Condensing unit disc fuse sizing

[Isaiah]

The unit nameplate has a MCA (min circuit amps) of 37A. Is this the value used to size the fuse x 1.25%?


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[infinity]

The MCA is used to size the branch circuit conductors. There should be a MaxOCPD rating for the fuse or circuit breaker.

 

[mtnelect]

As a HVAC contractor, can't believe this question keeps coming up ?

 

[LarryFine]

You need do no math.

Wire ampacity must at least meet MCA.

Fuse (if specified) or breaker must not exceed MOC.

 

[Crash117]

As far as electricians are concerned, you only need to have the MCA and MOCP. These numbers are required to be on the unit label. The MCA gives you info needed to size the wire for the unit. The MCOP tells you fuse or breaker size. In your case, MCA 37a tells you that you must have a wire with 37a as the minimum ampacity. Anything larger is fine but unnecessary. Best choice is 8 AWG for this application. If the MCOP says 50amp, you can use either 50amp breaker or fuses. If the unit says MCOP 50 amp fuse, then you can only use fuses. This is more common for mini split type systems than traditional AC units. Most traditional AC units also say type HACR breakers, which most breakers are. The info for these units is already factored with proper adjustments. Just read the labels and it will tell all.

 

[Eddie702]

It amazes me that a lot of electricians freak out when you tell them the breaker and wire size do not always match as in the above example with compressors or with motors.

 

[Isaiah]

As far as electricians are concerned, you only need to have the MCA and MOCP. These numbers are required to be on the unit label. The MCA gives you info needed to size the wire for the unit. The MCOP tells you fuse or breaker size. In your case, MCA 37a tells you that you must have a wire with 37a as the minimum ampacity. Anything larger is fine but unnecessary. Best choice is 8 AWG for this application. If the MCOP says 50amp, you can use either 50amp breaker or fuses. If the unit says MCOP 50 amp fuse, then you can only use fuses. This is more common for mini split type systems than traditional AC units. Most traditional AC units also say type HACR breakers, which most breakers are. The info for these units is already factored with proper adjustments. Just read the labels and it will tell all.

Many thanks to all


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[Isaiah]

As far as electricians are concerned, you only need to have the MCA and MOCP. These numbers are required to be on the unit label. The MCA gives you info needed to size the wire for the unit. The MCOP tells you fuse or breaker size. In your case, MCA 37a tells you that you must have a wire with 37a as the minimum ampacity. Anything larger is fine but unnecessary. Best choice is 8 AWG for this application. If the MCOP says 50amp, you can use either 50amp breaker or fuses. If the unit says MCOP 50 amp fuse, then you can only use fuses. This is more common for mini split type systems than traditional AC units. Most traditional AC units also say type HACR breakers, which most breakers are. The info for these units is already factored with proper adjustments. Just read the labels and it will tell all.

The vendor installation manual denotes “ELB (earth leakage CB) MCCB (molded case CB)”
does this mean the power source can be either one?
This CU is fed from a local 480V, 3Ph, 3W motor control center with a thermal mag CB. The bucket also contains an electronic overload device-but not sure where the trip is set.
Should the TM CB be swapped out for a magnetic only CB?


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[Eddie702]

I would think you would keep the TM CB but not the electronic overload. The Condensing unit should have built in overload protection.

 

[don_resqcapt19]

The vendor installation manual denotes “ELB (earth leakage CB) MCCB (molded case CB)”
does this mean the power source can be either one?
This CU is fed from a local 480V, 3Ph, 3W motor control center with a thermal mag CB. The bucket also contains an electronic overload device-but not sure where the trip is set.
Should the TM CB be swapped out for a magnetic only CB?


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Mag only breakers are only permitted as a part of a listed combination motor starter and would not be permitted for your application.

 

[busman]

Many years ago an HVAC contractor told me I was doing "Mickey Mouse Install" because I put a compressor on #14 wire with 30A fuses. Completely legal install. There was no way I could convince them otherwise. They had wire and breaker sizes firmly burned in their little brains.

Mark

 

[Isaiah]

Many years ago an HVAC contractor told me I was doing "Mickey Mouse Install" because I put a compressor on #14 wire with 30A fuses. Completely legal install. There was no way I could convince them otherwise. They had wire and breaker sizes firmly burned in their little brains.

Mark

So you’re saying the 30A fuse was only for short circuit and Internal overload protected the wire?


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[busman]

So you’re saying the 30A fuse was only for short circuit and Internal overload protected the wire?


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Correct. That's the reason MCA and MOP are different.

 

[infinity]

So you’re saying the 30A fuse was only for short circuit and Internal overload protected the wire?

Are you sure that the fuse was providing overload protection and not short circuit and ground fault protection?

 

[Isaiah]

Are you sure that the fuse was providing overload protection and not short circuit and ground fault protection?

I said the fuse is for the latter, ie SC/GF... The unit has an internal thermal overload that protects the cable


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[Eddie702]

Many years ago an HVAC contractor told me I was doing "Mickey Mouse Install" because I put a compressor on #14 wire with 30A fuses. Completely legal install. There was no way I could convince them otherwise. They had wire and breaker sizes firmly burned in their little brains.

Mark

I guess they only read the code book up to the start of article 430 and did not go any farther

 

[Jraef]

Should the TM CB be swapped out for a magnetic only CB?

NEVER! You can never use a mag-only CB in any manner OTHER THAN as part of a factory assembled and listed combination motor starter.

ELCB would indicate this unit was meant for some country other than in North America. ELCB is an IEC equivalent of what we would call a GFEP (Ground Fault Equipment Protection). But that to me raises a red flag as to whether it is listed by anyone (like UL, ETL etc.), which also might explain the lack of proper labeling by our standards.