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Old 07-17-2014, 09:59 PM   #1
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Max HVAC Breaker Size


Yo. If I run #10 Thhn in conduit to an A/C disconnect and fuse it with a 40 amp 2 pole hacr breaker and 30 amp fuses in disconnect will this pass NEC code?

Thanks in advance

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Old 07-17-2014, 10:01 PM   #2
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Max HVAC Breaker Size


Maybe, what does the nameplate call for?

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Old 07-17-2014, 10:08 PM   #3
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Originally Posted by Cletis
Yo. If I run #10 Thhn in conduit to an A/C disconnect and fuse it with a 40 amp 2 pole hacr breaker and 30 amp fuses in disconnect will this pass NEC code? Thanks in advance
No it is a violation
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Old 07-17-2014, 10:14 PM   #4
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Max HVAC Breaker Size


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Maybe, what does the nameplate call for?
nameplate says 30amps max fuse size
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Old 07-17-2014, 10:46 PM   #5
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Max HVAC Breaker Size


Then use a 30 amp breaker! Depending on the distance of the wire run 10gauge is undersized for 40 A. The breaker protects the wire, the fuses in the disconnect protect the device. This no-nonsense in supplying 40 A if you only need 30.
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Old 07-17-2014, 10:48 PM   #6
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Then use a 30 amp breaker! Depending on the distance of the wire run 10gauge is undersized for 40 A. The breaker protects the wire, the fuses in the disconnect protect the device. This no-nonsense in supplying 40 A if you only need 30.
Yeah, but whats the ampacity of #10 thhn in conduit? isnt there a next size up rule too ?
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Old 07-18-2014, 06:58 AM   #7
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Yeah, but whats the ampacity of #10 thhn in conduit? isnt there a next size up rule too ?
#10 AWG is good for 35 amps in your situation, The name plate says 30 AMP MAX OCP.... you have to change out the 40 amp breaker to a 30....
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Old 07-18-2014, 07:01 AM   #8
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Then use a 30 amp breaker! Depending on the distance of the wire run 10gauge is undersized for 40 A. The breaker protects the wire, the fuses in the disconnect protect the device. This no-nonsense in supplying 40 A if you only need 30.
We are talking AC units here, In most cases, you will have a much higher rated breaker than the actual wiring is rated for, this is because the breaker is NOT protecting the wire, the AC unit is... It is not out of the norm to have a #10 AWG protected by a 50 amp breaker, but you have to go off the AC nameplate, the manufacture already did all of the required calculations for you.
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Old 07-18-2014, 08:09 AM   #9
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The breaker is protecting the wire from low resistance (immediate trip) faults only.
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Old 07-18-2014, 09:16 AM   #10
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No it is a violation
Can you explain why? The fuses bring the ocpd down to the stated 30 amps.
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Old 07-18-2014, 09:22 AM   #11
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Can you explain why? The fuses bring the ocpd down to the stated 30 amps.
The #10 on a 40 amp breaker.
Nameplate calls for 30 amp max fuses.

Just my take.
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Old 07-18-2014, 09:24 AM   #12
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Some would say the fuses at the disco make the conductors supplying the disco feeders. Thus limiting the OCPD to 30 amps.
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Old 07-18-2014, 09:41 AM   #13
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Didn't think about it that way. Too early.
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Old 07-18-2014, 10:46 AM   #14
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Max HVAC Breaker Size


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Originally Posted by Cletis View Post
Yo. If I run #10 Thhn in conduit to an A/C disconnect and fuse it with a 40 amp 2 pole hacr breaker and 30 amp fuses in disconnect will this pass NEC code?

Thanks in advance
Don't make this complicated. First you read the unit condensers nameplate. It will state minimum circuit ampacity and maximum ocpd.
In some cases a BCSC (branch circuit selection current) will be on the nameplate.

Maximum OCPD may be stated as a circuit breaker/fuse in which case you may apply it to a breaker panel or fuse panel. If you have a breaker panel that supplies the unit and the nameplate states max fuse size then install a fusible a/c disconnect with correct fuses (not any ole fuse works). Other wise install the max circuit breaker (HCAR type) in the panel and size the conductors to carry the MCA ampacity and your done. Use a non fusible disconnect located near the unit and don't forget the service receptacle requirement.
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Old 07-18-2014, 11:08 AM   #15
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Some would say the fuses at the disco make the conductors supplying the disco feeders. Thus limiting the OCPD to 30 amps.
Not the proper procedure for determining branch circuit conductor sizing and opcd to a single residential ac unit.

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