Motor conductor sizing

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mbrooke

mbrooke

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Why do we size motor branch circuits based on 430.25? What is so special about these tables? The heaters will protect the wire anyway.
 
kwired

kwired

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you sure 430.25 applies to what you are asking?

If you main question is why the tables at end of 430 - I think the idea for typical motors listed in those tables you will have sufficient conductor regardless the efficiency of the motor as long as it is the same horsepower.
 
mbrooke

mbrooke

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kwired said:
you sure 430.25 applies to what you are asking?

If you main question is why the tables at end of 430 - I think the idea for typical motors listed in those tables you will have sufficient conductor regardless the efficiency of the motor as long as it is the same horsepower.
Click to expand...

My understanding is that the branch circuit is sized off those tables.


Name plates show specif amperage, so why wouldn't that be good enough for sizing?
 
kwired

kwired

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mbrooke said:
My understanding is that the branch circuit is sized off those tables.


Name plates show specif amperage, so why wouldn't that be good enough for sizing?
Click to expand...

Generally it kind of is good enough. Where there is question is if the nameplate is borderline on ampacity between two conductor sizes. We also must use 125% of motor current most cases so that helps with not having too little of conductor, but I think we do this partly because overload protection often can be 125%, so if you are typically running a motor into service factor it really don't matter anyway.

Then there is the fact that we use 90C conductors at 75C rating most of the time, so the terminations is sort of where the compromise might be.
 
C

Carultch

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mbrooke said:
My understanding is that the branch circuit is sized off those tables.


Name plates show specif amperage, so why wouldn't that be good enough for sizing?
Click to expand...

If the is a chance that the motor is replaced in the future, with a less efficient motor of the same mechanical output power (i.e. horsepower rating), you would need to have your circuit sized for motors of that particular HP rating in general. Not just the motor you intend to initially use.

FLA = "full load actual". The full load amperes from the nameplate of the device.
FLC = "full load code". The full load current, that the NEC gives for motors in general of any given horsepower rating.
 
H

Hv&Lv

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Carultch said:
FLA = "full load actual". The full load amperes from the nameplate of the device.
FLC = "full load code". The full load current, that the NEC gives for motors in general of any given horsepower rating.
Click to expand...

FLA = full load amps
FLC = Full load current.

both are used interchangeably by many. FLA is on the motor. FLC is in the NEC. It used to be FLA
FLC is theoretical, FLA is actual, but not what the “a” means..
 
mbrooke

mbrooke

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Carultch said:
If the is a chance that the motor is replaced in the future, with a less efficient motor of the same mechanical output power (i.e. horsepower rating), you would need to have your circuit sized for motors of that particular HP rating in general. Not just the motor you intend to initially use.

FLA = "full load actual". The full load amperes from the nameplate of the device.
FLC = "full load code". The full load current, that the NEC gives for motors in general of any given horsepower rating.
Click to expand...

I thought code didn't dictate future what ifs? :huh:
 
mbrooke

mbrooke

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Hv&Lv said:
FLA = full load amps
FLC = Full load current.

both are used interchangeably by many. FLA is on the motor. FLC is in the NEC. It used to be FLA
FLC is theoretical, FLA is actual, but not what the “a” means..
Click to expand...

I think you missed it :p:p
 
tom baker

tom baker

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As mentioned, the table values are conservative. Your statement "the heaters will protect the wire anyway" is incorrect. An old electricians saying is "the breaker protects the wire, the overloads protect the motor". We often ignore the part title in article 430 - motor short circuit and ground fault protection is for the motor branch circuit.
In a motor circuit the protection is split into two parts, short circuit and overloads
The branch circuit is sized at 125 % (minimum) of the table value to allow for inrush.
Overloads are sized at 120% or so. A motor can run at 115 % overload if it has a SF of 1.15 (not good for the motor!) and the overloads won't trip.
 
mbrooke

mbrooke

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Technician
tom baker said:
As mentioned, the table values are conservative. Your statement "the heaters will protect the wire anyway" is incorrect. An old electricians saying is "the breaker protects the wire, the overloads protect the motor". We often ignore the part title in article 430 - motor short circuit and ground fault protection is for the motor branch circuit.
In a motor circuit the protection is split into two parts, short circuit and overloads
The branch circuit is sized at 125 % (minimum) of the table value to allow for inrush.
Overloads are sized at 120% or so. A motor can run at 115 % overload if it has a SF of 1.15 (not good for the motor!) and the overloads won't trip.
Click to expand...

I have to disagree- the function of the heaters is exactly to protect the wire from overload. The breaker protects against short circuits and ground faults.
 
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