Advanced Calculations Drill 1 for
the 2011 NEC
© Ted “Smitty” Smith 2012
• What is the minimum sized service required for a machine shop if the
shop is equipped with the following:
• 50 feet of multioutlet assembly expected to be used simultaneously
• 50 duplex receptacles at 125V
• The following motors
–
–
–
–
Single phase, 208V, ¼ hp, 2.9 A varying duty motor, 15 mins
Single phase, 208V, 1 ½ hp, 9.2 A intermittent duty motor, 5 mins
Three phase, 208V, 5 hp, 14.9 A 15 minute, short time duty motor, 15 mins
(3) single phase, 120V, ½ hp, 120V, 11.1 A, intermittent duty motors, 30 mins
• A 208V three phase, 41.6 A rated roof mounted HVAC system
• The machine shop is 15000 square feet with 3000 square feet classified
office and 12,000 square feet classified warehouse.
• The service will be 208V, three phase.
• 175 ampere service
•
•
•
•
Lighting= 10500 + 3000=
13500 va
Recepts= 9000+ 9000= 18000 -10000=8000 x.5 =4000
10000+ 4000=
14000 va
Motors =
6441.3 va
•
723.84 va
1626.5 va
1198.8 va
1198.8 va
1198.8 va
Roof Mounted HVAC 41.6 x 1.732 x 208=
14986.6 va
Total = 54874.64 / 208 /1.732= 152.32 amperes
Motor Calc
• What is the minimum sized THHN wire that
can be used to feed the following motor:
• Three Phase, squirrel cage, 30 HP, 480 Volts,
33.6 amperes FLC, 1.15 marked service factor
and Design K.
• 6 AWG THHN
• Based on the 60 degree column of 310.16
based on 110.14(C)
Welding Calculation
• What is the minimum sized THHN copper
conductor that can be used to feed six arc
welders with the following ratings:
• 1. 50 A at 30 Duty Cycle
• 2. 60 A at 40 duty cycle
• 3. 100 at 10 duty cycle
• 4. 100 A at 10 duty cycle
• 5. 75 A at 50 duty cycle
• 6. 44.6 A at 60 duty cycle
• 250 KCMIL
• Determined by 6301.11 (A )and (B)
• Table 310.16 Column 60 degrees C based on
110.14(C)
Load Calc
• How many 15 ampere lighting circuits will be
required for a single family dwelling that is
2650 square feet to meet the minimum
requirements of the National Electric Code?
• 5
• 2650 x 3= 7950/120=66.25 amperes/ 15= 4.41
• Total of 5 circuits
Load Calcs
• What is the demand on the main service
entrance of a multifamily dwelling for 15
electric clothes dryers that are rated at 4.4 kW
at 240 volts?
• 32250 VA
• 5000 x 15= 75000
• 75000 x .43= 32250 VA
Load Calcs
• What is the demand on the main service
entrance of a multifamily dwelling for the
lighting and receptacle loads for 20
apartments if each apartment is 1300 square
feet?
• 55950 VA
•
•
•
•
•
•
3 x 1300 x 20= 78000
4500 x 20= 90000
78000 + 90000= 168000- 3000= 165000
165000- 117000= 48000 117000 x .35= 40950
48000 x .25= 12000
12000 + 40950 + 3000= 55950 VA
Motor Calc
• What is the maximum sized instantaneous trip
breaker that can be used to protect the
following motor for short circuit protection:
• 50 HP, 208V, three phase, squirrel cage motor,
116.8 amperes FLC, marked service factor of
1.15?
• 1200 Amperes
• Based on T430.52 and 240.6 for standard sized
fuses and circuit breakers.
• What is the calculated load on the branch
circuit of a three phase, 480V, 50 HP squirrel
cage motor that operates a ore rolling chute?
The motor has a nameplate rating of 61.6
amperes. The motor is rated to operate on
intermittent duty and is rated for 60 minute
operation.
• 55.4 amperes
• 430.22(E)
• What is the minimum sized THHN conductor
needed to feed a short time duty rated motor
that is single phase, 208V, 5 HP, 26.9 amperes,
and is rated to operate 5 minutes? The motor
is rated at 75 degrees C.
