Firguring breakers for split phase vs 3 phase

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ptevendale

Member
Location
St. Thomas, Virgin Islands
I am installing a 11.4 KW 3 phase Fronius inverter.

The max output is 11,400 watts. The output voltage is 208 3 phase.

If it was 240 split phase I would take 11,400 W / 240 v equaling 47.5 amps multiply that by 125% totaling 59.375 so I would use a double 60 amp.

It is 208 3 phase. If I take 11,400 W / 208 v equaling 54.81 amps. Do I divide by 3 and then multiply by 125%?
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
ptevendale said:
I am installing a 11.4 KW 3 phase Fronius inverter.

The max output is 11,400 watts. The output voltage is 208 3 phase.

If it was 240 split phase I would take 11,400 W / 240 v equaling 47.5 amps multiply that by 125% totaling 59.375 so I would use a double 60 amp.

It is 208 3 phase. If I take 11,400 W / 208 v equaling 54.81 amps. Do I divide by 3 and then multiply by 125%?
Click to expand...

The line current at 208 volt 3 phase of 11.4 KW would be 31.64 amps. Will that remind you of how to calculate this?
 
Smart $

Smart $

Esteemed Member
Location
Ohio
ptevendale said:
I think I found the answer:

11,400 W / (square root of 3 X 208 v) = 31.64 amps X 125% = 39.55 amps...therefore a 40amp breaker
Click to expand...
The breaker rating should be determined using the inverter's rated maximum output current on nameplate times 125%.

Also, if you think about it, ~208V ? sqrt(3) = 360, exactly 120V ? 3 phases (such as in a 208Y/120V 3? 4W system).
 
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