# breaker size/amps



## Bazooka Joe

Hi folks,
...I have built a second floor addition on to my house, and am in the process of getting an hvac contractor to install a second system. Im looking for information re: wire size/amps for the 220 volt circuit for the system and have found that the manufacturers fail to post that on their websites. I want to install a 3 to 3 1/2 ton system. Is a 40 amp circuit typical for this?


----------



## mdshunk

Air conditioning compressors follow special rules for wire and breaker sizing. If you don't know the specs on a unit, you're safe by running #10 to a 3, 3-1/2, and 4 ton unit. When the unit is finally on site, the data plate will say "maximum fuse or breaker size", so size your breaker to that.


----------



## thom

I run 8/2 w/g to my compressor units. 40A 2-pole breaker. Separate circuit for Furnace/air handler. 110V 20A circuit.


----------



## mdshunk

thom said:


> I run 8/2 w/g to my compressor units. 40A 2-pole breaker.


If that's what you do all the time, you're violating some codes at least a portion of the time.


----------



## mahlere

mdshunk said:


> If that's what you do all the time, you're violating some codes at least a portion of the time.


not if he uses the 8/2 to feed a fused disconnect and sizes the fuses to the condensor...


----------



## mdshunk

mahlere said:


> not if he uses the 8/2 to feed a fused disconnect and sizes the fuses to the condensor...


Doubt it... look at the user's profile.


----------



## MechanicalDVR

Call the rep in your area for the equipment you are using and they will either tell you or fax you a cut sheet on the units. Then size the wires by code in your area.


----------



## mdshunk

MechanicalDVR said:


> Call the rep in your area for the equipment you are using and they will either tell you or fax you a cut sheet on the units. Then size the wires by code in your area.


That's the best case scenario. I know that when I wire a spec, the brand of the outdoor unit often isn't even known yet. I'd rather have a cut sheet, but #12 for up to 2 ton, #10 for 3-4 ton, and #8 for 5 ton continue to work for any manufactuer, now that we're all on at least 13 SEER. You don't need to pick your overcurrent size until the unit is on site, so that can hang in limbo without any problem.


----------



## thom

mdshunk said:


> If that's what you do all the time, you're violating some codes at least a portion of the time.


OK MD, explain it. The compressor units call for a 40A circuit. Why would I not run #8?


----------



## mdshunk

thom said:


> OK MD, explain it. The compressor units call for a 40A circuit. Why would I not run #8?


Right there you're showing what you don't know. "The compressor units calls for a 40A circuit" is completely false. They don't call for anything. It will say two things... "minimum circuit ampacity" and "maximum fuse or breaker size". Compressors have a totally different set of rules. You size your wire to the "minimum circuit ampacity" (or bigger, if you want to be wasteful) and you size your breaker to the "maximum fuse or breaker size" (or smaller, if you want to tempt a call-back). With this in mind, you can see #12 with a 30 or 40 amp breaker, for instance, and it would be completely legal.


----------



## thom

Well MD, clearly you know more than I re: electric codes but I understand the situation differently.

If it says minimum capacity/ampacity 29.9 I would say that's calling for a 30A circuit. If it says maximum breaker size 50A than I would say that means That I could install up to a 50A breaker.

Nowhere does it say I can run a 50A breaker on #10 wire. I could run #10 on that circuit but it would require a 30A breaker. If I installed a 50A breaker I would need to run #6. 

What I really don't understand is why you would run #10 wire prior to seeing the unit if there was a possibility that it would require (have a minimum ampacity) greater than 30A. Seems to me that the money you would save by using #10 wire instead of #8 from the 100 jobs that work out fine would be more than eaten up on the one job you ended up ripping out sheetrock across the house to rewire the circuit, and then you would be dealing with a really pissed off GC.


----------