• 10 AWG
• 29.59 amperes
Conductor Ampacity
• A 1/0 AWG THHN conductor installed in an
ambient temperature of 103 degrees F. will
have an ampacity of _________.
Copyright 2005 Ted "Smitty" Smith
Conductor Ampacity
• 154.70 Amperes
Conductor Ampacity
What is the ampacity of a 250 kCMIL aluminum
THW conductor installed in an ambient
temperature of 44 degrees Celsius?
Copyright 2005 Ted "Smitty" Smith
Conductor Amapacity
• 168.100 amperes
Box Fill
• What is the volume allowance required for the
following junction box which has a total of
nine 12 AWG THHN conductors entering the
junction box, one of which is an equipment
grounding conductor. The box will contain one
duplex receptacle, has no internal clamps, and
no luminairie studs or hickeys.
Box Fill
• 24.75
Conductor Ampacity
• A load of 23 amperes is being fed by a circuit
consisting of 10 AWG THHN copper conductors.
The conductors are installed in a conduit that is
installed on a rooftop in the direct sunlight. The
conduit is installed 12” above the roof from the
bottom of the conduit. The average ambient
temperature for the area in which the conduit is
installed is 87 degrees F. The conduit contains a
total of three current carrying conductors. Is this
an acceptable installation?
Copyright 2005 Ted "Smitty" Smith
Conductor Ampacity
• Yes, it is rated to carry 32.8 amperes.
Raceway Fill
• Three 10 AWG THHN, four 6 AWG THW and
five 8 AWG TW conductors are installed in a
one and one half inch electrical metallic
tubing conduit that is 280 feet in length. Is this
an acceptable installation?
Copyright 2005 Ted "Smitty" Smith
Raceway Fill
• Yes, the wires make up a volume of .5722 and at 40%
fill an EMT 1 1/2” conduit is .814.
Copyright 2005 Ted "Smitty" Smith
Conductor Ampacity
• What is the ampacity of 2 AWG TW aluminum
conductor installed in a conduit that is
installed in a boiler room with an ambient
temperature of 103 degrees F and the
conductor is installed in a conduit with a total
of 17 current carrying conductors?
Conductor Amapacity
• 30.75 Amperes
Box Fill
• A 4 x 1 ½ square metal box is installed with
two duplex receptacles. The box has a total of
three 12 AWG MC cables entering the box,
each MC cable is a 12/2 with ground. The box
contains no internal cable clamps, and no
luminairie studs or hickeys. Is this an
acceptable installation?
Box Fill
• No, the conductors and devices require a
volume allowance of 24.75 and Table
314.16(A) lists a 4 x 1 ½” metal box as having a
volume of 21.0
•
•
•
•
What is the minimum sized service, minimum sized aluminum grounded conductor
and minimum sized copper grounding electrode conductor required for a machine
shop if the shop is equipped with the following:
100 feet of multioutlet assembly not expected to be used simultaneously
75 duplex receptacles at 125V
The following motors
–
–
–
–
Single phase, 208V, ½ hp, 1.9 A varying duty motor, 5 mins
Single phase, 208V, 5 hp, 28.7A intermittent duty motor, 30 mins
Single phase, 208V, 10 hp, 48.6 A 5 minute, short time duty motor, 15 mins
(6) single phase, 120V, 1 hp, 120V, 14.8 A, intermittent duty motors, 30 mins
•
•
A 208V three phase, 58.7 A rated roof mounted HVAC system
The machine shop is 36000 square feet with 5000 square feet classified office and
29000 square feet classified warehouse.
•
The service will be 208V, three phase. The service entrance conductors will be
THHN aluminum. All circuits are specified to be rated at 75 degrees C.
