tracking down wire size through NEC book

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Sparky_

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Greetings,

I have drawings for a solar system for a commercial system - 3 phase wye.

I started in section 690 of the code -

calculated the maximum current = 79 Amps - (2 Fronius Inverters w/ max current = 31.6 amps:
(31.6+31.6)*1.25)= 79 amps

Then Section 690 tells me to go to section 240 for conductor sizing but I don't see within section 240 a place specifying conductors going up to and beyond this current.

How can I tell from the NEC code what conductor size to use for 79 amps in a solar arrangement?

Also the system has a 40 amp breaker panel with 2 Fronius Solar inverters feeding the panel. The maxium output current from 1 inverter is 31.6 amps.

Is this arrangement ok?

Thanks
Sparky_
 
Sparky,
We are fortunate enough to have a PV guru as a member, but as he has not yet posted, I will try to answer.
690.33 references 690.9 which then references Art 240.
240.4 tells us that conductors shall be protected against overcurrent in accordance with their ampacities specified in 310.15 unless otherwise permitted in 240 (A) thru (G)
310.15 would be your guide, in conjunction with 110.14
Assuming 75? terminations, a 40 amp conductor would normally be a #8, a 79 amp conductor,a #4 (not taking any derating factors into consideration)
 
Sparky_ said:
Greetings,

I have drawings for a solar system for a commercial system - 3 phase wye.

I started in section 690 of the code -

calculated the maximum current = 79 Amps - (2 Fronius Inverters w/ max current = 31.6 amps:
(31.6+31.6)*1.25)= 79 amps

Then Section 690 tells me to go to section 240 for conductor sizing but I don't see within section 240 a place specifying conductors going up to and beyond this current.

How can I tell from the NEC code what conductor size to use for 79 amps in a solar arrangement?

Also the system has a 40 amp breaker panel with 2 Fronius Solar inverters feeding the panel. The maxium output current from 1 inverter is 31.6 amps.

Is this arrangement ok?

Thanks
Sparky_
Click to expand...

Well 690 says that the 240.5(B) sizing shall be permitted... Since 50amps is the limit of the what 240.5(B) lists I would say it would have to be sized IAW standard wire sizing of T310.16...
But it is only my opinion. After reading through 690 I can't find anything to indicate other wise. It doesn't say it "shall be sized" IAW 240.5(B)... Just shall be permitted.
 
Not clear which cables you are considering. There are at least 3 situations:

A. The DC PV source circuits. The combination of 125% in 690.8(A)(1) and the additional 125% in 690.8(B)(1) means you need an ampacity in these wire of 156% of the short circuit current. This rounds up to a standard OCPD rating unless Art 240.4(B) or (C) applies (not likely). Then you need a wire with ampacity based on the 156% when derated for terminal temperature, type of wire, conduit fill (if in conduits) and temperature effects.The wire must be protected by the fuse, move up in size as necessary.

B. Inverter to AC Combiner. Based on inverter rated maximum output * 125%. Same rules as above. Some AHJ consider this a busbar and subject to the 120% rule of Art 690.64(B)(2).

C. AC Combiner to main panel/SES. Based on ratings of the OCPDs in the AC Combiner. Some AHJ consider this a busbar and subject to the 120% rule of Art 690.64(B)(2).
 
Inverter to AC combiner: 31.5 x 125% = 39.4 amps Assuming THHN Cu conductors, use of 75 degree terminals limits ampacity to the 75 degree column of table 310.16, so you need to use #8 but can use 55 amps as a starting point if you need to apply de-rating.

Size of AC combiner: 31.5 + 31.5 = 63 amps. A 60 amp panel x 120% = 72 amps. I think a 40 amp panel is not big enough. Also, your AHJ might not like applying the 120% rule here, so I would use a 100 amp panel which costs a few dollars more than a 60 amp. I would feed in with 50 amp cir breakers. And feed out with a 80 amp cir br

AC combiner to main panel: #4 limited to 85 amps, but 95 amps for de-rating.

