PV Feeders and next size up OCPD

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electro7

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I have 4 inverters: two at 120.3A and 90.2 max output current.

Total current: 421A x 1.25 (continous load) = 526.25.

Next size up breaker 600A.

Its a 3 phase 480V system and the interconnect is around 700ft away. I am running 3 sets of 3/0 thwn-2 conductors in individual conduits.

3/0 thwn-2 base ampacity is 225A at 90°C.
225A x 3= 675.

My question is if this installation is code compliant? 600A breaker with 3 sets of 3/0 thwn-2 copper conductors.

Since there is 4 current carrying conductors per conduit the ampacity of the wire goes down to 180A.
180A x 3 (sets of conductors)= 540A.

Is it okay to have that ampacity (540A) protected by the 600A OCPD?

I think this is okay since the total load is 421A (or would I use the 526.25A for continuous load to apply the ampacity of the wire?)

I think I am good on voltage drop being under 3%.

Next question is what size EGC do I run per conduit- do I use 250.122 for 600A OCPD wire size per conduit?

Thanks ahead of time.


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electro7 said:
3 hots and a nuetral, no?

277/480V, wye circuit.

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Click to expand...

The neutral doesn't count as a CCC for purposes of this calculation in this example. It likely is an example where you get to run a reduced size neutral per 705.28(C), if the manufacturer allows it.
 
You do have the correct interpretation of the next size up rule. The wire and termination amps both need to exceed the previous size down, in order for them to be permitted to "round up" to the actual OCPD size you are using. If you have a non-standard OCPD, or an OCPD greater than 800A, the next size up rule doesn't apply, and you need at least as much ampacity as the OCPD you are using.

The EGC gets sized from Table 250.122, based on the OCPD you are using. This is your minimum local EGC, asssuming your ungrounded conductors are no larger than the NEC requires for ampacity. If you upsize above the minimum size that has sufficient ampacity for the intended installation, then you need to proportionally upsize the kcmil of the EGC by the same ratio.

In the event that upsizing prompts a change in the number of sets, your starting point for this calculation is the minimum size in the number of sets and configuration that you are installing. For instance, suppose you build a 400A circuit with 2 sets of 400 kcmil Cu in separate raceways. Rather than upsizing the EGC by a ratio of (2*400)/600 = 1.333, you instead upsize the EGC by a ratio of (2*400)/(2*168) = 2.38, because 3/0 Cu is the minimum size in the 2 sets in individual raceways.
 
In your example, I would advise against using 3 sets, unless either:
A: you have to use 3 sets
or
B: you know for sure that equipment on both sides has 3 terminals per phase.

From what I've seen, 600A equipment usually has 2 terminals per phase. 3-terminals per phase in 600A exists in some equipment, but it is not as common. It is much easier to get 2 sets to fit in equipment built for 2 sets, as you can use a simple splice/reducer on each conductor, if you've upsized for voltage drop and need to locally adapt to fit your equipment. To change between 3 sets and 2 sets, you would need a 5-terminal tap connector, in order to bring all 3 sets together, and convert them into 2 sets to terminate on equipment.

When voltage drop governs sizing, the ability to curtail voltage drop is nearly proportional to the kcmil, so you can expect to have about the same amount of wire material, no matter how you distribute it among parallel sets. By contrast, when ampacity governs sizing, the amps per kcmil is a lot higher in smaller wires. So paralleling for ampacity has a significant advantage, but the advantage is a lot less when voltage drop governs sizing.
 
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The neutral doesn't count as a CCC for purposes of this calculation in this example. It likely is an example where you get to run a reduced size neutral per 705.28(C), if the manufacturer allows it.
Click to expand...
Okay, am I looking at 310.15(B)(5)(a) for this?
I don't see 705.28(C) in my 2017 code book. Is that 2020?

In the event that upsizing prompts a change in the number of sets, your starting point for this calculation is the minimum size in the number of sets and configuration that you are installing. For instance, suppose you build a 400A circuit with 2 sets of 400 kcmil Cu in separate raceways. Rather than upsizing the EGC by a ratio of (2*400)/600 = 1.333, you instead upsize the EGC by a ratio of (2*400)/(2*168) = 2.38, because 3/0 Cu is the minimum size in the 2 sets in individual raceways.
Click to expand...
I am lost here. Can you explain what the numbers are referring to?

In your example, I would advise against using 3 sets, unless either:
A: you have to use 3 sets
or
B: you know for sure that equipment on both sides has 3 terminals per phase.
Click to expand...
Unfortunately there were 2.5" conduits already ran so we need to go with the three sets. I ordered the 600A breaker to accommodate the three sets.

Thanks Carultch!
 
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