•
300A ampere service, 500 kcmil AL SE conductors, 500 kcmil AL grounded conductor, 2 AWG copper grounding electrode
conductor
•
•
•
Lighting= 17500+7250=
Recepts= 17100 VA- 10000= 7100*.5=3550
10000+ 3550=
– Motors =
Single phase, 208V, ½ hp, 1.9 A varying duty motor, 5 mins
•
•
•
•
•
•
•
24750 va
13550 va
434.72 va
–
Single phase, 208V, 5 hp, 28.7A intermittent duty motor, 30 mins
5074.16 va
–
Single phase, 208V, 10 hp, 48.6 A 5 minute, short time duty motor, 15 mins
12130.56 va
–
(6) single phase, 120V, 1 hp, 120V, 14.8 A, intermittent duty motors, 30 mins
16623.36 va
Roof Mounted HVAC 58.7 x 1.732 x 208=
21147.03
Total = 93709.83 / 208 /1.732= 260.12 amperes= 300A service
300A at 75C Aluminum = 500 kcmil Service Entrance Conductors
Neutral Loads Lighting= 24750 + Recep= 13550 + 120V motors= 16623.36
Total Neutral Load= 54923.36va /120V= 457.69 A. 220.24 Requires the neutral to be no larger than the ungrounded
conductors so this would require 500 KCMIL grounded conductor
T250.66 Requires a 2 AWG copper grounding electrode conductor if the service entrance conductors are 500 kcmil
aluminum.
Raceway Fill
• Is it allowable to add three 10 AWG THHN
conductors to an existing conduit that is 150
feet in length and currently has eight 6 AWG
THWN conductors and one 12 AWG THHN
conductor installed? The conduit is a 1 ¼” inch
electrical metallic tubing.
Raceway Fill
• No. The total area used by the conductors
once the three 10 AWG conductors are added
will be .4822 inches and a 1 ¼” EMT at 31% fill
has an allowance of only .464 inches.
Conductor Ampacity
• A 39.6 ampere load is fed with a 6 AWG THW
conductor that is installed in a conduit with a
total of 7 current carrying conductors. The
conduit is installed in a room that has an
ambient temperature 38 degrees Celsius. Is
this an acceptable installation?
Conductor Ampacity
• Yes it is acceptable. The conductor is rated at
40.04 amperes.
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• What is the minimum sized service required for a machine shop if the
shop is equipped with the following:
• 50 feet of multioutlet assembly expected to be used simultaneously
• 50 duplex receptacles at 125V
• The following motors
–
–
–
–
Single phase, 208V, ¼ hp, 2.9 A varying duty motor, 15 mins
Single phase, 208V, 1 ½ hp, 9.2 A intermittent duty motor, 5 mins
Three phase, 208V, 5 hp, 14.9 A 15 minute, short time duty motor, 15 mins
(3) single phase, 120V, ½ hp, 120V, 11.1 A, intermittent duty motors, 30 mins
• A 208V three phase, 41.6 A rated roof mounted HVAC system
• The machine shop is 15000 square feet with 3000 square feet classified
office and 12,000 square feet classified warehouse.
• The service will be 208V, three phase.
• 175 ampere service
•
•
•
•
Lighting= 10500 + 3000=
13500 va
Recepts= 9000+ 9000= 18000 -10000=8000 x.5 =4000
10000+ 4000=
14000 va
Motors =
6441.3 va
•
723.84 va
1626.5 va
1198.8 va
1198.8 va
1198.8 va
Roof Mounted HVAC 41.6 x 1.732 x 208=
14986.6 va
Total = 54874.64 / 208 /1.732= 152.32 amperes
Motor Calc
• What is the minimum sized THHN wire that
can be used to feed the following motor:
• Three Phase, squirrel cage, 30 HP, 480 Volts,
33.6 amperes FLC, 1.15 marked service factor
and Design K.
• 6 AWG THHN
• Based on the 60 degree column of 310.16
based on 110.14(C)
Welding Calculation
• What is the minimum sized THHN copper
conductor that can be used to feed six arc
welders with the following ratings:
• 1. 50 A at 30 Duty Cycle
• 2. 60 A at 40 duty cycle
• 3. 100 at 10 duty cycle
• 4. 100 A at 10 duty cycle
• 5. 75 A at 50 duty cycle
• 6. 44.6 A at 60 duty cycle
• 250 KCMIL
• Determined by 6301.11 (A )and (B)
• Table 310.16 Column 60 degrees C based on
110.14(C)
Load Calc
• How many 15 ampere lighting circuits will be
required for a single family dwelling that is
2650 square feet to meet the minimum
requirements of the National Electric Code?
