minimum ampacity motors

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powerplay

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Hi everyone...

On a walk in cooler compressor, the compressor says a minimum ampacity of 18A. I have been told it is the minimum "wire" size and not the FLA which requires 125% sized wire....seems the same as i was told for some rooftop units, that is that they are for wire sizing, not calculating. is there a rule of thumb when to apply wire sizing other than going big and size the wire to 125% as one suggested, which can be a costly pain trying to make connections with #10 copper. I'd love to hear why they already do the calculations in some cases and not others.

Thanks again!
 
If the unit states minimum circuit ampacity then I believe it has the 125% built in. In either case the wire size would not change. A #12 will work for refrigeration -- look at Table 310.16 asterisks. Then go to 240.4(D). This states unless specifically allowed by 240.4(G) then you must use #12 at 20 amps however refrigeration is specified in 240.4(G) so if the wire and terminals are rated 75C or better then we can use the 75C col. of #12 which is 25 amps.
 
How is the "minimum ampacity" labeled. Minimum ampacity could be referring to the overload device, the short circuit/ground-fault protection device, the conductor size, or something else. Often the label will tell you "minimum circuit ampacity" and "maximum fuse/circuit breaker". There must be more information than you have given...
 
#14 would be allowed for the 18 MCA, even at the 60 deg rating. #14 (copper) has an ampacity of 20 at both 60 & 75 deg ratings.

Usually you will see the MCA and MOCP for a manufactured piece of equipment, such as your cooler compressor, when there are multiple motors or motors located within an enclosure. The manufacturer does the math for you.
 
I agree #14 will work if indeed the mca is 18 amps. Isn't it true that most motors you need the 125% but for a/c units the math is done for you. I wasn't sure about refrigeration.
 
I run my own selections for my roof top units and split systems using a carrier selection program, and the wire size it says to use is extremely small in my opinion. (see attached). I know there is a different table for AC equipment, but I would never use it.

The MCA on the attachment however is about 18 amps (same as yours), and it says to use a #14 Wire.
 
Npstewart said:
I run my own selections for my roof top units and split systems using a carrier selection program, and the wire size it says to use is extremely small in my opinion. (see attached). I know there is a different table for AC equipment, but I would never use it.

The MCA on the attachment however is about 18 amps (same as yours), and it says to use a #14 Wire.
Click to expand...

Interesting that it gives the wire size based on 60C or 75C-- I had not seen that.
 
Yup. By the way, thats for a condensing unit for a split AC system.

Also, if you do the math, you will see that the MCA has the 125% ALREADY put in there.

Compressor RLA = 14.1 x 1.25 = 17.6
Fan Motor FLA = 1.2

17.6 + 1.2 = 18.8 AMPS

Which is correct because the MCA for this equals 18.8 amps

from there, it automatically chooses the correct size wire based on that table in the NEC
 
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Npstewart said:
I run my own selections for my roof top units and split systems using a carrier selection program, and the wire size it says to use is extremely small in my opinion. (see attached). I know there is a different table for AC equipment, but I would never use it.

The MCA on the attachment however is about 18 amps (same as yours), and it says to use a #14 Wire.
Click to expand...

Is that the current at 208V or 230V. I don't see how it's going to be the same current at both voltages, as 208V is 10% lower than 230V.

I beleive they are using the fact that the #14 at 90deg C is 25A. For derating purposes you can use this value. The actual current cannot be higher than the rated current at 75deg C after it is derated. In this case 80% of 25A is 20A, your load is 18.8A, therefore #14 works.
 
kingpb said:
I beleive they are using the fact that the #14 at 90deg C is 25A.
Click to expand...

Kingpb, not quite. They are using the fact that #14 is 20A at both 60 & 75 deg. 20A is larger than the minimum of 18.8, therefore, #14 works for either 60 or 75 deg. There is no need to start with the 90 deg rating, and no derating going on.
 
I also believe there is no derating happening. I saw no mention of ambient temps above 30 degrees C or more than 3 current carrying conductors in conduit.
Regards,
Mark
 
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