ac breaker size

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billdozier

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gulf coast
Hey guys if you have an ac unit that has a minimum ckt ampacity of 22a and a max fuse size of 40a what size breaker would you use? In my understanding you would have to take the 23a and times it by 125% for start up. This would give you roughly just over 29a. So you could run 10 awg wire. You could then use anywheres from a 30a to a 40a breaker. Is this correct?
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
Hey guys if you have an ac unit that has a minimum ckt ampacity of 22a and a max fuse size of 40a what size breaker would you use? In my understanding you would have to take the 23a and times it by 125% for start up. This would give you roughly just over 29a. So you could run 10 awg wire. You could then use anywheres from a 30a to a 40a breaker. Is this correct?

Bill the min cir. amp has the 125% in it already. I always use the max fuse/ cb allowed. You can actually run a #12 wire for this unit and install a 40 amp breaker. see art. 240.4(G)
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
Hey guys if you have an ac unit that has a minimum ckt ampacity of 22a and a max fuse size of 40a what size breaker would you use? In my understanding you would have to take the 23a and times it by 125% for start up. This would give you roughly just over 29a. So you could run 10 awg wire. You could then use anywheres from a 30a to a 40a breaker. Is this correct?

If the nameplate says max fuse, then the branch circuit device must be a fuse. It can only be a breaker if the nameplate says fuse/breaker.

That said, your feeder device can be a breaker. I would vote for a 40A breaker.
 

billdozier

Senior Member
Location
gulf coast
Dennis that article then kicks me back to 440.22(a) and (b) which talks about 175% of the branch circuit selection. keeping in mind 440.22(c) I could take the 22a and times it by 175 and get the breaker size correct? However since I have a nameplate rating of 40a I can eliminate the math correct? And simpily install a 40a breaker? Also is the 125% always factored in to minimum ckt ampacity ratings?
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Dennis that article then kicks me back to 440.22(a) and (b) which talks about 175% of the branch circuit selection. keeping in mind 440.22(c) I could take the 22a and times it by 175 and get the breaker size correct? However since I have a nameplate rating of 40a I can eliminate the math correct? And simpily install a 40a breaker? Also is the 125% always factored in to minimum ckt ampacity ratings?

Yes, in my first post I stated the 125% is factored into the mca. The 175% is for the OCP but again the math is done for you. Use the nameplate ratings and you are good to go.
 

billdozier

Senior Member
Location
gulf coast
awesome man got into it with guy on job today. I guess techinacally he was right 30a was big enough. I felt we should have gone with at least a 35a breaker. But I was factoring in 125% start up. Oh well win some lose some and split hairs on a few
 

Dennis Alwon

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Location
Chapel Hill, NC
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Retired Electrical Contractor
awesome man got into it with guy on job today. I guess techinacally he was right 30a was big enough. I felt we should have gone with at least a 35a breaker. But I was factoring in 125% start up. Oh well win some lose some and split hairs on a few

First time the a/c doesn't start and trips the breaker the HVAC guy will say the breaker is too small. Save yourself a call back and put in a 40 amp.

Remember the unit itself has overload protection. The CB is for short circuit and ground faults only.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Don't really know. I hear stories about old equipment having higher starting current due to age/wear, etc. but I've yet to see any proof of that.

I got called back on a new unit because I used a 35 amp breaker instead of a 40-- don't ask why I had a 35 amp and not a 40 on the truck-- The service tech went back after 6 months and I had to use a 40 amp. Now how long before the unit had to be changed? I don't know. :smile:
 

220/221

Senior Member
Location
AZ
Curious as to why you would use a 30 amp breaker.


It said min 22, max 40 so ......30 seemed to fit right in there, almost in the middle :cool:

I mean, I'm not going to put it on a 20 because 20 is smaller than 22. 30 is the next size and it's greater than 22 so....

Is this a trick question?



Edit.....I don't believe I've ever even seen a 25 or 35 amp breaker. That may explain my logic
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
It said min 22, max 40 so ......30 seemed to fit right in there, almost in the middle :cool:

I mean, I'm not going to put it on a 20 because 20 is smaller than 22. 30 is the next size and it's greater than 22 so....

Is this a trick question?

You are allowed to use a 40 amp breaker and I was curious why you chose 30 amp. I usually use the max that is allowed.
 

220/221

Senior Member
Location
AZ
You are allowed to use a 40 amp breaker and I was curious why you chose 30 amp. I usually use the max that is allowed.

I was reading it as "what size circuit/breaker".

I was thinking #10 wire because it was big enough and a 30 amp breaker to protect the #10 and be in the min/max range.

Then someone said you could run @12's on a 40 and my head exploded.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I was reading it as "what size circuit/breaker".

I was thinking #10 wire because it was big enough and a 30 amp breaker to protect the #10 and be in the min/max range.

Then someone said you could run @12's on a 40 and my head exploded.

Well you can...
 
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