• 5
• 2650 x 3= 7950/120=66.25 amperes/ 15= 4.41
• Total of 5 circuits
Load Calcs
• What is the demand on the main service
entrance of a multifamily dwelling for 15
electric clothes dryers that are rated at 4.4 kW
at 240 volts?
• 32250 VA
• 5000 x 15= 75000
• 75000 x .43= 32250 VA
Load Calcs
• What is the demand on the main service
entrance of a multifamily dwelling for the
lighting and receptacle loads for 20
apartments if each apartment is 1300 square
feet?
• 55950 VA
•
•
•
•
•
•
3 x 1300 x 20= 78000
4500 x 20= 90000
78000 + 90000= 168000- 3000= 165000
165000- 117000= 48000 117000 x .35= 40950
48000 x .25= 12000
12000 + 40950 + 3000= 55950 VA
Motor Calc
• What is the maximum sized instantaneous trip
breaker that can be used to protect the
following motor for short circuit protection:
• 50 HP, 208V, three phase, squirrel cage motor,
116.8 amperes FLC, marked service factor of
1.15?
• 1200 Amperes
• Based on T430.52 and 240.6 for standard sized
fuses and circuit breakers.
• What is the calculated load on the branch
circuit of a three phase, 480V, 50 HP squirrel
cage motor that operates a ore rolling chute?
The motor has a nameplate rating of 61.6
amperes. The motor is rated to operate on
intermittent duty and is rated for 60 minute
operation.
• 55.4 amperes
• 430.22(E)
• What is the minimum sized THHN conductor
needed to feed a short time duty rated motor
that is single phase, 208V, 5 HP, 26.9 amperes,
and is rated to operate 5 minutes? The motor
is rated at 75 degrees C.
• 10 AWG
• 29.59 amperes
Conductor Ampacity
• A 1/0 AWG THHN conductor installed in an
ambient temperature of 103 degrees F. will
have an ampacity of _________.
Copyright 2005 Ted "Smitty" Smith
Conductor Ampacity
• 154.70 Amperes
Conductor Ampacity
What is the ampacity of a 250 kCMIL aluminum
THW conductor installed in an ambient
temperature of 44 degrees Celsius?
Copyright 2005 Ted "Smitty" Smith
Conductor Amapacity
• 168.100 amperes
Box Fill
• What is the volume allowance required for the
following junction box which has a total of
nine 12 AWG THHN conductors entering the
junction box, one of which is an equipment
grounding conductor. The box will contain one
duplex receptacle, has no internal clamps, and
no luminairie studs or hickeys.
Box Fill
• 24.75
Conductor Ampacity
• A load of 23 amperes is being fed by a circuit
consisting of 10 AWG THHN copper conductors.
The conductors are installed in a conduit that is
installed on a rooftop in the direct sunlight. The
conduit is installed 12” above the roof from the
bottom of the conduit. The average ambient
temperature for the area in which the conduit is
installed is 87 degrees F. The conduit contains a
total of three current carrying conductors. Is this
an acceptable installation?
Copyright 2005 Ted "Smitty" Smith
Conductor Ampacity
• Yes, it is rated to carry 32.8 amperes.
Raceway Fill
• Three 10 AWG THHN, four 6 AWG THW and
five 8 AWG TW conductors are installed in a
one and one half inch electrical metallic
tubing conduit that is 280 feet in length. Is this
an acceptable installation?
Copyright 2005 Ted "Smitty" Smith
Raceway Fill
• Yes, the wires make up a volume of .5722 and at 40%
fill an EMT 1 1/2” conduit is .814.
