LOAD CALCULATION QUESTION.

[Santa49]

Customer wants to add a shop, pool equipment and 2000 square foot living area to his existing 200 amp service. He has supplied to me his electric billing which includes every month of KWH usage for the last 2 1/2 years. He wants to know how many amps he can safely add to this 200 amp service without overloading. This is actually backwards from the way I've done load calculations in the past. Can anyone shed some light on this for me as to how to convert KWH usage to Amps?

 

[petersonra]

There is a provision to calculate loads based on actual usage. Take a look closely at this provision. I don't believe a normal utility bill will cut it, but the utility may have better information that would.

 

[charlie b]

KWH values are meaningless. They represent the total amount of energy used. You need the maximum amount of energy drawn during any month during a 12 month period. You need max KW values.

No, you cannot infer amps from KWH.

An analogy would be saying that you drove 200 miles on a highway with a 60 mph speed limit, and did it in four hours, and attempting to use that to prove that you were innocent of the speeding violation for which you were ticketed. However, you could have stopped for lunch and for gas, then drove at 90 mph the rest of the trip. For determining the availability of spare capacity for adding load to an existing building, what matters is how fast it draws power during its peak periods.

 

[LarryFine]

I'd want to know the peak momentary current.

 

[retirede]

Start with an NEC load calculation and see what it looks like.

 

[ptonsparky]

Start with an NEC load calculation and see what it looks like.

Yes.

The old billing won’t include that new resistant Ht unit, etc, so start over.

 

[Santa49]

Thanks everyone for such a speedy response to my situation. Everyone was on the same page and it was about what I had come to myself. I have never had much luck or confidence with using the KWH readings for load calculations. I believe we were trying to cut corners to save some time. Next step for me is to actually visit the job site or come up with a set of plans and come up with my own calculations. At least I would feel more comfortable with the end result. Thanks again.