I want to use some #4 USE to feed a 75 amp load. I'm trying to figure out how long my run can be before I have to bump up to a larger size wire. When using the free calculators from Mike Holt and Southwire, they both say that #4 is only good for 70 amps. But according to 310.15(B)(16) it's good to 85 amps. What am I missing here?
Voltage drop calculator
- Thread starter sketchy
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- Connecticut
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- Engineer
The calculators are using the 60deg ampacity because of 110.14(C)(1)(a)(2).
If you know that your equipment is listed for use with 75 deg conductors (which most equipment is,) you could use the 85A for #4 USE based on 110.14(C)(1)(a)(3).
If you know that your equipment is listed for use with 75 deg conductors (which most equipment is,) you could use the 85A for #4 USE based on 110.14(C)(1)(a)(3).
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
I can't check your numbers without a couple extra details, specifically the voltage level and the number of phases. But if you are asking about two different calculation tools giving you the same answer, I don't see why that should be a surprise. On the other hand, if you are asking why you can get the same answer from two calculation tools and a different answer from the table, that too should be no surprise. The table just gives a maximum limit on the conductor's ability to carry current. It does not tell you how many amps the conductor can carry for a given distance before running into voltage drop issues.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I haven't tried the Southwire site or the MHE calculator. But I have a spreadsheet VD calculator that someone gave me long ago. It has no way of accounting for direct buried runs, and I don't know how that would impact a spreadsheet calculator. I put in your values and came up with a distance of 358.7 feet.
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