wiring 12 amp fan

USA - AWG 10 gives you a 2.4% voltage drop. A fine print note in the National Electrical Code states (paraphrasing) that a voltage drop of 3% or less provides "reasonable efficiency of operation".

AWG 12 gives you a 4% voltage drop.

Your choice.

Ed

Assuming that you are using NM-B cable (romex).... The No. 12 voltage drop is more like 3.5 per cent. The reason for this is with a load of only 12 amperes the conductor temperature is approximately 39 degrees C instead of 75 degrees C assuming an ambient of 30 degrees C. The 4 per cent voltage drop assumes that the operating temperature of the conductor is at a maximum of 60 degrees C. allowed for NM-B cable because of code requirements in 334.80. With a load of 25 amperes and an ambient of 30 degrees C the conductor temperature should be near its maximum allowable temperature of 60 degrees C and there would be an approximate voltage drop of 4 per cent.

Ref:

334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60=B0C (140=B0F) conductor temperature rating. The 90=B0C (194=B0F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60=B0C (140=B0F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.

Gerald Newton

3% branch circuit or 5% combined branch circuit and feeder. [Note Henry also posted on alt.home.repair]

Bear in mind this is a guideline, not a NEC rule. It is in a "fine print note" not in the code. From a code standpoint the only recourse an inspector has is to look at the equipment manufacturer's instructions and try to use 110.3(B) to coerce compliance. You also have to take into account what the utility is giving you. Here in SW Florida I get about 124v at the panel so I can tolerate some voltage drop and still have my equipment running in the recomended voltage range.

If the OP wants a sanity check, try running his pump on a 100' orange cord (16ga or 14ga). See what voltage he is getting at the pump end.