USITT Electrical Workshop
Welcome
Basic Electricity
201
Copyright (C) 1997, 2001, 2013 USITT
Ken Vannice
Leviton/NSI/Colortran
USITT Representative NEC Panel 15
Electrical SME Chair
Recognized ETCP Trainer
The National Electrical Code made simple
Two separate principles
A. Protection of equipment (objective)
B. Reduction of the likelihood that equipment will be stressed (subjective)
A. An objective approach to feeding dimmer packs
Article 520 of the NEC requires:
Three-phase, four-wire packs must have a double-size neutral bus, except
if the pack is field-convertible to single-phase, three-wire, in which case it
needs to be only large enough for the single-phase case.
Power supply cable neutral shall be sized considering the neutral as a currentcarrying conductor.
Supply cable shall be multi-conductor, and have ampacity suitable for the
(total possible) load and not less than the rating of the connectors.
Supply receptacle outlet shall be protected for its rated ampacity.
Pack nameplate rating:
80A, 120/208V, 3Ph, 4Wire, 60Hz or
125A, 120/240V, 1Ph, 3Wire, 60Hz
For the three-phase case:
Our cable has four current-carrying conductors, 3 phase conductors and one
neutral conductor, the grounding conductor hopefully doesn’t carry current
NEC Section 400-5 states that we need to derate the 3-conductor values in
Table 400-5(A) or (B) by 80%. We can either multiply the table values by 0.8
or our requirement value by 1.25. 80 x 1.25 = 100 Table 400-5(A) does not
contain 100A so we must look in Table 400-5(B). The 60oC Column F
requires 2AWG cable.
Finishing the discussion, we must have 80A connectors and an 80A circuit
breaker.
Note that everything is protected for 80A including the conductors in the cable
that inter-heat each other.
Table 400.5 (A)
Size
(AWG)
18
16
14
12
10
8
6
4
2
A
(3 Current
Carrying Cond.)
7
10
15
20
25
35
45
60
80
B
(2 Current
Carrying Cond.)
10
13
18
25
30
40
55
70
95
B. The subjective approach to feeding dimmer packs
The previous example is the most desirable situation and should be a goal,
especially if the actual load is not known. Then reality sets in. Connectors
with 80A ratings aren’t very available and we don’t have 80A available either.
NEC Section 220-10 (a) is entitled Ampacity and Computed Load. It continues
by requiring the feeders to be large enough to supply the load served. It also
requires that the computed load be no less than the sum of the loads of all
the circuits supplied.
Here is the subjective part. Who calculates the loads? You.
Who agrees or disagrees with your calculations? The electrical inspector.
Hookup
Dimmer
1
2
3
4
5
6
7
8
9
10
11
12
Production: The Sandbox
Location Unit# Type
1st. Elect. 26
6” Fresnel
1st. Elect. 35
6” Fresnel
2nd. Elect. 48/49 2-6’ Strip
1st. Elect. 30
6” Ellips
1st. Elect. 28
6” Fresnel
1st. Elect. 37
6” Fresnel
2nd. Elect. 48/49 2-6’ Strip
2nd. Elect. 51
8” Fresnel
1st. Elect. 29
6” Fresnel
1st. Elect. 38
6” Fresnel
2nd. Elect. 48/49 2-6’ Strip
Floor
89
Lamp
Date: 4/1/97
Use
Color
Area 1L #009
Area 1R #003
Red Cyc #027
UR Spec #328
Area 2L #009
Area 2R #003
Grn Cyc #090
Sun Spl #021
Area 3L #009
Area 3R #003
Blue Cyc #068
Pract.
None
Ph-A Ph-B Ph-C
1000
1000
1200
1000
1000
1000
1200
2000
1000
1000
1200
100
Total Watts
4200 5200 3300
Total Amps
35
43
28
Feeder Req’d: 50A, 120/208V, 3Ph, 4W
50A is kind of tight but 60A is quite available. Remember lamp bulbs have a
tolerance of +/- 10% or so.
60A x 1.25 = 75A From Column A of Table 400-5(A) 2AWG is required.
The connector and circuit breaker must be rated for 60A.
Again note that the system is protected. If someone changes all the 1kW’s
to 2 kW’s the breaker will protect the equipment and wiring.
Other ‘subjective’ thoughts:
The NEC defines continuous load as one likely to be on continuously for three
hours. In several places it talks about 100% non-continuous load plus 125% of
continuous load. This essentially derates circuits to 80% or 1920W on a 20A
branch circuit. In our application we rarely have all the dimmers on at full
intensity for over 3 hours. We do need to derate circuits such as worklights,
rehearsal lights, stage apron lecture lights, auditorium lecture/classroom house
lights and the like. Furthermore, this 80% derating factor is on the use of the
circuit and not the 80% derating factor applied to multiple conductors.
The subjective part of the NEC requirements sounds like a game between you
and a mythical inspector, but it is not. If there is a problem and an inspector
finds a violation after the fact, your insurance is as good as gone. Furthemore,
if you undersize your circuits your lights will go off in the middle of the show.
