Three Phase Solar Calculation.......Help!

[ShawnB]

Hi Guys,

I'm embarrassed to say I'm confused on the proper way to size the Service and Feeder of a three phase Solar Array. The array is 24kw and the inverter output voltage is 208v three phase.

Specifically my question is when sizing the OCPD and wires do I use the 1.73 multiplier? Or, do I just use the output current rating of the inverter?

If I have ten inverters all feeding back to the main service how do I size the main service? Do I add together the the output currents of the inverters and size the service to that? Does the multiplier of 1.73 apply to that?

Example) 24,000W/208V = 115A. 115X10(inverters) = 1153A So service would have to be 1200 Amps?

Example 2) 24,000W/(208X1.73) = 66.8A. 66.8Ax10(inverters) = 668A

Thank you in advance for any clarity you can lend.

 

[Smart $]

Welcome. :thumbsup:



What is the rated output current of each inverter?

Is each inverter 1Ø or 3Ø?

 

[GoldDigger]

And if single phase, will they be connected line to line or line to neutral?

 

[ShawnB]

As of right now I do not know the output current of the inverter other than that they are 24KW. I need to figure out a service and feeder cost estimation.

As I understand, they are three phase 208v output.

I hope that helps

Thank you!

 

[jaggedben]

The rated output current of the inverter is the only number relevant to the code requirements. So you really need to be sure of that, you can't just divide power by voltage.

If the output of each individual inverter is three-phase, then you do not need to do any calculations involving 1.73.

 

[ShawnB]

Thanks jagged

So based on the limited information I have available, you would feel safe using a preliminary calculation using the 24KW/208v, to arrive at 66A. And, using that Amperage to determine branch circuit, feeders and services? Do I understand you correctly?

I appreciate your help!

 

[ShawnB]

Correction

115A

Ooops

 

[Smart $]

Thanks jagged

So based on the limited information I have available, you would feel safe using a preliminary calculation using the 24KW/208v, to arrive at 66A. And, using that Amperage to determine branch circuit, feeders and services? Do I understand you correctly?

I appreciate your help!

Correction

115A

Ooops

If 3Ø output current rating is equivalent to 24kVA, then 67A.

 

[ShawnB]

Smart$,

Are you sure about that? To derive at 66A you need to take 208X1.73=359 and use that as the divisor.

Jagger above advised this was not needed.

 

[electrofelon]

Hi Guys,

I'm embarrassed to say I'm confused on the proper way to size the Service and Feeder of a three phase Solar Array. The array is 24kw and the inverter output voltage is 208v three phase.

Specifically my question is when sizing the OCPD and wires do I use the 1.73 multiplier? Or, do I just use the output current rating of the inverter?

If I have ten inverters all feeding back to the main service how do I size the main service? Do I add together the the output currents of the inverters and size the service to that? Does the multiplier of 1.73 apply to that?

Example) 24,000W/208V = 115A. 115X10(inverters) = 1153A So service would have to be 1200 Amps?

Example 2) 24,000W/(208X1.73) = 66.8A. 66.8Ax10(inverters) = 668A

Thank you in advance for any clarity you can lend.

If a device is three phase then 99.9% of the time, "current", "line current", "output current", etc will refer to the current on each phase so you are all set as far as sizing your conductors (except of course for the continuous load factor, derating for conditions of use, voltage drop, etc). If you are giving KW and want line current, that is when you use the 1.732. Dont forget the 1.25% for continuous loads, and volltage drop.

 

[Smart $]

Smart$,

Are you sure about that? To derive at 66A you need to take 208X1.73=359 and use that as the divisor.

Jagger above advised this was not needed.

I'm quite certain jaggedben's comment was in regards to using the nameplate output current value... not the PV kW rating.

As noted by electrofelon, when all three lines of a 3Ø system are involved in a current calculation, the square root of three is part of the equation.

 

[jaggedben]

Thanks jagged

So based on the limited information I have available, you would feel safe using a preliminary calculation using the 24KW/208v, to arrive at 66A. And, using that Amperage to determine branch circuit, feeders and services? Do I understand you correctly?

No, I would feel 'safe' only knowing the inverter model and the rated ouput current as stated on the datasheet. And the confusion in this thread just reinforces that thought.

Your calculation of inverter output current from kW power would indeed be okay only for a preliminary ballpark estimate, in which 66A would be the correct answer. Your busbar and conductor requirements are unlikely to change by more than one size if the actual inverter rated current is different from that, but again don't draw up plans or build the thing til you know. In fact I wouldn't necessarily even bid based upon it. One change in size for this scale system can be a lot of money saved or lost. Or at least bid one size higher than you think need if they won't / can't tell you the inverter model.

Smart$ is correct regarding what meant in my previous post. If you are working with the rated 3ph output of the inverter, you have no further involvment of the 1.73 factor for calculating busbar and conductor sizes. If you are trying to calculate current from power then you do need to divide by 1.73, but calculating current from power is what I told you not to do.

 

[ggunn]

What they said. Get the maximum rated current from the data sheet for the inverter and use it to determine conductor size and OCPD. Do not use any other number. Most of the time if you calculate max current from the power rating (correctly, of course) it will be very close to the number on the data sheet, but I have seen it differ by quite a bit. If you are confused about the sqrt3 in the power to current calculation, then all the more reason to get the number from the data sheet.

BTW, what inverter are you using? From the 24kW power rating I thought at first it might be a Sunny Tripower, but the Tripower series is 480V only.