wire size

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normbac

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i am wiring a house and they do not have the ac condensor yet, but want it wired now. no specs on the unit but I was told it was a 5 ton unit is it true that you can approximate 10 amps per ton in order to size the wire ie; 5 ton 50 amp # 6 wire ?? TIA
 
Re: wire size

The unit on the side of my house is 3 tons and has a minimum circuit ampacity of 19 amps. You could wire it with #14 if you wanted to. I'm not sure if the 10 amps/ton is accurate but your #6 will definitely not be too small. So it might be a wise choice given you don't have the unit in front of you.
 
Re: wire size

Please disregard my last post, my 3 ton unit has a minimum circuit ampacity of 23.3 amps not 19. I was thinking about an old unit and just ran out to check the nameplate. This comes out to about 7.8 amps/ton so your 10 amp/ton might not be a bad estimate. Sorry for the screw up.


Trevor
 
Re: wire size

My normal generic design for 5 ton residential is a 60/2 breaker with #6 copper. This is based on older Carrier loads. Smaller wire might or might not be acceptable. In this area the common interpretation is that the MCA value can only be used to size the wire from the last breaker or fuse in the circuit. I always call out a fused switch at the unit (although the contractor usually puts in non-fused 'cause it's cheaper). With a fused switch at the unit the conductors between the panel and the fused disconnect have to be protected at the conductor's ampacity, so that wire is sized to the breaker rating rather than to MCA. Other jurisdictions may have other interpretations....

Martin
 
Re: wire size

With a fused switch at the unit the conductors between the panel and the fused disconnect have to be protected at the conductor's ampacity, so that wire is sized to the breaker rating rather than to MCA. Other jurisdictions may have other interpretations....
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By definition I would agree with you. Installing an OCPD at the unit disconnect makes the conductors between the disco and the panel feeders. This has been a topic of debate on this forum before. It is because of this that we never install an OCPD next to the unit unless it required by the nameplate. From a contractors perspective unless it's in the spec, it's not needed and a waste of money.
 
Re: wire size

Remember that as of January 1st of 2006, all AC units sold will have a minimum efficiency of 13 SEER. With that in mind, you could run a 14/2 and be safe.

However, the AC contractor may be installing an unit from old stock or from a clearance sale that many HVAC distributors are having due to the new federal legislation.

I would try to call the AC contractor and find out.

Worst case scenario would be to run a 10/2. The last 5 ton unit I wired had an MCA of 29 amps.
 
Re: wire size

(Remember that as of January 1st of 2006, all AC units sold will have a minimum efficiency of 13 SEER. With that in mind, you could run a 14/2 and be safe.)

I read that in mike holts book but cant seem to understand it been doing this for so many years #14 or #12 for an ac condensor baffles my brain I have allways sized acording to amperage, could you elaborate a little on this. tia
 
Re: wire size

I read that in mike holts book but cant seem to understand it been doing this for so many years #14 or #12 for an ac condensor baffles my brain I have allways sized acording to amperage, could you elaborate a little on this. tia
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AC units are rated by the manufacturer for their Minimum Circuit Ampacity (MCA) and their Maximum Fuse or Circuit Breaker size. The MCA rating, is for sizing the conductors feeding the unit. This number already has the 125% factored in to the MCA rating by the manufacturer so the ampacity that appears on the nameplate is the actual ampacity that you will use to size the conductors.

Next you would go to 310.16 and look up what conductor size you would need.

From the 75 degree column you will see that #14 has an ampacity of 20 amps and #12 has an ampacity of 25 amps. So if the unit has a nameplate MCA of 20 amps or less you can use #14 THHN, MCA of 25 amps or less #12 THHN can be used and so on. The limitations of 240.4(D) do not apply to conductors sized for AC units as outlined in article 440.

THe OCPD can be any size between the MCA and Maximum OCPD rating on the nameplate.
 
Re: wire size

Best to be over than under.This could end up being a heat pump.Question is was this a bidded house and if so what did you plug into this for cost.I do recall in Mike Holts vidio course about a #14 being breakered at 30 amps
 
Re: wire size

(Question is was this a bidded house and if so what did you plug into this for cost.)

This is a t&m job 3000 sq ft added to existing 2000 sq ft GC homeowner. I took it as a favor for an old friend, did t&m because they didnt know what they were doing, had alot of repair on existing structure. Thanks for all replies, guess I have been oversizing ac condensors :confused:
 
Re: wire size

Originally posted by infinity:
THe OCPD can be any size between the MCA and Maximum OCPD rating on the nameplate.
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I would offer your very good explanation a slight correction. Our minimum OCPD would have to be based on the size listed on the unit, not the MCA.


For instance, I was checking out one of those Trane ultra-efficient units they have on display at Home Depot, and the MCA was 13 amps, and the minimum and maximum OCPD size was 20 amps.
 
Re: wire size

Originally posted by normbac:
Thanks for all replies, guess I have been oversizing ac condensors :confused:
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Perhaps, but there are advantages to using a larger size wire, namely less voltage drop and less I^2*R losses.

What is important is to install the correct sized OCPD as listed on the unit.
 
Re: wire size

Originally posted by peter d:
Remember that as of January 1st of 2006, all AC units sold will have a minimum efficiency of 13 SEER. With that in mind, you could run a 14/2 and be safe.
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Things are better, but not that much better. My 4 Ton 14 SEER unit has an MCA of 29. Since I used NM cable and didn't know the exact MCA at the time, I ran #8 cu. Now that I know its 29A MCA and not 31, I could have run #10. If you're putting in a 5 ton and using NM cable, I'd use #8. If you are using a wire that can be used at 75C, then #10 may be OK but I wouldn't go that small if the wire is more than 50' in length.
 
Re: wire size

Originally posted by peter d:
Originally posted by infinity:
THe OCPD can be any size between the MCA and Maximum OCPD rating on the nameplate.
Click to expand...
I would offer your very good explanation a slight correction. Our minimum OCPD would have to be based on the size listed on the unit, not the MCA.


For instance, I was checking out one of those Trane ultra-efficient units they have on display at Home Depot, and the MCA was 13 amps, and the minimum and maximum OCPD size was 20 amps.
Click to expand...
This is making an assumption that all units have both the minimum and maximum listed on the nameplate. Not all of them do, so the OCPD can be any size between the MCA and maximum OCPD listed on the unit. Unless of course it does have a specific minimum ampacity as you've mentioned.
 
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