Motor breaker sizing and conductor sizing

[Grendal]

I don't have my NEC handy but I know that the minimum conductor size is 1.25 times the motor amps.

The breakers can be sized up to 2.5 (or was it 2.25) times the motor amps.

Question: does the motor conductors need to be the same amperage as the breakers? since the inrush is only short in duration is it ok for the conductors to be undersized compared to the circuit breaker.

Any help would be appreciated.

 

[infinity]

You have kind of answered your own question. The conductors are sized at 125% of the ampacity listed in the tables in Article 430 not the nameplate value. Your OCPD is sized according to the type of device you're using. For example in most applications a time delay fuse can be sized at 175% of the motor FLA or 250% for an inverse time circuit breaker. {Table 430.52}

 

[kwired]

I don't have my NEC handy but I know that the minimum conductor size is 1.25 times the motor amps.

The breakers can be sized up to 2.5 (or was it 2.25) times the motor amps.

Question: does the motor conductors need to be the same amperage as the breakers? since the inrush is only short in duration is it ok for the conductors to be undersized compared to the circuit breaker.

Any help would be appreciated.

The 250% for the breaker (which can be increased in some instances if it will not allow the motor to start) is short circuit and ground fault protection only. The motor overload protection has a dual purpose function - it protects the motor and the conductors from overload conditions.

 

[iwire]

Question: does the motor conductors need to be the same amperage as the breakers?

No.

 

[mike9593]

Overcurrent Question

Overcurrent Question

What about if you have (1) 1 hp 120v motor with a FLA of 16? 125% = 20, However you really want to install a 25 or 30 amp breaker. Does 240.4 (d) (5) prevent you from doing this if you use # 12? If so then you must increase conductor size to accomodate?

 

[ActionDave]

What about if you have (1) 1 hp 120v motor with a FLA of 16? 125% = 20, However you really want to install a 25 or 30 amp breaker. Does 240.4 (d) (5) prevent you from doing this if you use # 12? If so then you must increase conductor size to accomodate?

Not if you take advantage of the first sentence in 240.4 (D) and flip over to 240.4 (G).

 

[electrofelon]

I don't have my NEC handy but I know that the minimum conductor size is 1.25 times the motor amps.

small (potential) correction: note that if it is multiple motors on a circuit or feeder, you only need 125% of the largest motor.

 

[kwired]

Not if you take advantage of the first sentence in 240.4 (D) and flip over to 240.4 (G).

Back up a little further 240.3 tells us to see art 430 for overcurrent protection for motor circuits.

430 ultimately leaves us with being allowed to have larger overcurrent devices then what would be usual for other applications for short circuit and ground fault protection, and yet still inherently has overload protection for conductors because we must also protect the motor from overloads.

 

[tom baker]

The breaker protects the wire
The overloads protect the motor
The motor branch circuit, short circuit and ground fault protection device (breaker) is sized larger than the wire to allow the motor to start.
The allowance for this is in Section 240.4 (G) which sends you to art 430 and its lists which parts.
Find a Sq D wholesale house and get yourself a Sq D Motor Data slide rule, just put in the motor size and pick the wire and 'breaker'.
Take a deep breath and put that 14 AWG on a 30 breaker, its OK.

 

[tom baker]

Also the conductor size is not always 125% it can be larger. See 430.22(E).

 

[GoldDigger]

The breaker protects the wire from short circuits and ground faults. Those failures will trip the breaker quite rapidly.
But a constant 20% to 50% overload of the actual wire ampacity will not be stopped by the breaker. That sort of overload will not come from a short circuit but rather from an internal failure in the motor OR subjecting the motor to undervoltage or excess mechanical load. The motor overloads will detect and cut off that low level overcurrent. So in a very real sense the overloads are also protecting the wires.

Tapatalk!