I have 3 - 6KW inverters that output 208V single phase each (L1 & L2 + N & Gnd). Per the spec sheet the maximum output current is 29A per inverter.. at 208V.
The electrical service that I will be connecting them to is 120/208V 3PH 4W (wye). If I connect the 1st inverter to A-B, 2nd to B-C and 3rd to A-C (at an AC combiner panel).. what will the load be on each phase?
My thought is 58A each, but I'm not so sure that is correct with a 3PH system.. and I'm not an engineer.
Once you have a balanced configuration of phase-to-phase inverters on a three-phase system, the calculation of per conductor current is total power/voltage/sqrt(3).
6 kW * 3 = 18 kW
18000W / 208V / sqrt(3) = 50A
If for some reason, the inverter datasheets have a larger current than power rating alone would perscribe, increase this by the above nominal current ratio. One example is a hypothetical 6 kW inverter having 32A at 208V. Because 6 kW at exactly 240V is really 29A. This hypothetical datasheet current is 10% higher than power rating alone would predict, and one strategic reason for this, is to avoid clipping during undervoltage operation. If you triplet three of these hypothetical inverters, the current becomes 3*6000W/sqrt(3)/208V * (32/29) = 55A.
If you DO NOT have a balanced current situation, the formal procedure for adding up phase-to-phase currents is as follows:
Add up all A-B currents. Call it Iab.
Add up all B-C currents. Call it Ibc.
Add up all C-A currents. Call it Ica.
Calculate current on conductor A, by using all terms pertaining to phase A, which are Iab and Ica.
Ia = sqrt(Iab^2 + Ica^2 + Iab*Ica)
Calculate current on conductor B, by using all terms pertaining to phase B, which are Iab and Ibc.
Ib = sqrt(Iab^2 + Ibc^2 + Iab*Ibc)
Calculate current on conductor A, by using all terms pertaining to phase A, which are Iab and Ica.
Ic = sqrt(Ibc^2 + Ica^2 + Ibc*Ica)
Add in any applicable phase-to-neutral currents to each of the above values.
Add in any nominal per phase currents from any applicable three phase circuits, to each of the above values.
Take the largest current, and that becomes the operating current for the entire mix of loads. All OCPDs, panels, disconnects, and wire corresponding to the aggregate feeder, are to be sized to that value. On all phases.