100 Amp breaker with #4 feeding A/C disconnect

J

J2H

Member
Location
Wisconsin
Occupation
Broadcast Engineer/Licensed Electrician
A client has an existing non-fused 100 amp disconnect for a A/C condenser that needs to be replaced. The unit calls for a max breaker size of 70 amps and min circuit ampacity of 55 amps. The conductors feeding the existing disconnect are #4 copper.

If I am interpreting the code correctly, it seems to me, that simply replacing the disconnect with an appropriately sized fused disconnect is not compliant and the breaker or conductors need to be replaced. Is this correct or is there an exception I am missing?

Somewhat adding to the complication, it seems that these particular I-line breakers are obsolete, but there should be enough room in the raceway to pull new conductors.
 
J2H said:
A client has an existing non-fused 100 amp disconnect for a A/C condenser that needs to be replaced. The unit calls for a max breaker size of 70 amps and min circuit ampacity of 55 amps. The conductors feeding the existing disconnect are #4 copper.

If I am interpreting the code correctly, it seems to me, that simply replacing the disconnect with an appropriately sized fused disconnect is not compliant and the breaker or conductors need to be replaced. Is this correct or is there an exception I am missing?

Somewhat adding to the complication, it seems that these particular I-line breakers are obsolete, but there should be enough room in the raceway to pull new conductors.
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What do you think the problem is?? #4 cu more than sufficient for the 55amp Mca it’s oversized , branch circuit conductors are sized to nameplate Mca , ground fault short circuit protection is sized by nameplate MOP , # 6 on a 70 would be code compliant as well
 
Jaybone812 said:
What do you think the problem is?? #4 cu more than sufficient for the 55amp Mca it’s oversized , branch circuit conductors are sized to nameplate Mca , ground fault short circuit protection is sized by nameplate MOP , # 6 on a 70 would be code compliant as well
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The existing breaker that feeds the disconnect is a 100 amp with #4 conductors. While the internal overloads on the unit should in theory protect the #4 particularly with the addition of fuses, I can’t think of a code rule that would permit that.
 
You need a 70 amp circuit breaker at the start of the circuit if you have a non-fused disconnect at the unit. So your intention is to leave the 100 amp circuit breaker?
 
J2H said:
A client has an existing non-fused 100 amp disconnect for a A/C condenser that needs to be replaced. The unit calls for a max breaker size of 70 amps and min circuit ampacity of 55 amps. The conductors feeding the existing disconnect are #4 copper.

If I am interpreting the code correctly, it seems to me, that simply replacing the disconnect with an appropriately sized fused disconnect is not compliant and the breaker or conductors need to be replaced. Is this correct or is there an exception I am missing?

Somewhat adding to the complication, it seems that these particular I-line breakers are obsolete, but there should be enough room in the raceway to pull new conductors. No
Click to expand...

J2H said:
The existing breaker that feeds the disconnect is a 100 amp with #4 conductors. While the internal overloads on the unit should in theory protect the #4 particularly with the addition of fuses, I can’t think of a code rule that would permit that.
Click to expand...
I missed the part of existing 100 amp breaker saw 100 amp non fused disconnect
 
J2H said:
The existing breaker that feeds the disconnect is a 100 amp with #4 conductors. While the internal overloads on the unit should in theory protect the #4 particularly with the addition of fuses, I can’t think of a code rule that would permit that.
Click to expand...
If you’re intention is to leave the 100 amp breaker you need 100 amp feeder to the disconnect then fuse it down to 70 at the disconnect
 
infinity said:
You need a 70 amp circuit breaker at the start of the circuit if you have a non-fused disconnect at the unit. So your intention is to leave the 100 amp circuit breaker?
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My intention, would be to leave the 100 amp breaker if replacing the 70 amp fused disconnect would be compliant. But I don’t believe that would be.

Existing install is clearly not compliant, and I want to correct that as part of the disconnect replacement.
 
Jaybone812 said:
If you’re intention is to leave the 100 amp breaker you need 100 amp feeder to the disconnect then fuse it down to 70 at the disconnect
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That’s exactly what I thought, I didn’t think there was any way this install was compliant.
 
It’s clear that this is not the original unit, so I suppose it’s possible that the MCA and breaker size matched the nameplate. Bunch of other things with the install such NM used on the load side of the disconnect etc.
 
J2H said:
It’s clear that this is not the original unit, so I suppose it’s possible that the MCA and breaker size matched the nameplate. Bunch of other things with the install such NM used on the load side of the disconnect etc.
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Of course that still means the current install is not compliant.
 
J2H said:
It’s clear that this is not the original unit, so I suppose it’s possible that the MCA and breaker size matched the nameplate.
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Yes, it is possible that the original unit had an MCA of say 80A and an MOCPD of at least 100A.

Interestingly, if this existing circuit supplied both your new unit (MCA 55A, MOCPD of 70A) and an additional 30A of load, then it would be compliant to supply the combined load with a #4 feeder (75C ampacity of 85A) protected at 100A. Of course, you'd need further branch circuit protection for the new unit and for the additional load.

Cheers, Wayne
 
Little Bill said:
With all the "Oops" messages, I'm trying to get to the point before the next one hits!
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Sorry about all the oops messages. I will be more careful about that going forward.

There is also, “the it’s worked fine these way for years”, so I wanted to be sure I was justified in my recommendation.
 
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