Advanced Calculations Drill 1 for the 2011 NEC © Ted Smitty Smith 2012.

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 vaRoof Mounted HVAC 41.6 x 1.732 x 208= 14986.6 vaTotal = 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?


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?


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?


Raceway Fill

• Yes, the wires make up a volume of .5722 and at 40% fill an EMT 1 1/2” conduit is .814.


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= 24750 va• Recepts= 17100 VA- 10000= 7100*.5=3550• 10000+ 3550= 13550 va

– Motors = Single phase, 208V, ½ hp, 1.9 A varying duty motor, 5 mins 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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