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#80954 - 06/12/02 08:17 PM 40 Amp CB on #10 NM for Central Air  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
I'm getting stuck on the code loop for increasing a breaker size to eliminate nuisance tripping due to inrush.
I know it's there.
The compressor has OL protection, the CB is only for SC & GF protection, etc. But I need formal justification.
Anyone?


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#80955 - 06/12/02 09:37 PM Re: 40 Amp CB on #10 NM for Central Air  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Redsy,
Look at 440.21 and .22 for the OCPD sizing and 440.33 for the conductor sizing.
Don(resqcapt19)


Don(resqcapt19)

#80956 - 06/12/02 10:13 PM Re: 40 Amp CB on #10 NM for Central Air  
Matt M  Offline
Member
Joined: Mar 2001
Posts: 93
Laporte, MN, USA
I'll take a shot at it Redsy,

The #10 wire has already been sized for 125% of the motor-compressor full load current plus 100% of all other loads, and it has no problem carrying the motor/compressor starting current for the short duration that it is present. Why install larger wire if it is not necessary?

Its really not all that different than other motor circuits. It is very common to see motor circuits protected with fuses or circuit breakers with ratings much higher than the ampacity of the circuit conductors. You said it yourself, the overcurrent protection is provided by the integral OL. It already has one device protecting the circuit from overloads with a rating well within the ampacity of the wire, just as a general purpose branch circuit normally only has a single device protecting it.


Branch circuit conductor size and overcurrent protection for air conditioning equipment is marked on the equipment nameplate in most cases. These values will have been computed by use of section 440.32 or 440.33 for conductor sizing and 440.22(a) for branch circuit short circuit and ground fault protection. The calculations have already been performed by the manufacturer and the results are listed on the equipment nameplate. It is not necessary to repeat these calculations again in the field. Matt

[This message has been edited by Matt M (edited 06-12-2002).]


#80957 - 06/13/02 08:06 AM Re: 40 Amp CB on #10 NM for Central Air  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Thanks, guys. I actually looked at those Articles, but wondered if there was anything eles. After rereading them, I think they are sufficient by themselves. This is another pesky Home Inspection Report that insists that the 40 amp CB is a violation, and recommends installing a 30 amp CB(the same report that says that an EGC is necessary for the operation of GFCI receptacles). I told the homeowner that in the case of AC equipment that this is acceptable. It looks like a new unit installed on original wiring. I'll look at the nameplate data, and ensure that the #10 is sufficient.

Thanks again!


#80958 - 06/14/02 09:11 AM Re: 40 Amp CB on #10 NM for Central Air  
George Corron  Offline
Member
Joined: May 2001
Posts: 717
Lorton, Va USA
Guys,
Maybe it would help if you could see a state interpretation. This is Virginias, go to page 6
http://www.dhcd.state.va.us/DBFR/trb/docs/intbook.pdf


#80959 - 06/14/02 10:20 AM Re: 40 Amp CB on #10 NM for Central Air  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Great, George.

Thanks!


#80960 - 06/14/02 12:13 PM Re: 40 Amp CB on #10 NM for Central Air  
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
White Plains, NY
From George's link, the minimum circuit amps was 16 amps. Why #14?


Ron

#80961 - 06/14/02 12:33 PM Re: 40 Amp CB on #10 NM for Central Air  
George Corron  Offline
Member
Joined: May 2001
Posts: 717
Lorton, Va USA
#14 from 310-16 is worth 20 amps. The interpretation correctly analyzes that the notes do not apply to motors/AC loads. I don't think it supercedes 110-14 (c)(1), but it doesn't make any difference one way or the other here.


#80962 - 06/14/02 08:59 PM Re: 40 Amp CB on #10 NM for Central Air  
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
White Plains, NY
I'm so used to 240-3(d) limiting #14 to a 15A breaker that I missed 240-(g) refering back to 440 Parts C & F


Ron

#80963 - 06/16/02 01:12 PM Re: 40 Amp CB on #10 NM for Central Air  
tdhorne  Offline
Member
Joined: Mar 2001
Posts: 345
Maryland, USA
I am not trying to contradict the general consensus here when I ask if it is worth running the larger wire when the difference is not a budget buster. If the name plate OCPD is forty amps and the circuit selection current is 24 amps am I safe in assuming that the overload protection provided in the unit is going to prevent any overloading of the number ten conductors.

If the load is overload protected by the fuses or circuit breaker would it still be safe to use a lower circuit selection current than the OCPD value or setting or is there a chance that the equipment will develop an overload that the breaker will tolerate to the failure point of the conductors.
--
Tom


Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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