Minimum Circuit Ampacity - Is this the wire amps OR the wire and power supply amps?

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Jim Mayer

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I will be installing an AC unit with a rated load amps (RLA) of 11.6 amps, a minimum circuit amps (MCA) of 22 amps and a max overcurrent protection (MOP) of 24 amps. I will need to install a 3 phase transformer to feed the unit. I realize that I size the wire for the MCA of 22 amps and use a 24 amp breaker. But, do I size the transformer for the RLA of 11.6 amps or the MCA of 22 amps? Thanks in advance...
 
The NEC allows for the use of the "next closest size" circuit breaker, which would be 25A... 240.4(B). The term "MOCP" by the way is never defined or mentioned in the NEC.

As to sizing the transformer, I would not use less than the MCA value (which to answer your title question, is the WIRE amps). MCA is derived from the total continuous FLA of everything that can run at once , plus 25% of the largest motor FLA and 25% of any other non-continuous load. I like to over size the transformer however, because if it is too small it can saturate on starting current for a motor, which then causes a voltage drop that might then cause issues with any control systems involved. For motors not controlled electronically, I use total kVA + 3x the largest motor HP as minimum transformer kVA. But you don't likely know that motor HP value, so that makes it tougher. At the very least, I would start with the transformer FLA that corresponds to the MCA and go to the next size larger.
 
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texie said:
24 amp would not be a standard NEC size. I think the OP needs to double check the name plate for the correct MOCP.
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From what I have seen, manufacturers, particularly ones outside the US, make no particular effort to coordinate their MOCP recommendations with US standard breaker values.
Now if the combination of MCA and MOCP does not allow a standard value to be used, you do have a valid complaint against the manufacturer.

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GoldDigger said:
From what I have seen, manufacturers, particularly ones outside the US, make no particular effort to coordinate their MOCP recommendations with US standard breaker values.
Now if the combination of MCA and MOCP does not allow a standard value to be used, you do have a valid complaint against the manufacturer.

Sent from my XT1585 using Tapatalk
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I agree with both and will add that even if it calls for a nonstandard but obtainable fuse size, you cannot use the next size up for when you have a nameplate MOCP.
 
181016-0847 EDT

Jraef:

Load current does not saturate a transformer core. Relative to load current on a transformer you treat the transformer equivalent circuit as a series inductance and resistance, and that is where the voltage drop comes from.

.
 
Jim Mayer said:
I will be installing an AC unit with a rated load amps (RLA) of 11.6 amps, a minimum circuit amps (MCA) of 22 amps and a max overcurrent protection (MOP) of 24 amps. I will need to install a 3 phase transformer to feed the unit. I realize that I size the wire for the MCA of 22 amps and use a 24 amp breaker. But, do I size the transformer for the RLA of 11.6 amps or the MCA of 22 amps? Thanks in advance...
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I do not know why others are talking about MOPD size and standard size OCPDs as that was not your question unless I misunderstood something.

I think RLA is only the compressor and not the fan so I do not think you can use that without adding in the fan load.

I believe that MCA is the addition of the compressor (largest motor) @ 125% plus all additional loads such as the fan. So, I think you could use that number x Voltage x 1.732 for the load on the xformer secondary winding.
 
florida-sparkey said:
I do not know why others are talking about MOPD size and standard size OCPDs as that was not your question unless I misunderstood something.

I think RLA is only the compressor and not the fan so I do not think you can use that without adding in the fan load.

I believe that MCA is the addition of the compressor (largest motor) @ 125% plus all additional loads such as the fan. So, I think you could use that number x Voltage x 1.732 for the load on the xformer secondary winding.
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You are right in that others aren't addressing OP's question.

His real question I believe is in the thread title "Minimum Circuit Ampacity - Is this the wire amps OR the wire and power supply amps?"

IMO MCA applies to the branch circuit conductors, would also apply to a service or feeder conductor that only supplies one branch circuit though that is a little rare with this kind of equipment.

Transformer usually can be loaded to 100% of it's rating. So he needs a transformer that equal or greater than RLA plus all other loads that can run simultaneously, no 125% needs factored into this, but does for conductor selection.
 
kwired said:
You are right in that others aren't addressing OP's question.

His real question I believe is in the thread title "Minimum Circuit Ampacity - Is this the wire amps OR the wire and power supply amps?"

IMO MCA applies to the branch circuit conductors, would also apply to a service or feeder conductor that only supplies one branch circuit though that is a little rare with this kind of equipment.

Transformer usually can be loaded to 100% of it's rating. So he needs a transformer that equal or greater than RLA plus all other loads that can run simultaneously, no 125% needs factored into this, but does for conductor selection.
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Even if the tranny is only loaded to 80% the efficiency would still be 95% or better for an average transformer.
 
The OP asked if MCA applies to the wire or the wire _and_ the transformer, and then asks about sizing the transformer.

The answer is that MCA applies to the _wire_, but as Jraef stated neither MCA nor RLA is suitable for sizing the transformer for a _motor_ load. As he described, size the transformer for correct motor starting.

Motors when starting draw quite a bit more than their running amps, and transformers place an impedance in the circuit which will cause voltage drop. This voltage drop can cause problems with starting , cause control equipment to drop out, or might cause no problem at all depending on the specific design of the load.

-Jon
 
jumper said:
Even if the tranny is only loaded to 80% the efficiency would still be 95% or better for an average transformer.
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Question wasn't about efficiency though, one should be able to load the transformer to 100%.

winnie said:
The OP asked if MCA applies to the wire or the wire _and_ the transformer, and then asks about sizing the transformer.

The answer is that MCA applies to the _wire_, but as Jraef stated neither MCA nor RLA is suitable for sizing the transformer for a _motor_ load. As he described, size the transformer for correct motor starting.

Motors when starting draw quite a bit more than their running amps, and transformers place an impedance in the circuit which will cause voltage drop. This voltage drop can cause problems with starting , cause control equipment to drop out, or might cause no problem at all depending on the specific design of the load.

-Jon
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Marginally sized transformer will give you less issues than a marginally sized generator as a general rule though. With generator you also have the output ability of the prime mover being bigger issue then the generator winding not being able to deliver in many cases.
 
From an NEC perspective, there isn't much guidance on sizing a transformer and what value to use (i.e. article 220 vs actual) In fact, there isn't even anything other than 110.3 that says you can't overload a transformer (although 450.3 puts an indirect limit on it). I think that mca/next size up from mca is probably good practice, but I also think one could go a little lower code compliantly.
 
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