30A fused disconnect on 60A circuit = violation?

[SteamDonkey]

Hi,

I'm working on a project where some rooftop units (with electric heat) are being replaced with new units using gas heat, thus requiring a smaller circuit. The existing circuits are 60A CB with #6 CU wire. The units will come factory equipped with 30A fused disconnect switches. The unit MCA is 28A, and MOCP is 30A.
As long as the disconnect is sized properly for the load it appears to be code compliant, but something about having a 60A breaker upstream of a 30A switch seems wrong. Am I overlooking something?

 

[Cincycaddy]

The 60A CB is there to protect the #6 wire. What's downstream doesn't matter. You could have a 30A disconnect with a 1A fuse and still be fine. The downstream disconnect and fuse are sized for your load. As long as your feeder is sized to carry your load you are ok.

 

[qcroanoke]

Hi,

I'm working on a project where some rooftop units (with electric heat) are being replaced with new units using gas heat, thus requiring a smaller circuit. The existing circuits are 60A CB with #6 CU wire. The units will come factory equipped with 30A fused disconnect switches. The unit MCA is 28A, and MOCP is 30A.
As long as the disconnect is sized properly for the load it appears to be code compliant, but something about having a 60A breaker upstream of a 30A switch seems wrong. Am I overlooking something?

As long as there are fuses at the units I don't see a problem.
But check your lugs in the disconnects. They may not accept #6.

 

[augie47]

As long as there are fuses at the units I don't see a problem.
But check your lugs in the disconnects. They may not accept #6.

Depends on how many strands you cut

 

[kwired]

What you have is a 60 amp feeder and a 30 amp branch circuit tapped off of it.

You actually have additional capacity in the feeder and could tap another 30 amp circuit off of it, maybe even more depending on full load current of your compressors and how much higher overcurrent protection is for allowing starting.

 

[SteamDonkey]

Thanks guys!

 

[Strathead]

Hi,

I'm working on a project where some rooftop units (with electric heat) are being replaced with new units using gas heat, thus requiring a smaller circuit. The existing circuits are 60A CB with #6 CU wire. The units will come factory equipped with 30A fused disconnect switches. The unit MCA is 28A, and MOCP is 30A.
As long as the disconnect is sized properly for the load it appears to be code compliant, but something about having a 60A breaker upstream of a 30A switch seems wrong. Am I overlooking something?

Wouldn't have it been really messed up for you if they installed #10 wire from that 60 amp breaker. That would also have been legal.

 

[kwired]

Wouldn't have it been really messed up for you if they installed #10 wire from that 60 amp breaker. That would also have been legal.

I'm sorry but I'm not seeing it, outside of having a feeder tap supplying the second OCPD, but then it will have length/size restrictions unless outdoors. MOCP was 30 had it been 60 then I could see it.

Something doesn't quite seem right with the 28 amp MCA and 30 amp MOCP though. I'd expect the OCPD to be at least 125% of the MCA or at least a little closer to 125% then this is.

 

[Strathead]

I'm sorry but I'm not seeing it, outside of having a feeder tap supplying the second OCPD, but then it will have length/size restrictions unless outdoors. MOCP was 30 had it been 60 then I could see it.

Something doesn't quite seem right with the 28 amp MCA and 30 amp MOCP though. I'd expect the OCPD to be at least 125% of the MCA or at least a little closer to 125% then this is.

I am not sure what you are saying. Mike has several articles on sizing feeders for HVAC equipment and motors. The conductor is sized for the MCA and the breaker or fuse is sized for the MOCP. I must admit that I don't do it, unless specified by others. I size the wire to the breaker.

 

[david luchini]

I am not sure what you are saying. Mike has several articles on sizing feeders for HVAC equipment and motors. The conductor is sized for the MCA and the breaker or fuse is sized for the MOCP. I must admit that I don't do it, unless specified by others. I size the wire to the breaker.

If the MOCP is 30, and the breaker is sized at 60, then the #10 wire (sized for MCA) from the breaker to the disconnect would not be allowed, unless it met one of the tap rules.

 

[kwired]

I am not sure what you are saying. Mike has several articles on sizing feeders for HVAC equipment and motors. The conductor is sized for the MCA and the breaker or fuse is sized for the MOCP. I must admit that I don't do it, unless specified by others. I size the wire to the breaker.

The answer is in your reply - "the breaker is sized for the MOCP", when I studied math in school 60 was greater then 30, did they change that in the last 30-35 years?