208 / 120 calculation...

[1793]

I?m not sure I know how to ask this question.

How does one calculate the load for a 208/120V 3 phase system?

Situation:
Nail Salon- approx 1,100 sq ft,-has 100amp panel. Would like to add a 50gal electric water heater. There is one in place now. There is a 3phase A/C unit. There are eight work stations, 4 pedicure chairs with circulating pumps/ motors. Washer and Dryer, and of course lighting.

I have not been able to Amp the conductors as they don?t want me in during the most busy time.

Are there any worksheet or programs, fee of course, similar to the ones for residential calculations available?

Load / 208x1.73= Load / 359.84 ?

 

[bob]

You may want to contact the utility and get a demand for the building if
it is billed with a demand charge. Regarding the program, I do not know of one for commercial loads. This is not a large load so you can caculate it the hard way.
Why can't you get a reading during the peak load? The NEC is not much help with this type of load. I think you need to assume everything is operating at the same time. The question is what is contineous load and what is not.

 

[1793]

This Nail Salon is part of a Strip Store. The building demand will not work for this situation.

I guess my main question was how to compensate for the 3 phase aspect of the calculation, or does that even matter?

 

[eric stromberg]

I guess my main question was how to compensate for the 3 phase aspect of the calculation, or does that even matter?

It doesn't really matter. If all you had were 120 Volt Loads, you could look at the three phase supply as being three 120 Volt supplies.

Power for three phase is:
P = V(L-L) * A(L-N) * sqrt(3)
...or...
P = V(L-N) * A(L-N) * 3

The first thing you should do is to calculate your total load in VA and then divide by [208*sqrt(3)] This will give you the Amperage on each phase if the load were perfectly divided. After you do this, you need to see how the loads will be distributed per phase.

...and/or...

Calculate your three phase and single phase loads separately. Use the three phase loads as your base load that is equal on each phase. Now, decide which phases you will put your single phase loads on and add accordingly.

Eric

 

[77401]

1793
I'm sure most will disagree with this but I'll try to explain my reasons...

From a business standpoint the customer wants another Water heater. Give them a Quote & hook it up. Stop thinking too hard about all of the Demands.
Do the job, get paid, move on!

From a safety point, you do have branch circuit protection & a Main breaker.
You should only hope that the 100amp main trips. When it does they'll be calling you.
Then you can simply sell them, with proof of a tripped main breaker, that they need a 200 amp service.
You tried to save them money by not upgrading the first time but it didnt't work. It didn't cost them more to have it done separately!
More money to you!