MCA decimals

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Tainted

Tainted

Senior Member
Location
New York
Occupation
Engineer (PE)
If the MCA of a motor circuit provided by the manufacturer is 30.25

Assuming 60C insulation, can I use #10 copper wire? does the decimal in the MCA matter?
 
Often HVAC people give me incorrect info or the unit that arrives does not match the one on the submittal, or they forgot to add the block heater or whatever thats called..
So when doing residential and working with NM cable where it may be difficult to change later its best to over shoot the size, unless you bidding a entire subdivision of new homes at once and you need to save every penny.
 
infinity said:
30 amps is less than 30.25 so IMO the #10 is too small.
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Do you think 220.5(b) applies to MCA values?

Also reason I used 60C is because there is an existing #10 cloth wiring with no insulation label. So 60C is worst case. Obviously it would be ideal to replace the wiring but it will be too invasive
 
Tainted said:
Do you think 220.5(b) applies to MCA values?
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No. That only applies within Article 220 for load calculations. There was a Public Input to move that rule to Article 110 so it would apply throughout the code and that was rejected. The language in the 2026 code specifically says load calculations.
120.5 (B) Fractions of an Ampere.
Load calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped.
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The calculation in this thread is an ampacity calculation and not a load calculation.

Note that Article 220 in previous codes became Article 120 in the 2026 code.
 
don_resqcapt19 said:
No. That only applies within Article 220 for load calculations. There was a Public Input to move that rule to Article 110 so it would apply throughout the code and that was rejected. The language in the 2026 code specifically says load calculations.

The calculation in this thread is an ampacity calculation and not a load calculation.

Note that Article 220 in previous codes became Article 120 in the 2026 code.
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Good point. What I think you're saying is that the MCA value is derived from the load calculation. Rounding down the 30.25 would not be allowed because it's considered double dipping
 
Tainted said:
Rounding down the 30.25 would not be allowed because it's considered double dipping
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If it's important, I would be inclined to check the listing standards to see if the load is actually being calculated to more than two significant figures. If not, rounding down would be justifiable on a technical basis, and then I'd check for any (irrational) code prohibitions.

Cheers,
Wayne
 
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