Motor Circuit breaker sizing

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Brinkmann

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In a motor controller we have (230/3/60) a 2 HP(6.8 FLA) motor and a 1/2 HP (2.2 FLA). For breaker sizing we use table 430.52.

2HP 6.8 * 2.50 = 17 or a 15 amp breaker (next lowest size.)

1/2 HP 2.2 * 2.50 = 5.5 or a 5 amp breaker.

OR does exception # 2 from 430.52 (C) (3) mean if the FLA is 8 or less we can use a 15 amp circuit breaker or does we still use the 250% rule? Is a 15 amp the minimum "standard" size?

Thanks!
 
Re: Motor Circuit breaker sizing

Brinkmann,

I think you are combining two different NEC rules that are mutually exclusive. Referencing the NEC 2002, because that is what I have access to right now, the 2.5 you are using corresponds to the 250% for an inverse time breaker. The exception you noted is for an instantaneous trip circuit breaker, which allows a maximum of 800%. If you are using an instantaneous trip then it would be:
2hp=6.8*8=54.4A, or 50A breaker
1/2hp=2.2*8=17.6A, or 17.5A breaker

You would now be selecting a breaker that exceeds 15A so the exception would not apply.

The values listed in the table are the maximum values, as a design decision you can chose a lower value. You could chose a 10A breaker for your 2hp motor. However, going to low in the selection makes the possibility of nuisance tripping more likely.
Tony
 
Re: Motor Circuit breaker sizing

Sorry for the confusion....you are correct.

We are using an inverse time circuit breaker.

So with the 1/2 HP motor would a 5 amp circuit breaker (maximum) be correct? Or is there a minumim "standard size" breaker (15A) ?

UL508A would allow a 15 amp to be used as it is their minimum size on the charts. This seems to conflict with the NEC sizing.
 
Re: Motor Circuit breaker sizing

Is there really that much of a difference between say a 10A breaker and a 50A breaker used only for ground-fault or short-circuit protection, assuming the motors have proper overload protection.
 
Re: Motor Circuit breaker sizing

When it comes down to it remember that breaker do not protect motors. A TM breaker will protect the cable from OL thermally an provide short circuit protection magnetically.
The way I see it is to not muddy up the water and to size the cable to supply the multiple motor loads to be served and then size the breaker to protect the cable. Each motor should have its own overload protection either with OLR or internal thermal protection.
The breaker provides what the NEC often has referred to as 'ground fault protection' not to be confused with GFCI or equipment GFP. Simply stated when a motor starts to fail a winding often is starting to fault to ground which then escalates. The magnetic trip element of the breaker is intended to pick this up and clear the fault.
So, should a 10 or 15a breaker be applied? What cable are you using? Smaller than #14? Probably #12.
Does this make sense or what are your thoughts about what athe breaker is for and the protection it is actually providing?
Thanks,
Dave
 
Re: Motor Circuit breaker sizing

Dave,
The breaker is for short circuit protection only. The motors have an overload assembly to monitor motor amperage. We try to size the breaker first and then size the cable for the breaker.

bhp...no in a short circuit mode, 10 amp or 50 amp makes no difference in function, they both trip. But what is the correct size based on the motor?

It's rare for us to use a small motor in our panels (1/2HP 3 phase) which has caused the confusion. On larger motors a max. of 250% of FLA works fine. In this case we are using a 15 amps CB, which by the UL standards is the smallest CB listed.
 
Re: Motor Circuit breaker sizing

As others have stated the circuit breaker or fuse is there to protect the wiring. Since the smallest wiring normally used is #14 and it is to be protected by a 15-amp breaker, you would never need to install anything smaller than 15-amp.

[ January 11, 2006, 04:13 PM: Message edited by: haskindm ]
 
Re: Motor Circuit breaker sizing

Try looking at Exception 1 to 430.52(C)(1). This says you can go up to the next standard size CB.
 
Re: Motor Circuit breaker sizing

See section 430.53(A) for motors not over 1 HP and not over 600V, you can suppy it with a 15 ampere breaker.
 
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