IMO, why use a AC combiner? Why not two separate runs to the main panel with #8 using two 50 amp cir br?

Unless you have a 400 amp service, I think you will need to use a 200 amp service box with a 100 amp main breaker- not so sure about this item. And not sure if you feed into 3 phase A-B both or A-B and B-C phases?

All items subject to verification by the guru....
 
Last edited:
Inverter to AC combiner: 31.5 x 125% = 39.4 amps Assuming THHN Cu conductors, use of 75 degree terminals limits ampacity to the 75 degree column of table 310.16, so you need to use #8 but can use 55 amps as a starting point if you need to apply de-rating.

Size of AC combiner: 31.5 + 31.5 = 63 amps. A 60 amp panel x 120% = 72 amps. I think a 40 amp panel is not big enough. Also, your AHJ might not like applying the 120% rule here, so I would use a 100 amp panel which costs a few dollars more than a 60 amp. I would feed in with 50 amp cir breakers. And feed out with a 80 amp cir br

AC combiner to main panel: #4 limited to 85 amps, but 95 amps for de-rating.

IMO, why use a AC combiner? Why not two separate runs to the main panel with #8 using two 50 amp cir br?

All items subject to verification by the guru....
 
Why use a AC combiner?

Why use a AC combiner?

In my experience most of the utilities involved with line-tie PV systems require a separate KWH meter and a PV AC Disconnect switch and are reluctant to allow separate meters and switches for two inverters when it is practical to use an AC Combiner.
 
Hey guys thanks for the help!!

Twoskinsoneman said:
Well 690 says that the 240.5(B) sizing shall be permitted... Since 50amps is the limit of the what 240.5(B) lists I would say it would have to be sized IAW standard wire sizing of T310.16...
But it is only my opinion. After reading through 690 I can't find anything to indicate other wise. It doesn't say it "shall be sized" IAW 240.5(B)... Just shall be permitted.
Click to expand...

What does "IAW" stand for? (I am a newbie)

augie47 - thanks I knew I needed to get to the tables in 310 but overlooked 240.4

I went from 690 to 240 but overlooked (several times acctually) 240.4.


mlnk - what does AHJ stand for?

Follow-up question:

where can I find the red flag rules for the 40-amp panel not being big enough?

Thanks
Sparky
 
Sparky_ said:
Hey guys thanks for the help!!



What does "IAW" stand for? (I am a newbie)

augie47 - thanks I knew I needed to get to the tables in 310 but overlooked 240.4

I went from 690 to 240 but overlooked (several times acctually) 240.4.


mlnk - what does AHJ stand for?

Follow-up question:

where can I find the red flag rules for the 40-amp panel not being big enough?

Thanks
Sparky
Click to expand...

In accordance with
Authority having jurisdiction
 
This solar system is installing a 40 amp panel box - between the solar inverters and a disconnect

in reading the responses - is this too small? mink I see recommends 100 amp.

How do I prove this by code?

Thanks again
-Sparky_
 
Installing a ?? amp panel box - between the solar inverters and a disconnect

Installing a ?? amp panel box - between the solar inverters and a disconnect

The original discussion was wire sizing, not panel sizing.

The panel, called an AC Combiner, comes under 690.64(B)(2).

The size of this panel is based on the breakers feeding it: 40A + 40A + 80A (assumed in the main panel) = 160A. This is where the 120% rule comes in, the panel can be fed with breakers up to 120% of its rating provided it is labeled as an AC combiner and permanently marked "LOADS NOT TO BE ADDED TO THIS PANEL" and the PV breakers located at the far end of the bus from the 80A feed. A 100A panel can have breakers adding up to 120A, a 125A panel is good for 150A, and a 150A panel is good for 180A. Looks like a 150A panel is needed here.

If loads are to be served, then the 120% allowance does not apply and a 200A panel will be needed to hold the two 40A breakers.


See the following for a good discussion and example:

http://homepower.com/files/webextras/HP134_p105_CC.pdf
 
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