Split Enphase Combiner Output

[hitehm]

Our problem:
Coming off the roof we have 3 Enphase 20A microinverter circuits landing in the Enphase combiner box, then leaving the combiner box on 1 60A circuit (52.94 actually) through the disconnect and what would normally land on a 60A backfed solar breaker in the MSP. However, the customer's MSP won't allow more than a 50A 2P breaker in the top slots where we need to land our 60A solar breaker.

Possible Solution: (And fyi, we are trying to avoid 2 separate combiner boxes with 2 separate discos)
Place a small 4 slot MLO subpanel after the disco before the MSP and split the one 240V 60A 2P PV circuit into two 240V 30A 2P feeder circuits that will land in the top 2 slots on two 30A backfed breakers in the MSP.
So the questions are:
Is there anything wrong electrically, code wise, safety wise or other that I'm missing?

 

[wwhitney]

Is there anything wrong electrically, code wise, safety wise or other that I'm missing?

Yes, your proposed configuration violates 240.8.

You can either bring two separate supplies off the roof, find a way to interconnect your 52.94A feeder elsewhere, or remove a few panels/microinverters to get your feeder down to 50A.

Cheers, Wayne

 

[hitehm]

Yes, your proposed configuration violates 240.8.

You can either bring two separate supplies off the roof, find a way to interconnect your 52.94A feeder elsewhere, or remove a few panels/microinverters to get your feeder down to 50A.

Cheers, Wayne

Thanks Wayne! I knew there had to be something I was missing code-wise and it totally makes sense as far as not guaranteeing they both trip if not as one unit. However, out of curiosity and to settle the watercooler debate, do you see anything electrically about it that wouldn't work under normal operation? To me, it's simply providing 2 electrically identical parallel paths for current for either single or 2P loads. Or basically a combiner box in the opposite direction if you think towards the PV source.

 

[wwhitney]

If the current divides equally on the two parallel paths, obviously it will work fine. But the currents will divide in proportion with the respective inverse impedances of the two paths, and so you'd have to ensure those are equal. And if the current division on the two different legs don't match, you'd get EMI and possibly inductive heating if the two separate feeders pass through separate openings in ferrous enclosures.

The installation would also violate the spirit of 310.10(G) (IIRC) on parallel conductors. Not quite the letter, as you are using OCPD to do the paralleling, but presumably that's why 240.8 exists.

Cheers, Wayne

 

[hitehm]

If the current divides equally on the two parallel paths, obviously it will work fine. But the currents will divide in proportion with the respective inverse impedances of the two paths, and so you'd have to ensure those are equal. And if the current division on the two different legs don't match, you'd get EMI and possibly inductive heating if the two separate feeders pass through separate openings in ferrous enclosures.

The installation would also violate the spirit of 310.10(G) (IIRC) on parallel conductors. Not quite the letter, as you are using OCPD to do the paralleling, but presumably that's why 240.8 exists.

Cheers, Wayne

Agreed, I did think of the inductive heat issue but wasn't that concerned with 300.3B and 300.20 b/c they would be grouped in the same raceway in a very short 8" run between the subpanel and MSP. I also agree that in retrospect, it does kind of get around 310.H (2017).

Thanks Again!

 

[jaggedben]

I don't understand why you'd have a 50A restriction unless it's because you're trying to use a quad breaker and there's minimal space left in the panel. If that's the case, then if you have space for an MLO subpanel between the combiner and main panel, you can move whichever circuit is taking up the space over the the subpanel to make space in the main, then put your 60A breaker to supply both the relocated circuit and the solar. I have no idea how using 2 combiners instead of one would help you have have a space issue, so maybe there's something I'm not understanding. Is there something else you're leaving out?

Another option with Enphase now is to use power control systems to limit the output to 40A on a 50A breaker. It's 'Main Panel Upgrade Avoidance' in installer toolkit under the PCS options. The customer will likely lose some energy production this way, but it's an alternative to actually downsizing the system.

