986
8627
10386
11847
13491
14954
16233
18314
19635
20934
23482
10
15KV
236
375
598
951
1478
2339
2948
3693
4617
5717
7109
8750
10642
12115
13973
15540
16921
19314
21348
23750
26608
Nonmagnetic
600V
5KV
236
236
375
375
598
598
951
951
1482
1481
2353
2349
2958
2958
3739
3724
4699
4681
5875
5851
7372
7328
9242
9164
11408
11277
13236
13105
15494
15299
17635
17351
19587
19243
22987
22381
25750
25243
29036
28262
32938
31919
15KV
236
375
598
951
1479
2344
2958
3709
4646
5771
7201
8977
10968
12661
14658
16500
18154
20978
23294
25976
29135
Electrical Plan Review
Peak Let-Through Charts
Peak let-through charts let you determine both the peak let-through current and the apparent prospective RMS
symmetrical let-through current. (These charts are commonly referred to as Current-Limitation Curves.)
LOW-PEAK® KRP-C Fuses
LOW-PEAK® LPJ Fuses
1000000
900000
800000
700000
600000
B
100000
90000
80000
70000
60000
B
600A
50000
500000
400A
40000
400000
100000
90000
80000
70000
60000
800A
601A
50000
AMPERE
RATING
40000
30000
20000
10000
9000
8000
7000
6000
5000
4000
200A
20000
100A
60A
10000
9000
8000
7000
6000
30A
5000
15A
4000
3000
AMPERE
RATING
2500A
2000A
1600A
1200A
2000
1000
900
800
700
600
500
400
300
3000
A
A
200
2000
PROSPECTIVE SHORT-CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES
PROSPECTIVE SHORT CIRCUIT CURRENT - SYMMETRICAL RMS AMPERES
11
200000
300000
60000
70000
80000
90000
100000
50000
40000
30000
20000
6000
7000
8000
9000
10000
4000
5000
3000
2000
600
700
800
900
1000
500
400
300
100
200000
300000
60000
70000
80000
90000
100000
50000
40000
30000
20000
5000
6000
7000
8000
9000
10000
4000
3000
2000
200
100
1000
1000
INSTANTANEOUS PEAK LET THRU CURRENT IN AMPERES
200000
INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES
6000A
5000A
4000A
3000A
300000
30000
Electrical Plan Review
Peak Let-Through Charts
FUSETRON® FRS-R Fuses
FUSETRON® FRN-R Fuses
B
200000
200000
AMPERE
RATING
300000
600A
400A
50000
40000
200A
30000
100A
20000
60A
10000
9000
8000
7000
6000
PEAK CURRENT IN AMPERES
100000
90000
80000
70000
60000
50000
30A
5000
200A
40000
30000
100A
20000
60A
10000
9000
8000
7000
6000
5000
4000
4000
3000
3000
30A
LOW-PEAK® LPS-RK Fuses
200000
50000
60000
70000
80000
90000
100000
40000
30000
20000
LOW-PEAK® LPN-RK Fuses
B
B
100,000
600
400
200
100
60
10,000
30
A
AMPERE
RATING
INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES
400,000
AMPERE
RATING
400,000
100,000
600A
400A
200A
100A
60A
10,000
30A
A
RMS SYMMETRICAL CURRENTS IN AMPERES
A-B=ASYMMETRICAL AVAILABLE PEAK (2.3 x SYMM RMS AMPS)
RMS SYMMETRICAL CURRENTS IN AMPERES
A-B=ASYMMETRICAL AVAILABLE PEAK (2.3 x SYMM RMS AMPS)
