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Are the Ampere-Feet Tables too conservative?

Dear Sir: I have been researching the origin of ampere feet tables and have been unable to find anyone that can answer that question. The reason for this is that the ampere feet tables do not match formulas for voltage drops. The cable size from the ampere feet tables is higher than the formula answers for the same parameters. This is a question where the voltage drop is critical. Can you tell me why there's a disparity between the ampere feet tables and the formulas?

Ed Nemie
Hobbyist - Chesapeake, Virginia
2007

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2007

Where are you looking at the tables that you refer to, Ed -- in the National Electrical Code? I do not know the answer but I would not discount the possibility that the tables allow for some "typical" amount of damage to the wire or a limited amount of heat rise or heating from other wires (as I recall there is an adjustment for more than x wires in a conduit or more than y percent full).

If this is not an application covered by the code, I think I'd use your own calculation for the voltage drop, based on factors you can evaluate, rather than a generalized table. But I wouldn't count on wire resistance for an engineered voltage drop; for that you should probably use a precision shunt.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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2007

Check out these links for a good explanation of the NEC on voltage drop calculations:

www.mikeholt.com/technical.php?id=technicalvoltagedrop1

www.mikeholt.com/technical.php?id=technicalvoltagedrop2

Kurt Sammons
- Inman, South Carolina