Ac Feed Question

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solo618

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Orange County, N.Y.
I Know in the code under the 90deg. column it says #10cu thhn is good for 40 amps,but does anybody actually do this or has anybody done it for a two pole 40amp feed to a residential ac condensor? Considering this thing will be running during the hottest days of the summer. I'm sorry i don't have any specs on the unit because i was asked this question by a friend and i told him i never use 10 wire for more than 30amps.He had this strange look on his face and when i asked him why he said he had a liscensed contractor do this in his house for his new ac unit.
 
Ambient temperature aside, AC unit conductors must be sized according to the MCA on the nameplate of the unit. The MOCPD listed on the unit will tell you the size of the maximum OCPD permitted for ground fault and short circuit protection.

So for example, an AC unit with an MCA of 35 amps and a MOCPD of 50 amps, #10 THHN conductors are permitted if the terminations are all rated for 75 degrees. An OCPD of any size between 35 and 50 amps is also permitted.
 
solo618 said:
I Know in the code under the 90deg. column it says #10cu thhn is good for 40 amps,but does anybody actually do this or has anybody done it for a two pole 40amp feed to a residential ac condensor? Considering this thing will be running during the hottest days of the summer. I'm sorry i don't have any specs on the unit because i was asked this question by a friend and i told him i never use 10 wire for more than 30amps.He had this strange look on his face and when i asked him why he said he had a liscensed contractor do this in his house for his new ac unit.
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When it comes to AC units, I will always install the maximum OCP the nameplate allows, and the smallest branch circuit, unless i feel voltage drop to be a serious issue... Not unusual to have #12's backed by a 30 amp breaker...
 
ac feed question

ac feed question

Thanks for the quick response, but Trevor could you really put a 50 amp breaker on that circuit you used for your example.That really blows my mind.I've been in this business for some time now but all of my time in commercial,industrial ,and transit and i never thought residential work could be so different.It really is a much different animal, almost a whole new business to me. Even though i understand your explanation i can't see how you could use a 50 amp breaker to protect #10wire.I would think an inspector would look into the panel and shoot that right down.Please don't think i'm questioning you,because i really respect the people in this forum. There really are some smart guys in here.i never assume to know anything thats why i like this forum because i still look to learn whatever i could.
 
solo618 said:
Thanks for the quick response, but Trevor could you really put a 50 amp breaker on that circuit you used for your example.That really blows my mind.I've been in this business for some time now but all of my time in commercial,industrial ,and transit and i never thought residential work could be so different.It really is a much different animal, almost a whole new business to me. Even though i understand your explanation i can't see how you could use a 50 amp breaker to protect #10wire.I would think an inspector would look into the panel and shoot that right down.Please don't think i'm questioning you,because i really respect the people in this forum. There really are some smart guys in here.i never assume to know anything thats why i like this forum because i still look to learn whatever i could.
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It's simple, the breaker you install at the panel is only providing ground fault and short circuit protection, the overload is built into the motor... if the motor goes out on overload, then its pretty hard for the wire to melt...
 
solo618 said:
TI would think an inspector would look into the panel and shoot that right down.
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They would only if they didn't understand the correct application of the NEC rule. Stickboy explained it quite well.

I've installed #12 NM cable on 30 amp breakers before for A/C units. It's code legal and safe. :)
 
ac feed

ac feed

ok stickboy,great explanation.i got it ,but i like to ask questions.The difference with this circuit is because it is feeding a motor that has an overload built in but if this circuit was feeding a diagnostic machine if you will or some other piece of equipment then this would not be exceptable.would it?
 
solo618 said:
T Even though i understand your explanation i can't see how you could use a 50 amp breaker to protect #10wire.I would think an inspector would look into the panel and shoot that right down.
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Check out 240.4(G)

ETA:
Might as well start at 240.4(D) ...it sends you to (G) anyway ;)
 
solo618

These guys know the code and are real helpful to me also. But that does not mean that the inspector is as knowledgeable as these guys. I have failed two inspections in two different jurisdictions because of this very reason.

Both said- "Where in the code does it say you use mca as sizing the conductor and mop as the breaker sizing?"

I said- "440.6 A for conductor sizing and 440.22 A for breaker size and that the manufactor done the calculations for us."

Still I was told I didnt know what I was talking about and I then bought two breakers and paid for two more inspections.

In my opinion, the nec could make this easy for everyone and just say to use mca for conductors and mop for breaker for ac equipment.

Or the local governments could start hiring educated, willing to listen people to do their ele. inspections. Instead of hiring Billy Bob John McCoy who was the cousin and nephew of the judge executive who got elected because seventy percent of the community is kin to him.

And yes I live in rural kentucky.
 
jcole said:
Still I was told I didnt know what I was talking about and I then bought two breakers and paid for two more inspections.
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That's where you and I differ. Everybody has a supervisor.
 
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