HVAC overcurrent protection

[wizardelect]

There are many connotations used in HVAC equipment manufacturing information, e.g. MCA, MFS, MOCP.

An RTU's recent data indicated an MCA of 38 amperes and an MFS of 50 amperes. Our application was replacement and the existing switchboard only had inverse-time circuit breakers. Based on the 38 MCA, we calculated that the existing 60 ampere breaker would be allowed per Table 430.52.

However (?), is this calculation correct or was there an alternate method to ensure that the correct overcurrent protection was used?

Ejohn

 

[Dennis Alwon]

I do believe the 60 amp breaker is code compliant if that is what you are asking.

I am not sure whether Table 430.52 would take precedent over art 440.22 or not. I suspect 440.22 is where you need to look which of course references art. 430.53

 

[480sparky]

If the nameplate listed only a maximum fuse size, would not using a breaker violate the listing?

 

[Dennis Alwon]

If the nameplate listed only a maximum fuse size, would not using a breaker violate the listing?

My understanding was there was a 50 amp fuses but again I may have assumed too much.

I am surprised that 50 amp would be the maximum fuse allowed with a unit that is 38 amps.

 

[raider1]

If the nameplate listed only a maximum fuse size, would not using a breaker violate the listing?

I agree. If the manufacturer has marked the nameplate with a maximum fuse or breaker size, you could not install a larger fuse or breaker or you would violate 110.3(B).

Chris

 

[480sparky]

My understanding was there was a 50 amp fuses but again I may have assumed too much.

I am surprised that 50 amp would be the maximum fuse allowed with a unit that is 38 amps.

The OP mentioned MOCP, but didn't give any breaker info, just MFS.