motor branch circuit minimum ampacity

Hello again.

I'm bruching up for the exam, and came across some conflicting information. this is an example question taken from several test prep sources:

motor load is 16 amps, what is smallest 75degree conductor you can use?

all examples size the load as 16 x 125% = 20A adjusted ampacity. but then one of my books says that is a # 12 AWG cu wire, while the other source says it is a 14 AWG cu wire. I found a couple of places on the internet that agree with the # 14, which agrees with my vague recollection of trade school, but when i read 240.4(D) in The Book, it directs me to choose the wire in accordance with sections 3 - 7 of article 430, and i'm having trouble pinning down exactly if it is the value given in table 310.15, 20A, or the adjusted value in 240.4(D), 15A, that is correct. Help!?!

appreciatively,
Jim
 

texie

Senior Member
Hello again.

I'm bruching up for the exam, and came across some conflicting information. this is an example question taken from several test prep sources:

motor load is 16 amps, what is smallest 75degree conductor you can use?

all examples size the load as 16 x 125% = 20A adjusted ampacity. but then one of my books says that is a # 12 AWG cu wire, while the other source says it is a 14 AWG cu wire. I found a couple of places on the internet that agree with the # 14, which agrees with my vague recollection of trade school, but when i read 240.4(D) in The Book, it directs me to choose the wire in accordance with sections 3 - 7 of article 430, and i'm having trouble pinning down exactly if it is the value given in table 310.15, 20A, or the adjusted value in 240.4(D), 15A, that is correct. Help!?!

appreciatively,
Jim
The correct answer is #14. The so called "small conductor rule" of 240.4(D) directs you to 240.4(G), which sends you to Table 240.4(G), which sends you to art 430 for conductor size. In other words the "small conductor rule" does not apply in the case of a motor.
 

kwired

Electron manager
Location
NE Nebraska
Hello again.

I'm bruching up for the exam, and came across some conflicting information. this is an example question taken from several test prep sources:

motor load is 16 amps, what is smallest 75degree conductor you can use?

all examples size the load as 16 x 125% = 20A adjusted ampacity. but then one of my books says that is a # 12 AWG cu wire, while the other source says it is a 14 AWG cu wire. I found a couple of places on the internet that agree with the # 14, which agrees with my vague recollection of trade school, but when i read 240.4(D) in The Book, it directs me to choose the wire in accordance with sections 3 - 7 of article 430, and i'm having trouble pinning down exactly if it is the value given in table 310.15, 20A, or the adjusted value in 240.4(D), 15A, that is correct. Help!?!

appreciatively,
Jim
Normally if the motor is a standard horsepower listed in tables 430.247-250 you use the current listed in appropriate table. If not a motor listed then you go by nameplate. So if 16 amps is what our current is (no matter how we came up with the 16), then next thing is determining if it is a continuous duty motor. If so the 1.25 multiplier comes into play. (most of the time it will be continuous but occasionally it will not be).

If no other ampacity adjustments are needed the minimum size conductor needed is 20 amps. A 14 AWG 75?C conductor is good for 20 amps according to 310.15(B)(16) in 2011 NEC.

240.4(D) says "unless specifically permitted in ...(G) ...."

240.4(G) sends you to art 430 for overcurrent protection for motor applications.

This 14 AWG conductor ultimately could have a 40 amp inverse time breaker or a 125 amp instantaneous trip breaker protecting it, or even higher if necessary to allow the motor to start.
 

texie

Senior Member
The correct answer is #14. The so called "small conductor rule" of 240.4(D) directs you to 240.4(G), which sends you to Table 240.4(G), which sends you to art 430 for conductor size. In other words the "small conductor rule" does not apply in the case of a motor.
I might add that as kwired said, you would normally use the tables in art 430, and in fact are required to in the real world. But since they give you the amps and no HP and motor type, you use the amps they give. Many times they will give you only the HP and motor type in an exam for a question like this so they are checking that you know to use the tables and size the conductor.
 

kwired

Electron manager
Location
NE Nebraska
I might add that as kwired said, you would normally use the tables in art 430, and in fact are required to in the real world. But since they give you the amps and no HP and motor type, you use the amps they give. Many times they will give you only the HP and motor type in an exam for a question like this so they are checking that you know to use the tables and size the conductor.
The tricky part is if they give you horsepower and nameplate amps for a question. Nameplate is meaningless if a standard size motor listed in tables in 430. The only thing nameplate amps is used for is motor overcurrent protection. All other calculations are based on tables in 430.
 
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