208 Single Phase motor VA for 3phase loading

[arc_flashed]

I am looking for some clarification on how the VA for a single phase motor is calculated and how that translates into the loading/current flow on my wires when I'm adding it to a 3-phase distribution.
I have a single phase (L-L, no neutral) 208vac, 2HP motor. NEC FLA = 13.2 amps.
This works out to 208v * 13.2a = 2746VA.
My question is how does this 2746VA add to the load on the three phase conductors I have on a 208vac 3-phase breaker?
The total loading on this panel is 6amps at 120vac L-N plus the 13.2 amps 208vac L-L. If I do the math:
6A @120v = 720VA (phase A)
13.2@208v = 2746VA (phase B and C)
25% motor = 687VA
Total panel VA = 4153VA

If I convert this 4153VA to 208VAC 3PH:
4153VA/208v/1.73 =11.54amps

This seems incorrect as it would indicate that a 15amp 3P 208v breaker would be sufficient based on the calculated FLA of the panel. However, looking at my loads I would need a 20amp breaker for my motor load which is higher than the main. In my calculation above do I need to account for 2X the VA of the single phase motor since the 13.2amp will be flowing out one leg and back on the other without a neutral to sum at the main breaker?

I am attempting to minimize the load/feed on this panel as it is almost 1000ft away from the source so every amp really matters with the voltage drop. That is why we are looking at a 3phase distribution but running the numbers they don't seem to be adding up.

I was also using the 3-phase load calculation for the voltage drop which I am now questioning also.

Thanks in advance for any help.

 

[wwhitney]

I am looking for some clarification on how the VA for a single phase motor is calculated and how that translates into the loading/current flow on my wires when I'm adding it to a 3-phase distribution.
I have a single phase (L-L, no neutral) 208vac, 2HP motor. NEC FLA = 13.2 amps.
This works out to 208v * 13.2a = 2746VA.
My question is how does this 2746VA add to the load on the three phase conductors I have on a 208vac 3-phase breaker?
The total loading on this panel is 6amps at 120vac L-N plus the 13.2 amps 208vac L-L. If I do the math:

Then assuming you put the L-N loads on the leg that does not power your L-L motor, and given that the NEC FLA is multiplied by 125% for conductor sizing purposes, your loads on the 3 legs are (6, 16.5, 16.5) amps, with 6A on the neutral.

If I convert this 4153VA to 208VAC 3PH:
4153VA/208v/1.73 =11.54amps

That conversion only works if the 4153 VA is from balanced loads. E.g. if all your L-N and L-L loads come in sets of 3 identical loads, with each set connected in a balanced fashion across all 3 legs. [Plus I guess the L-N loads would need to have power factor 1.]

I am attempting to minimize the load/feed on this panel as it is almost 1000ft away from the source so every amp really matters with the voltage drop. That is why we are looking at a 3phase distribution but running the numbers they don't seem to be adding up.

I was also using the 3-phase load calculation for the voltage drop which I am now questioning also.

For voltage drop, use nameplate numbers on the motor, rather than the NEC FLA with its 125% factor. For motors there's the possibility that startup current is actually the critical voltage drop case, which is something I don't know how to compute.

Otherwise, you just want to use the actual running current to calculate the voltage drop. Check voltage drop for each load separately using the single phase formula, since you have two distinct single phase loads.

Cheers, Wayne