Motor conductor sizing.

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bdktoys

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We have a differences of opinions in our office. Your advice will be welcome.
When sizing a single motor circuit. Can the conductors from a distribution panelboard to a combination motor starter be sized at 125% of the Full load current of the motor or must it be sized to the circuit breaker in the distribution panelboard. That circuit breaker can be sized up to 250% of the motor full load current. I didn't think that the conductors on the line side of the combination motor starter had to exceed the 125% of the motor load...
 
I've seen this discussed many times due to the definition of a "branch circuit". Since your motor is protected by a combination starter which has OCP, there is a decent argument that the branch circuit originates at the starter since that is the last OCP device, and the panel breaker supplies a feeder, however, following 430.62 would allow the feeder OCP to be sized by the motor in any case.
In the case of other types of equipment, such as an air conditioner, the discussion gets more interesting.
 
NEC 430.22 state's that the 125% is a minimum not a maximum. I think you'll find this in other types a loads where the ampacity of the branch circuit is no less that 125%. The overload, not short circuit/ground fault, is sized at a maximum.
 
Gus,

Do you consider the protection in a motor starter " overcurrent protection" or more like

" overload protection", this is in reference to definition of branch circuit.
 
erickench said:
NEC 430.22 state's that the 125% is a minimum not a maximum. I think you'll find this in other types a loads where the ampacity of the branch circuit is no less that 125%. The overload, not short circuit/ground fault, is sized at a maximum.
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Eric,

The overload is sized at minimum.

The overcurrent is sized at maximum
 
The short circuit and ground fault protection can be any size that is equal to or less than the maximum permitted by the code. The same is true of the overload protection. Note: sizing these devices too small will result in nuisance trips, but not code violations.
 
Question, If the motor FLA is 15, at 125 % is 18.75 amps. Installing a wire size is #12 AWG. Now I can size overcurrent at 250%(15X250%) 37.5 amps. How do you want the ground sized? 12 or 10 Does 250.122 apply.
 
1- For branch breaker size and feeder breaker size please follow the link which I have posted for you. it explains every thing step by step.


2- For ground wire size also please follow the NEC which has already been determined each ground size for each cable size.
 
jamesoftn said:
Question, If the motor FLA is 15, at 125 % is 18.75 amps. Installing a wire size is #12 AWG. Now I can size overcurrent at 250%(15X250%) 37.5 amps. How do you want the ground sized? 12 or 10 Does 250.122 apply.
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That's interesting. The answer seems to be yes and yes :). 250.122(D) tells is to size it by the branch circuit ground fault protector (or equivalent in the case of an IT breaker) but it also refers us back to 250.122(A) which states in part: "but in no case shall they be required to be larger than the circuit conductors supplying the equipment." so it appears in your case, 250.122 calls for a #10, but a #12 is all thats required to match your circuit conductors (which, incidentally, could be #14)
 
jamesoftn said:
Question, If the motor FLA is 15, at 125 % is 18.75 amps. Installing a wire size is #12 AWG. Now I can size overcurrent at 250%(15X250%) 37.5 amps. How do you want the ground sized? 12 or 10 Does 250.122 apply.
Click to expand...

250.122 may apply but the egc would never have to be any larger than the nongrounded conductors. Article 250.122(A)

BTW, you can actually use #14 AWG based on the asterisk which takes us to 240.4(D)

Darn, Gus you beat me to it. My sentiments exactly.
 
Frank(benaround), NEC 430.32(A) state's that the overload is sized at a maximum. It has to be this way. Ampacity at a minimum and overload at a maximum.:)
 
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