35 amp breaker

14962 Views 4 Replies 4 Participants Last post by nap, Mar 20, 2008

monte sparky

Discussion Starter · Mar 20, 2008 (Edited)

Is it ok to land #10awg thhn on a 35amp breakers to feed a small motor pulling a full load of 7.8 amps? I was told today by my forman that i was unable to do this. The breaker size and wire size was givin to me on drawings.

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480sparky

· #2 · Mar 20, 2008

10 THHN's ampacity from T310.16 is 40, so it's entirely possible.

A calculation using Art. 430 would tell you if it's OK, though.

Your foreman is probably unfimiliar with 240.4(G).

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nap

· #3 · Mar 20, 2008 (Edited)

actually, to properly size the max OCPD, the motors HP is supposed to be used and then the max OCPD is figured using the appropriate chart (for this motor, probably 430.248)

and then apply the calc and direction from 430.52.

without knowing what the motor hp or type of motor, I would still guess the breaker is too large. With an inverse time breaker, even under the exception in that section, you can only size the breaker at 400% (absolute max). This is only dependant on the lower rating of 250% of chart rating and then upsized allowable to the next standard breaker of250%. Utilizing the actual nameplate (which actually is not the method given by the code) that would mean that you could not use over a 30 amp breaker but actually should not be over a 20 amp breaker unless it will not hold on start up of the motor. So in either case, the breaker size is too big.

so motor type and hp and we can figure this out.

the OCPD is not dependant on the wire size but by the direction of article 430 and the wire size is controlled by article 430 as well.

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Super_33

· #4 · Mar 20, 2008

monte sparky said:
Is it ok to land #10awg thhn on a 35amp breakers to feed a small motor pulling a full load of 7.8 amps? I was told today by my forman that i was unable to do this. The breaker size and wire size was givin to me on drawings.

why are you putting a motor that pulls 8 amps on a 35 amp breaker?

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nap

· #5 · Mar 20, 2008

actually, it is quite possible he is legally allowed to use a 30 amp breaker and maybe a 35, depending on the size of the motor. This is to account for inrush currents.

he did state that he followed the drawings provided to him for this installation so I would think some engineer designed the system and he was merely following the engineers directions.

monty, engineers do make mistakes and sometimes they are just plain wrong. Had to argue with too many so they would let me install a system properly. What pisses me off is if I installed it as they drew it (wrong) they would have shown me the disclaimer on nearly every drawing that states it is the contractors responsibility to verify the installation and install it per applicable codes.

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