 

[hitehm]

I don't understand why you'd have a 50A restriction unless it's because you're trying to use a quad breaker and there's minimal space left in the panel. If that's the case, then if you have space for an MLO subpanel between the combiner and main panel, you can move whichever circuit is taking up the space over the the subpanel to make space in the main, then put your 60A breaker to supply both the relocated circuit and the solar. I have no idea how using 2 combiners instead of one would help you have have a space issue, so maybe there's something I'm not understanding. Is there something else you're leaving out?

Another option with Enphase now is to use power control systems to limit the output to 40A on a 50A breaker. It's 'Main Panel Upgrade Avoidance' in installer toolkit under the PCS options. The customer will likely lose some energy production this way, but it's an alternative to actually downsizing the system.

Hey Jag, it's this particular Square D panel we've dealt with before and the inspector failed us when we used a 60A. The SE cables come in from the bottom so to stay within 120% rule we need to land at the top where the tandems normally go and there are 2 different labels that you can find in the panel. The old label specifically says those slots are limited to 50A for 2pole breakers and doesn't designate a type, the newer label has a table that says HOM type up to 100, HOMT (for tandem) up to 50A. Obviously we're not using tandem so we would be good but ours has the label that restricts the slots regardless.

 

[ggunn]

... to stay within 120% rule we need to land at the top where the tandems normally go...

Bottom feed panel?

 

[jaggedben]

Hey Jag, it's this particular Square D panel we've dealt with before and the inspector failed us when we used a 60A. The SE cables come in from the bottom so to stay within 120% rule we need to land at the top where the tandems normally go and there are 2 different labels that you can find in the panel. The old label specifically says those slots are limited to 50A for 2pole breakers and doesn't designate a type, the newer label has a table that says HOM type up to 100, HOMT (for tandem) up to 50A. Obviously we're not using tandem so we would be good but ours has the label that restricts the slots regardless.

Interesting. Want to post a pic of panel and/or label? I recall a Homeline meter main with a 50A separate service disconnect, and a bottom fed MLO meter main. Not sure if I've seen this one.

 

[hitehm]

Interesting. Want to post a pic of panel and/or label? I recall a Homeline meter main with a 50A separate service disconnect, and a bottom fed MLO meter main. Not sure if I've seen this one.

Yes, bottom feed Homeline panel and I will post a pic soon.

 

[hitehm]

Yes, bottom feed Homeline panel and I will post a pic soon.

Sorry, but the best pic we have as the full label that's not torn is the Spanish side, but I'm sure you can make it out. I also have the English label but it's torn. This is the old label we have. Notice the area at top with the tandems and notice the bottom says 50A max for ANY breakers in that location.

 

[hitehm]

Here is another label indicating the same thing

 

[hitehm]

Part # is SU3040M200F

 

[jaggedben]

I see the issue. It's a shame when manufacturers make mistakes on their labels.

 

[Zee]

couple solutions:
1. skip the Enphase Combiner Box (CB) altogether and land your micro-inverter branch circuits all in the MSP on 3@20A brkrs or 1@20A + 1@40A.
(In the latter case, combine 2 @ 20A circuits in a cheap, small sub panel first).
Then install an Envoy separately. You dont NEED an ENPHASE CB. It's just convenient.
Variation: use an Enphase CB to combine only 2 @ 20A circuits instead of a standard sub panel.

2. Are you sure the 3 @ 20A circuits are each maxed out. 16A of micro amps each?
Otherwise, if you only have 40A of total micro amps, which at 125% is 50A ...... you can combine all 3 circuits to a 50A brkr.
It is the actual inverter amps that matter in the final PV breaker sizing - not the sum of brkrs.

3. Variation of above:
choose the next smaller size of micro inverter to hopefully reduce each branch circuit current to 13.3 A max.
(3 @ 13.3A = 40A) ---> 40A @ 125% = 50A

 

[Zee]

Side Q: Does your utility actually require separate AC disconnects still? Code allows for the PV brkr to serve as ACD.

 

[electrofelon]

Just curious, how are you meeting the 120% rule? Is the main breaker downsized?