12
300,000
200,000
100,000
10,000
1,000
300,000
100,000
10,000
1,000
1,000
INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPERES
4000
AVAILABLE RMS CURRENT IN AMPERES
AVAILABLE RMS CURRENT IN AMPERES
1,000
5000
6000
7000
8000
9000
10000
1000
200000
60000
70000
80000
90000
100000
50000
30000
40000
20000
6000
7000
8000
9000
10000
4000
5000
1000
3000
1000
2000
2000
3000
A
A
2000
1000
600A
400A
2000
PEAK CURRENT IN AMPERES
100000
90000
80000
70000
60000
B
400000
300000
AMPERE
RATING
400000
Electrical Plan Review Work Sheet
Short-Circuit Calculations (Transformer)
Short-Circuit at Secondary Terminals of Transformer (Infinite Primary)
A) FROM TABLES
B) CALCULATED
Amperes
STEP 1 Find Transformer Full-Load Amperes
(3 Phase)
KVA x 1000 =
IFLA =
EL-L x 1.73
x 1000 =
x 1.73
Amperes
x 1000 =
Amperes
=
Amperes
(1 Phase)
KVA x 1000 =
IFLA =
EL-L
STEP 2 Find Multiplier:
M=
100
=
TRANS %Z
100
STEP 3 Find Short-Circuit Current:
ISCA = Transformer F.L.A. x M
=
Short-Circuit Amperes
=
x
Short-Circuit at Secondary Terminals of Transformer (Infinite Primary)
Amperes
A) FROM TABLES
B) CALCULATED
STEP 1 Find Transformer Full-Load Amperes
(3 Phase)
KVA x 1000 =
EL-L x 1.73
x 1000 =
x 1.73
Amperes
(1 Phase)
KVA x 1000 =
IFLA =
EL-L
x 1000 =
Amperes
=
Amperes
IFLA =
STEP 2 Find Multiplier:
M=
100
=
TRANS %Z
100
STEP 3 Find Short-Circuit Current:
ISCA = Transformer F.L.A. x M
=
x
Short-Circuit Amperes
=
13
Electrical Plan Review Work Sheet
Short-Circuit Calculations (Three o)
Short-Circuit Current at
1.73 x L x IL-L-L
A) f =
C x EL-L
B) f = 1.73 x
x
x
C) Multiplier “M” =
1 =
1+
1+f
=
1
D) SCA = M x lL-L-L =
=
x
=
Amperes
=
Amperes
Short-Circuit Current at
1.73 x L x IL-L-L
A) f =
C x EL-L
B) f = 1.73 x
x
x
C) Multiplier “M” =
1 =
1+f
1+
=
1
D) SCA = M x lL-L-L =
=
x
Short-Circuit Current at
1.73 x L x IL-L-L
A) f =
C x EL-L
B) f = 1.73 x
x
x
C) Multiplier “M” =
1 =
1+
1+f
D) SCA = M x lL-L-L =
=
1
=
=
x
14
Amperes
Electrical Plan Review Work Sheet
Short-Circuit Calculations (Three o)
Short-Circuit Current at
1.73 x L x IL-L-L
A) f =
C x EL-L
B) f = 1.73 x
x
x
1 =
1+f
1+
C) Multiplier “M” =
=
1
D) SCA = M x lL-L-L =
=
x
=
Amperes
=
Amperes
=
Amperes
Short-Circuit Current at
1.73 x L x IL-L-L
A) f =
C x EL-L
B) f = 1.73 x
x
x
C) Multiplier “M” =
1 =
1+f
1+
=
1
D) SCA = M x lL-L-L =
=
x
Short-Circuit Current at
1.73 x L x IL-L-L
A) f =
C x EL-L
B) f = 1.73 x
x
x
C) Multiplier “M” =
1 =
1+f
1+
D) SCA = M x lL-L-L =
=
1
=
x
15
Electrical Plan Review Work Sheet
Short-Circuit Calculations (Single o, Line-To-Line)
Short-Circuit Current at
2 x L x lL-L
A) f =
C x EL-L
B) f = 2 x
x
x
C) Multiplier “M” =
