Tanning salon service size

[Todd Z]

Maybe easier to see, but without the mathematical rigor in a simpler, non-balanced situation:

Consider only two loads, A to C and B to C. Each 80 amps.
The current on A will be 80 amps, the current on B will be 80 amps, but because the two loads are pulling currents on the C wire which are out of phase the resultant total current on C will be the vector sum rather than the arithmetic sum. Specifically 80 x (1.73 / 2) = 69. That gives you the same numbers as growler's calculation by watts (which includes the 1.73 factor that comes into the three phase wattage calculation for the same reason!)

Ok I'm following you somewhat. So since the two different loads on C are 120 degrees out of phase, you can't just add them together. I do recall the .866 mutilpier from either college or my apprenticeship but didn't remember its function. So basically if use my original sum of the loads and multiply by .866 then that will be the total current on the service conductors?

 

[GoldDigger]

Ok I'm following you somewhat. So since the two different loads on C are 120 degrees out of phase, you can't just add them together. I do recall the .866 mutilpier from either college or my apprenticeship but didn't remember its function. So basically if use my original sum of the loads and multiply by .866 then that will be the total current on the service conductors?

Not quite. That will only apply if the system is perfectly balanced. (Remember in my example the currents on A and B did not get reduced.)

 

[david luchini]

Maybe easier to see, but without the mathematical rigor in a simpler, non-balanced situation:

Consider only two loads, A to C and B to C. Each 80 amps.
The current on A will be 80 amps, the current on B will be 80 amps, but because the two loads are pulling currents on the C wire which are out of phase the resultant total current on C will be the vector sum rather than the arithmetic sum. Specifically 80 x (1.73 / 2) = 69. That gives you the same numbers as growler's calculation by watts (which includes the 1.73 factor that comes into the three phase wattage calculation for the same reason!)

I should think you would get 139A on C, not 69A. (assuming, of course, that the loads have the same power factor)

 

[GoldDigger]

I should think you would get 139A on C, not 69A. (assuming, of course, that the loads have the same power factor)

Have to think about that for a moment....
Yes, slipped again. Thanks David.

But in any case, it is less than 160.

 

[Todd Z]

Thanks everybody for the info. Turns out another contractor came in and changed the 300A main to a 350A. The panel bus was rated at 400A and it was fed with 350kcm. Atleast this was a good learning experience.