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Max HVAC Breaker Size

11K views 82 replies 15 participants last post by  electures 
#1 ·
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
 
#3 ·
Cletis said:
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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#5 ·
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.
 
#6 ·
Yeah, but whats the ampacity of #10 thhn in conduit? isnt there a next size up rule too ?
 
#14 · (Edited)
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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#17 ·
Think of it this way,

What if I ran a 240V branch cir with #10 thhn in conduit to an outdoor 3r subpanel and then put 30amp fuses or breakers to the A/C unit. Would that be ok?? If so, whats the difference in that v.s. the original case ?
 
#22 ·
Originally Posted by JohnDS
Thanks guys.
Iwire, you have raised another question. In this case, Why does installing a fused disconnect as opposed to a non-fused disconnect determine that the wire from disconnect to panel be upsized to 8awg?

Sent from my SGH-T999 using Tapatalk
By definition the conductors between the panel and the fused disconnect are feeder conductors. They would have different rules than the branch circuit conductors between the fused disco and the unit.

There has been some argument that the fuses could be supplemental protection and therefore the conductors would not be feeder conductors.
 
#23 ·
brric said:
Some would say the fuses at the disco make the conductors supplying the disco feeders. Thus limiting the OCPD to 30 amps.
Bingo. Since the disco is fused the conductors supplying it are feeders. Have to protect them at their rated amps cities. #10 romex = 30A. #10 THHN = 35A.
 
#24 ·
Stubbie said:
Not the proper procedure for determining branch circuit conductor sizing and opcd to a single residential ac unit.
They are not branch circuit conductors to the fused disco. They are feeders.
 
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#31 ·
Well, it passed with flying colors. I used 240.6 for my 40 amp 2 pole in main panel and the 30 amp slo blo fuses were my circuit protection for the A/C.

Another Green Tag !!! :thumbup:
 
#32 ·
Cletis said:
Well, it passed with flying colors. I used 240.6 for my 40 amp 2 pole in main panel and the 30 amp slo blo fuses were my circuit protection for the A/C. Another Green Tag !!! :thumbup:
I wouldn't have passed it with the 40. But that's just me.
 
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#34 ·
Ok, lets mix it up a bit here; suppose that in the OPs example, the length of the #10s between the 40 amp breaker and the fused disconnect was less that 25', and it was in conduit.

It seems to me that the tap rules would apply here and it'd be compliant.

But Cletis didn't state the conductor length or the type of installation, so we don't know if that's the reason it passed.
 
#45 ·
Ok Cletis time to shoot straight your continuing to dig a deeper and deeper hole :yes:

Though I find nothing wrong with posing hypothetical installations to improve ones understanding of code and wiring configurations but it is a little disingenuous to not be up front about it.
 
#36 ·
micromind said:
Ok, lets mix it up a bit here; suppose that in the OPs example, the length of the #10s between the 40 amp breaker and the fused disconnect was less that 25', and it was in conduit. It seems to me that the tap rules would apply here and it'd be compliant. But Cletis didn't state the conductor length or the type of installation, so we don't know if that's the reason it passed.
It is not a tap when it originates from a breaker. Don't matter how you look at it. It is a non compliant installation.
 
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#40 · (Edited)
I suppose the simply way of answering the tap rules is if an ocpd is required at the point of the tap then the conductors do not meet the requirements of the tap rules. Once the ocpd is installed it is no longer a tap.

(B) Feeder Taps.​
Conductors shall be permitted to be tapped,

without overcurrent protection at the tap, to a feeder as specified​
in 240.21(B)(1) through (B)(5). The provisions of​
240.4(B) shall not be permitted for tap conductors.
 
#41 · (Edited)
Cletis said:
Well, it passed with flying colors. I used 240.6 for my 40 amp 2 pole in main panel and the 30 amp slo blo fuses were my circuit protection for the A/C. Another Green Tag !!! :thumbup:
240.6 will get you to a 35a breaker. Nice try though. Now you have a 35a breaker protecting a 30a disco.
 
#42 ·
Stubbie said:
I suppose the simply way of answering the tap rules is if an ocpd is required at the point of the tap then the conductors do not meet the requirements of the tap rules. Once the ocpd is installed it is no longer a tap. (B) Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.
Bingo.
 
#43 ·
beenthere said:
Only for the condenser fan motor and compressor. Their internal over loads only open between their common winding and their run and start windings. A partial short to ground can still occur before the internal over load. Yes, I know the NEC feels its enough protection for the wire.


In a hermetically sealed motor application, the breaker we supply only provides ground fault and short circuit protection, the motor itself contains overloads that protect the conductors, the manufacture supplies the calculations on the nameplate, why do you think you can run #12 awg supplied by a 30 amp circuit breaker?
 
#47 ·
I know what an internal over load does. And have seen more then one set of wires over heated on an A/C. And I am talking about the wiring from the panel box breaker, not the wires inside the A/C only.

To answer your question. Because the starting amp draw is only for a few seconds at most.
 
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#46 ·
That is true and a great explanation by Mike and crew ... my view is the feeder is terminating on the ocpd or fused disconnect ... there are no tap conductors. Once you install the ocpd or fused disconnect terminated directly to the feeder conductors the tap rules no longer apply.
 
#50 ·
beenthere said:
I know what an internal over load does. And have seen more then one set of wires over heated on an A/C. And I am talking about the wiring from the panel box breaker, not the wires inside the A/C only. To answer your question. Because the starting amp draw is only for a few seconds at most.

I have no idea what you are suggesting, but if you have come across burnt conductors, I'm going with loose connections.
 
#54 ·
No loose connections. Compressor FLA of 15.8, but drawing 17 amps, plus the condenser fan drawing 1.2 amps for 6 plus hours. Tends to over heat a 12 gauge wire.

The internal over load of an A/C compressor is cooled by the returning refrigerant, and doesn't trip just because the compressor is drawing 10 to 15% higher amperage then it should.
 
#51 ·
Jump-start said:
FWIW, I have seen the breaker trip before the overload many times when a condenser fan stalls. IMO, (I know code allows it) but the overloads in compressors are not the same as a good ole breaker. Separate mounted commercial overloads are held to higher standards from experience.
Seriously? Because we have all the proper testing equipment that UL has.... :-/
 
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