Copyright 2005 Ted "Smitty" Smith
Conductor Ampacity
• What is the ampacity of 2 AWG TW aluminum
conductor installed in a conduit that is
installed in a boiler room with an ambient
temperature of 103 degrees F and the
conductor is installed in a conduit with a total
of 17 current carrying conductors?
Conductor Amapacity
• 30.75 Amperes
Box Fill
• A 4 x 1 ½ square metal box is installed with
two duplex receptacles. The box has a total of
three 12 AWG MC cables entering the box,
each MC cable is a 12/2 with ground. The box
contains no internal cable clamps, and no
luminairie studs or hickeys. Is this an
acceptable installation?
Box Fill
• No, the conductors and devices require a
volume allowance of 24.75 and Table
314.16(A) lists a 4 x 1 ½” metal box as having a
volume of 21.0
•
•
•
•
What is the minimum sized service, minimum sized aluminum grounded conductor
and minimum sized copper grounding electrode conductor required for a machine
shop if the shop is equipped with the following:
100 feet of multioutlet assembly not expected to be used simultaneously
75 duplex receptacles at 125V
The following motors
–
–
–
–
Single phase, 208V, ½ hp, 1.9 A varying duty motor, 5 mins
Single phase, 208V, 5 hp, 28.7A intermittent duty motor, 30 mins
Single phase, 208V, 10 hp, 48.6 A 5 minute, short time duty motor, 15 mins
(6) single phase, 120V, 1 hp, 120V, 14.8 A, intermittent duty motors, 30 mins
•
•
A 208V three phase, 58.7 A rated roof mounted HVAC system
The machine shop is 36000 square feet with 5000 square feet classified office and
29000 square feet classified warehouse.
•
The service will be 208V, three phase. The service entrance conductors will be
THHN aluminum. All circuits are specified to be rated at 75 degrees C.
•
300A ampere service, 500 kcmil AL SE conductors, 500 kcmil AL grounded conductor, 2 AWG copper grounding electrode
conductor
•
•
•
Lighting= 17500+7250=
Recepts= 17100 VA- 10000= 7100*.5=3550
10000+ 3550=
– Motors =
Single phase, 208V, ½ hp, 1.9 A varying duty motor, 5 mins
•
•
•
•
•
•
•
24750 va
13550 va
434.72 va
–
Single phase, 208V, 5 hp, 28.7A intermittent duty motor, 30 mins
5074.16 va
–
Single phase, 208V, 10 hp, 48.6 A 5 minute, short time duty motor, 15 mins
12130.56 va
–
(6) single phase, 120V, 1 hp, 120V, 14.8 A, intermittent duty motors, 30 mins
16623.36 va
Roof Mounted HVAC 58.7 x 1.732 x 208=
21147.03
Total = 93709.83 / 208 /1.732= 260.12 amperes= 300A service
300A at 75C Aluminum = 500 kcmil Service Entrance Conductors
Neutral Loads Lighting= 24750 + Recep= 13550 + 120V motors= 16623.36
Total Neutral Load= 54923.36va /120V= 457.69 A. 220.24 Requires the neutral to be no larger than the ungrounded
conductors so this would require 500 KCMIL grounded conductor
T250.66 Requires a 2 AWG copper grounding electrode conductor if the service entrance conductors are 500 kcmil
aluminum.
Raceway Fill
• Is it allowable to add three 10 AWG THHN
conductors to an existing conduit that is 150
feet in length and currently has eight 6 AWG
THWN conductors and one 12 AWG THHN
conductor installed? The conduit is a 1 ¼” inch
electrical metallic tubing.
Raceway Fill
• No. The total area used by the conductors
once the three 10 AWG conductors are added
will be .4822 inches and a 1 ¼” EMT at 31% fill
has an allowance of only .464 inches.
Conductor Ampacity
• A 39.6 ampere load is fed with a 6 AWG THW
conductor that is installed in a conduit with a
total of 7 current carrying conductors. The
conduit is installed in a room that has an
ambient temperature 38 degrees Celsius. Is
this an acceptable installation?
Conductor Ampacity
• Yes it is acceptable. The conductor is rated at
40.04 amperes.