The “big enough” rule
NEC Section 520-9 essentially says that receptacle voltage and current ratings
shall not be less than the circuit rating. Table 210-21(b)(2) shall not apply.
80A
80A
80A x 1.25 = 100A
80A Bus
80A
100A
80A x 1.25 = 100A
80A Bus
100A
100A
80A x 1.25 = 100A
80A Bus
Do not assume that receptacles are protected at their rating. Check the
rating of the circuit breaker or fuse.
Do not assume that the voltage available at the receptacle is the same
as the rating of the receptacle. Check the voltage of the source.
Single-Phase, Three-Wire (Split Phase)
Pack nameplate rating: 125A, 120/208V, 1Ph, 3Wire, 60Hz
Our cable has three current-carrying conductors plus the grounding
conductor. Three current-carrying conductors do not require the values
in Tables 400-5(A) and (B) to be derated. Table 400-5(A) contains no values
above 95A so we must look in the 60oC Column F of Table 400-5(B). The
table indicates that 1AWG is required.
125A
200A
125A
125A Bus
Note that if the pack didn’t have the ability to be strapped for split phase, two
of the three phases would have to be connected to one of the split phase legs.
For full use of the pack that leg would have to be rated for 160A. Since the
neutral bus is only rated for 80A the main breaker could only be rated for 80A
resulting in the pack loosing much of its capability.
Hookup
Dimmer
1
2
3
4
5
6
7
8
9
10
11
12
Production: The Sandbox
Location Unit# Type
1st. Elect. 26
6” Fresnel
1st. Elect. 35
6” Fresnel
2nd. Elect. 48/49 2-6’ Strip
1st. Elect. 30
6” Ellips
1st. Elect. 28
6” Fresnel
1st. Elect. 37
6” Fresnel
2nd. Elect. 48/49 2-6’ Strip
2nd. Elect. 51
8” Fresnel
1st. Elect. 29
6” Fresnel
1st. Elect. 38
6” Fresnel
2nd. Elect. 48/49 2-6’ Strip
Floor
89
Lamp
Date: 4/1/97
Use
Color
Area 1L #009
Area 1R #003
Red Cyc #027
UR Spec #328
Area 2L #009
Area 2R #003
Grn Cyc #090
Sun Spl #021
Area 3L #009
Area 3R #003
Blue Cyc #068
Pract.
None
Ph-A
1000
1000
1200
1000
1000
1000
Ph-C
1200
2000
1000
1000
1200
100
Total Watts
6200
6500
Total Amps
52
55
Feeder Reqd: 60A, 120/240V, 1Ph, 3W
From Table 400-5(A) Column A 4AWG is appropriate.
In summary the breaker needs to be 60A and the connector needs to be at least
60A because the 4AWG cable is 60A.
“Tap” Rules
200A
200A
125A
125A Bus
The general rule is that the overcurrent protection device must be located
where the wiring changes size.
Section 520-53(h) of the NEC contains two exceptions which are known as
the “tap” rules.
The first one states that if the conductors are not over 10ft. long between the
supply and switchboard or subsequent overcurrent device the overcurrent
device protecting the conductors may be 4 times larger providing the following
conditions are met:
1. The supply conductors shall terminate in a single overcurrent device no
larger than the rating of the conductors.
2 thru 6. The supply conductors shall meet other requirements that reduce
the likelihood of damage.
This does not mean that the conductors may be used at 4 times their rating!!!
10ft. Max.
200A
200A
125A
125A
125A Bus
10ft. Max.
200A
200A
125A
125A
125A Bus
The second rule is similar except the length is 20ft., the overcurrent device
may be only twice the rating and the restrictions are greater.
That’s All Folks
Non-Resistive Loads
• In order to simplify the previous discussion we assumed
that the load was resistive. What if that is not true?
• The formula is: E = I x Z.
• The Impedance (Z) is made up of a resistive component, a
inductive component, and a capacitive component.
• Resistive loads: E = I x R. Resistive loads use energy and
provide power to do useful work.
Non-Resistive Loads
• Inductive loads: E = I x XL
• XL, the inductive reactance, is a function of
frequency and the inductance of the load. Inductive
loads store energy in a magnetic field.
• Capacitive loads: E = I x XC
• XC, the capacitive reactance, is a function of
frequency and the capacitance of the load.
Capacitive loads store energy in an electrostatic field.
Effect of Loads
• In the case of Resistive
loads, the current is in
line with the voltage.
• In the case of Inductive
loads, the current lags
(occurs later than) the
voltage.
• In the case of
Capacitive loads, the
current leads the
voltage.
Voltage and also
1.5
Resistive Current
Q
1
0.5
0
-0.5
-1
-1.5
Inductive Current
(lagging)
Load Math
• When the current is not in line with the
voltage, a more complex math involving the
concept of vectors is required.
Power Factor = cos Q
pf of 1 is resistive
Q
Power in Watts
pf of .9 lagging is inductive
Reactive Power in VARS
+ for Inductive
- for Capacitive