1 =
1+
1+f
=
1
D) SCA = M x lL-L =
=
x
=
Amperes
=
Amperes
=
Amperes
Short-Circuit Current at
2 x L x lL-L
A) f =
C x EL-L
B) f = 2 x
x
x
C) Multiplier “M” =
1 =
1+f
1+
=
1
D) SCA = M x lL-L =
=
x
Short-Circuit Current at
2 x L x lL-L
A) f =
C x EL-L
B) f = 2 x
x
x
C) Multiplier “M” =
D) SCA = M x lL-L =
1 =
1+f
1+
=
1
=
x
16
Electrical Plan Review Work Sheet
Short-Circuit Calculations (Single o, Line-To-Line)
Short-Circuit Current at
2 x L x lL-L
A) f =
C x EL-L
B) f = 2 x
x
x
C) Multiplier “M” =
1 =
1+
1+f
=
1
D) SCA = M x lL-L =
=
=
x
Amperes
Short-Circuit Current at
2 x L x lL-L
A) f =
C x EL-L
B) f = 2 x
x
x
C) Multiplier “M” =
1 =
1+f
1+
D) SCA = M x lL-L =
=
1
=
Amperes
=
x
Short-Circuit Current at
2 x L x lL-L
A) f =
C x EL-L
B) f = 2 x
x
x
C) Multiplier “M” =
D) SCA = M x lL-L =
1 =
1+f
1+
=
1
=
x
=
17
Amperes
Electrical Plan Review Work Sheet
Short-Circuit Calculations (Single o, Line-To-Neutral)
Short-Circuit Current at
2 x L x lL-N*
A) f =
C x EL-N
B) f = 2 x
x
=
x
C) Multiplier “M” =
1 =
1+
1+f
1
D) SCA = M x lL-N =
=
x
=
Amperes
=
Amperes
Short-Circuit Current at
2 x L x lL-N*
A) f =
C x EL-N
B) f = 2 x
x
=
x
C) Multiplier “M” =
1 =
1+f
1+
1
D) SCA = M x lL-N =
=
x
Short-Circuit Current at
2 x L x lL-N*
A) f =
C x EL-N
B) f = 2 x
x
=
x
C) Multiplier “M” =
1 =
1+f
1+
D) SCA = M x lL-N =
1
x
=
Amperes
=
*See Note 2 on Page 8
18
Electrical Plan Review Work Sheet
Short-Circuit Calculations (Single o, Line-To-Neutral)
Short-Circuit Current at
2 x L x lL-N*
A) f =
C x EL-N
B) f = 2 x
x
=
x
C) Multiplier “M” =
1 =
1+
1+f
D) SCA = M x lL-N =
1
=
x
=
Amperes
=
Amperes
=
Amperes
Short-Circuit Current at
2 x L x lL-N*
A) f =
C x EL-N
B) f = 2 x
x
=
x
C) Multiplier “M” =
1 =
1+
1+f
D) SCA = M x lL-N =
1
=
x
Short-Circuit Current at
2 x L x lL-N*
A) f =
C x EL-N
B) f = 2 x
x
=
x
C) Multiplier “M” =
1 =
1+f
1+
D) SCA = M x lL-N =
1
x
=
*See Note 2 on Page 8
19
Electrical Plan Review
Grounding & Bonding of Service Equipment
Bonding
Why? Section 250-90
What? Section 250-92
How? Section 250-94
Material: Section 250-102(a)
Size: Section 250-102(c)
MAIN DISTRIBUTION PANEL
METER
BASE
NEUTRAL
Equipment
Grounding
Conductor
Material: Section 250-118
Install: Section 250-120
800/800
Grounded Neutral Service
Entrance Conductors to Pad
Mount Transformer
Neutral Grounded Conductor
Size: Sections 230-42, 220-22
When Serving As Grounding
Conductor:
What? Section 250-24(a)
Size: Section 250-24(b)
Section 250-66
IMPORTANT:
Effective Bonding and
Grounding Required:
Section 250-2(d)
Section 250-90
Section 250-96(a)
Must have capacity
to conduct safely
any fault current
likely to be
imposed on it.
