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#98780 - 06/06/06 02:45 PM HVAC conductor sizing
Gee Offline
Junior Member

Registered: 10/26/05
Posts: 9
Loc: baton rouge LA, USA
Hey guys, a question:

I have a 480v, 3 phase rooftop unit that we installed #10 conductors as per plans.
Now the units arrived and has a Min.Circuit Ampacity of 30.2 amps.
Is the #10 conductor acceptable using 310-16 value of 35 amps ?
If this is acceptable, only if the OCP is sized at 35 amps ?
I am fairly certain that even if these are acceptable, I still will be required to change the disconnect to a 60 amp unit w/ 35 amp fuses.

Thanks in advance

2014 / 2011 NEC & Related Books and Study Guides
#98781 - 06/06/06 03:06 PM Re: HVAC conductor sizing
HotLine1 Offline


Registered: 04/03/02
Posts: 6804
Loc: Brick, NJ USA
OK, what's the data on the AC nameplate?
Min circ amps? (30.2)
Max OCP??

Unit nameplate data governs the MOCP.

Yes, IF you need 35 amp fuses, then a 60 amp FD is req.

Refer to the "*" under 310.16 to go to 240.4

As to the 'plans', time to request a change order and pull in #8 & change the disco.


#98782 - 06/06/06 03:19 PM Re: HVAC conductor sizing
caselec Offline

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
John, why do the #10’s need to be replaced with #8’s? As long as the MCA is 35 amps or less #10 THWN is acceptable for this installation.

Curt Swartz

#98783 - 06/06/06 03:25 PM Re: HVAC conductor sizing
Gee Offline
Junior Member

Registered: 10/26/05
Posts: 9
Loc: baton rouge LA, USA
Min cir- 30.2
Max fuse - 35
I am sure I have to checge the disconnect. But can I get by with the 35 amp allowance for #10 for HVAC loads.
Does *240.4(D)> 240.4(G)table ( Air-condition refrigeration) > 440,part VI allow the # 10 to be protected at 35 amps ?
I am confused to say the least.
Seems like one day after 20 years I maybe can actually fuse # 10 at 35 amps as per 310-16. If not why do they even list 35 amps for #10

#98784 - 06/06/06 04:12 PM Re: HVAC conductor sizing
George Little Offline

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
#10 wire is fine and 240.4(G) allows protection per Article 440. I suspect that he/she might end up with an protection larger than 35a. based on what the nameplate of the AC says. This larger protector is not really overload protection for the wire but merely short circuit and ground fault protection. It no doubt will be a 60a. switch unless he is using a breaker. The actual overload protection is internal in the AC and in an overload situation this would permit the branch circuit conductors from being damaged.
Just as a side note, the horsepower rating of a 30a. switch would not be adaquate for this large an AC.

[This message has been edited by George Little (edited 06-06-2006).]
George Little

#98785 - 06/06/06 04:37 PM Re: HVAC conductor sizing
HotLine1 Offline


Registered: 04/03/02
Posts: 6804
Loc: Brick, NJ USA
Sorry, but YES you can use #10 per 440.
20 years and you get to do it! (440)
I just didn't go to 440.
Don't forget the Change Order for the 60 amp FD

Yes, #10 is OK by NEC.

Based on Gee's response nameplate is 35 A MOCP, and he said "fuse".

However, and this is OPINION, not NEC:
Consideration has to be given to length of circuit conductors, ambient temp of circuit (on roof, in bar joists, etc) for conductor sizing. None of this was asked by GEE, nor asked by me in my reply. I would do this install with minimum #8, possibly #6.



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