PV Disconnect Current Calculation

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I'm working on a PV system which has a Utility disconnect that serves a 3phase 208V panelboard. Connected to that panel board is (5) 10KW 3 Phase Inverters and (2) 3.8KW Single Phase Inverters. We are required to attach a label on the disconnect stating the max current. I calculate ~160Amps. My boss with 30years experience gets 176Amps. He says I need to calculate the current as the 3 Phase Load (139Amps) plus the Single Phase load (37Amps). I argued that since the (2) single phase loads would be wired on A,B,C,A, that should not matter.

What gives? If this is the case, I've sized feeders, panelboards, everything incorrectly for 6 years!
 

James@CHA

Member
I'm working on a PV system which has a Utility disconnect that serves a 3phase 208V panelboard. Connected to that panel board is (5) 10KW 3 Phase Inverters and (2) 3.8KW Single Phase Inverters. We are required to attach a label on the disconnect stating the max current. I calculate ~160Amps. My boss with 30years experience gets 176Amps. He says I need to calculate the current as the 3 Phase Load (139Amps) plus the Single Phase load (37Amps). I argued that since the (2) single phase loads would be wired on A,B,C,A, that should not matter.

What gives? If this is the case, I've sized feeders, panelboards, everything incorrectly for 6 years!

How is the system configured? If the disconnect is on the output of the inverters then the maximum current would likely be the sum of the continuous rated output of each inverter.
 
A 3 pole disconnect feeds a 3phase 4w panelboard which feeds (5) 3 phase 10KW inverters and (2) 3.8KW single phase Inverters.
 

rcwilson

Senior Member
Location
Redmond, WA
Assume you had three 3.8 kW single phase inverters connected AB-BC-CA. 3 x 3.8 kW = 11.4 kW.
Add the 5, 3-phase, 10 kW inverters: 5 x 10 kw = 50 kw. 50 + 11.4 = 61.4 kW three phase load.

Calculate the current for this balanced 3-phase load load: I = 61.4 x 1000 /(208 x 1.732) = 170.5 A .

Now remove the single phase inverter connected B-C to get back to your configuration. The current on A phase will still be 171 amps because nothing has changed on that phase. That is the current for the sizing calculation, assuming the inverters operate at 1.0 power factor so 1 kW = 1 kVA.

I think your 160 amp calc has the implicit assumption that the 7.6 kW single phase loads are distributed equally across all three phases: I= (50 + 7.6)*1000/(208*1.732) = 160A.

Your boss's conservative calculation assumes the single-phase and three-phase inverters' currents are in phase and add arithmetically instead of vectorially. (Phase-phase currents are 30 degrees out from phase currents):

50 kW, 3phase, 208V = 138.8 A.
3.8 kw, 1 phase, 208V = 18.3 A, for two units, I = 36.6 A .

Current on the highest loaded phase, assuming currents are in phase = 138.8 + (2 x 18.3) = 175.3 Amps.

While not technically correct, it is conservative.
 
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