Supplemental
Ground
(If Required)
Section 250-50(a)
Neutral Disconnecting Means
Section 230-96
Main Bonding Jumper: Section 250-28, 250-96
Material: Section 250-28(a)
Size: Section 250-28(d)
Connect: Section 250-28(c)
Grounding Electrode Conductor
Size: Section 250-66
Material: Section 250-62
Install: Section 250-64
Enclosure: Section 250-64(e)
What: Section 250-24
Grounding
Electrode
System
Section 250-50
Connection to Electrode
Sections 250-68, 8, 70
Bonding of Metal Water Pipes
Section 250-104
20
Electrical Plan Review
Typical Component—Short Circuit Current Ratings
Component
Short-circuit current rating, kA
Clock-Operated Switch
5,000
HVAC Equipment
110-120V
9.8 or less
9.9-16.0
16.1-34.0
34.1-80.0
Over 80.0
Single-Phase-Amps
200-208V
5.4 or less
5.5-8.8
8.9-18.6
18.7-44.0
Over 44.0
3-Phase-Amps
220-240V
2.0 or less
2.1-3.5
3.6-9.0
9.1-22.0
Over 22.0
200-208V
2.12 or less
2.13-3.7
3.8-9.5
9.6-23.3
Over 23.3
Meter Socket Base
Motor Controller,
Rated in Horsepower (kW)
a. 0-50 (0-37.3)
b. 51-200 (38-149)
c. 201-400 (150-298)
d. 401-600 (299-447)
e. 601-900 (448-671)
f. 901-1600 (672-1193)
Photoelectric Switches
Receptacle (GFCI Type)
Receptacle
(Other Than GFCI Type)
Snap Switch
Terminal Block
Thermostat
220-240V
4.9 or less
5.0-8.0
8.1-17.0
17.1-40.0
Over 40.0
254-277V
—
6.65 or less
—
—
Over 6.65
440-480V
—
1.8 or less
—
—
Over 1.8
550-600V
—
1.4 or less
—
—
Over 1.4
200
1,000
2,000
3,500
5,000
200
1,000
2,000
3,500
5,000
10,000
5,000
10,000
18,000
30,000
42,000
85,000
5,000
10,000
2,000
5,000
10,000
5,000
Wire
Copper, 75° Thermoplastic Insulated Cable
Copper
Wire Size
75° Thermoplastic
#14
#12
#10
#8
#6
#4
Maximum Short-Circuit Withstand Current in Amperes
For
1/2 Cycle
For
1 Cycle
For
2 Cycles
For
3 Cycles
2,400
3,800
6,020
9,600
15,200
24,200
1,700
2,700
4,300
6,800
10,800
17,100
1,200
1,900
3,000
4,800
7,600
12,100
1,000
1,550
2,450
3,900
6,200
9,900
21
Electrical Plan Review
Work Sheet Problem—Main Distribution Panel
PRIMARY FUSE
1
300 KVA Transformer by Utility
120/208 Volt
3 Phase, 4-Wire
2% Impedance
1 METER
(2) 3" C. each with 4-500 kcmils/XHHW - 20 Feet
4 - #8 THHN, 3/4" C. - 10 Feet
8
EMP
800/800
3
200/200
4 - #3/0 THHN, 2" C. - 60 Feet
1/2" C. - 30'
FLUOR.
3 - #12 THHN
2
Ground Buss
FIXTURE
2 MAIN SWITCH
9
LPA
3
4
200/150
4 - #1/0 THHN, 1-1/2" C. - 15 Feet
LPC
4
100/100
4 - #3 THHN, 1-1/4" C. - 20 Feet
LPB
5
6
100/90
AC-1
3 - #3 THHN, 1" C. - 35 Feet
6
7
100/70
3 - #4 THHN, 1" C. - 35 Feet
AC-2
7
8
9
3 - #8 THHN, 3/4" C.- 4 Feet
5
10
100/
200/
7-1/2
Combination
Motor
Controller
22