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Re: SINGLE PHASE AND 3 PHASE LOAD CALCULATION

Jim T. is correct about the way you add the loads. However, I am not completely confident in the way you derived the loads. You gave the numbers in terms of “kilowatts” (KW), not in terms of “kilovolt amperes” (KVA). The two are never the same. They have the same numerical value for resistive loads. But the value of KVA, for a given component’s load, is higher than the value of its KW, if the load is a motor. So I would ask how you derived the values you are giving us.

For example, to get the motor load, did you take a value off the nameplate? For the purposes of NEC load calculations, you cannot use the manufacturer’s amp or watt rating. Instead, for single phase motors, you find the horsepower, look up the amps in Table 430.148, and multiply by the appropriate voltage rating, taking that value of voltage from the Table. That gives you KVA for that motor. Similarly, for three phase motors, you find the horsepower, look up the amps in Table 430.150, and multiply by the appropriate voltage rating, taking that value of voltage from the Table, and multiply that result by 1.732 (i.e., the square root of 3). That gives you KVA for that motor.

To get the required generator rating, you need to know if the owner plans on connecting all of the loads, or just a portion. Generators are rated in terms of “KW, at a power factor of (some value).” Divide that KW by that power factor, and you get the generator’s rating in terms of KVA. You need a generator that can handle the intended load (in KVA).

Charles E. Beck, P.E., Seattle

Comments based on 2008 NEC unless otherwise noted.

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