How Objective Is The N.E.C.?

drb9

If one electrician says the load calculation is 250 amps (and wants to do a heavy-up to 400 amps), and another comes in at 150 amps (and feels that existing 200 amp service is sufficient), is one clearly wrong? Are there judgment calls in reaching this determination?

The difference seems to be hard-wired electric heaters that either do or don't get counted at the same time as a/c units (which, presumably) would be used during different times of the year. Does the N.E.C. not provide a clear answer for this?

USAF sparky

Those heaters could have a hight startup current that one electrician is taking into account and the other isn't. And no the AC and Heater would not be used at same time so this does not affect the calculation. The heater will pull and sustain higher amps than the AC so this will be what needs to be calculated.

Magnettica

why would you have high start up current on a resistive load?

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kbsparky

The lesser of the heating or A/C loads can be ignored when performing load calculations. It's called non-coincident loads.

If the one electrician is including both loads, he may be trying to upsell that 400 Amp service.

OTOH, if you are dealing with Heat Pumps, then both those loads may be to be considered, since the outside compressors can operate at the same time as the resistance strip heaters

mdshunk

The code is clear about calculating the heating and cooling... pick the bigger of the two. In the case of heat pumps, the load is both.

That said, there are some games you can play with the load calc, but it's pretty hard to come up with a 100 amp disparity between two. Realistically, the cost difference between a 200 amp and a 320 amp service is not really that much.