Backfeeding 170KwDC into a single phase system

[SolarNinja]

Hello,


I'm currently dealing with the local utility to allow me to tap into their overhead pole mount transformer. This is a property
with 3 parcels and 3 existing meters, of which only 1 can be fed into 18kW roof mount, the rest of my solar 152kW groundmount will need to go into separate meters (aggregate metering with SDG&E). 3phase power is out of the question, there's no infrastructure from utility for 3phase, it's on indian reservation only single phase.

Here's what I'm thinking for the 152kw groundmount

(9) 15kW fronius single phase inverters with Max. OCDP 80amps - That's 720amps of backfeed OCPD

I'm currently working with a service planner that tells me I can add 1 new single 600 amp overhead service.

My question is, and please share your thoughts;

Can I install a main lug 600 amp switch gear and backfeed (9) 80 amp breakers with no main breaker?

Because the sum of the OCPDevices would be 9*80 = 720 amps
600 amp switchgear buss * 1.2 = 720 amps.

So that in my mind is perfect, but there's no main, only a 600 amp service disconnect switch. Will this fly with AHJ?

Thank you in advance.

 

[Carultch]

Hello,


I'm currently dealing with the local utility to allow me to tap into their overhead pole mount transformer. This is a property
with 3 parcels and 3 existing meters, of which only 1 can be fed into 18kW roof mount, the rest of my solar 152kW groundmount will need to go into separate meters (aggregate metering with SDG&E). 3phase power is out of the question, there's no infrastructure from utility for 3phase, it's on indian reservation only single phase.

Here's what I'm thinking for the 152kw groundmount

(9) 15kW fronius single phase inverters with Max. OCDP 80amps - That's 720amps of backfeed OCPD

I'm currently working with a service planner that tells me I can add 1 new single 600 amp overhead service.

My question is, and please share your thoughts;

Can I install a main lug 600 amp switch gear and backfeed (9) 80 amp breakers with no main breaker?

Because the sum of the OCPDevices would be 9*80 = 720 amps
600 amp switchgear buss * 1.2 = 720 amps.

So that in my mind is perfect, but there's no main, only a 600 amp service disconnect switch. Will this fly with AHJ?

Thank you in advance.

If there is a fusible disconnect or other master OCPD upstream of this panelboard, than that counts as the main breaker for protecting the busbar of the panelboard. Should it be a mixed use panelboard, i.e. loads + solar, the "120% rule" would still apply with the upstream main OCPD counting in place of the main breaker.

Another way you can justify protection of a panelboard, is if the sum of the breakers, excluding the main supply, do not exceed the busbar ampacity. 2014 rule.


If there isn't an upstream master OCPD on the utility feed, then each branch breaker in the panelboard is a service disconnect. And you'd be limited to the standard 6, unless another can be justified by an exception.

 

[SolarNinja]

If there is a fusible disconnect or other master OCPD upstream of this panelboard, than that counts as the main breaker for protecting the busbar of the panelboard. Should it be a mixed use panelboard, i.e. loads + solar, the "120% rule" would still apply with the upstream main OCPD counting in place of the main breaker.

I think i get what you're saying, this will not be a mixed use panel and is reserved for feed in only. Is there such things as a built in fused switchgear? Will that fuse count as part of the 120% rule, therefore limiting me to less Solar?

I think I like the idea of each inverter OCPD acting as a service disconnect, but I have (9) inverters. So if 6 is my limit, how would I be able to backfeed a potential of 720 amps? A line or load side tap? Or maybe sub panels? Thanks

 

[jaggedben]

I'm not an AHJ, but if I were one, I would say you can't backfeed 720A of power on a 600A service. Period. Doesn't matter one iota how you hook them up. 705.12(A) may not explicitly state that the sum of sum of all interactive sources counts against what you can backfeed the service, but that's a loophole. Common sense and engineering sense says you can't overload the service conductors.

Best case scenario, I let you use 125% of the rated output instead of the OCPD. At 62.5*1.25*9 = 703A. Still not good enough.

I'm not your AHJ, but in my humble opinion, unless your AHJ is a fool, you either have to downsize the inverter power, or somehow cajole (or pay for) the utility to rate your service higher.

My professional opinion is that you should increase your DC to AC ratio. You're seemingly only at 1.12, which is not very high, so consult the Fronius spec sheet and see how much more DC current those bad boys can take.

 

[jaggedben]

Another thing...

Even if your AHJ was a fool, and let you apply the 120% rule load side while ignoring the implications of the limited service rating...

Your switch gear would have to be rated 1100A or more.

 

[electrofelon]

this is an interesting pickle. If you were not subject to the 6 disconnect rule, then you MAY be ok as 62.5*9=563 and the panelboard bus is not subject to the 125% continuous load adder (unless you consider the busing service entrance conductors which an argument can be made for). Your service entrance conductors would still need 125% of that so then you would be at the mercy of how the poco defines the service size with no single main - but anyway your 9 disconnects are still a deal breaker unless they will let you have two seperate groups of 6 or less, you could maybe try that out

A few other options come to mind. One is to use a 100% rated main breaker. Id have to cogitate on that a little more as to whether that would get you where you need to be. The other is to go 277V to lower your current - I think there are some single phase inverters that do that. The poco may no want to give you a "non standard" 277V single phase service however.

 

[SolarNinja]

I'm not an AHJ, but if I were one, I would say you can't backfeed 720A of power on a 600A service. Period. Doesn't matter one iota how you hook them up. 705.12(A) may not explicitly state that the sum of load side connections counts against what you can backfeed the service, but that's a loophole. Common sense and engineering sense says you can't overload the service conductors.

Best case scenario, I let you use 125% of the rated output instead of the OCPD. At 62.5*1.25*9 = 703A. Still not good enough.

I'm not your AHJ, but in my humble opinion, unless your AHJ is a fool, you either have to downsize the inverter power, or somehow cajole (or pay for) the utility to rate your service higher.

My professional opinion is that you should increase your DC to AC ratio. You're seemingly only at 1.12, which is not very high, so consult the Fronius spec sheet and see how much more DC current those bad boys can take.

Excellent point(s)

Looks like the other option from Utility is (2) 400 amp service drops. The option would then be to use (10) 12.5 kW inverters because the 15k inverters max output power is only 13.5kW and the 12.5kW inverter max output is 12.5kW. So maximizing the output on the 12.5kW seems like a better option. Each 12.5kw inverter can handle upto 15kWdc max, therefore my total PV system would be 150kw w/ (10) 12.5kW inverters.

If this is the case, then how to go about feeding 5*52.1*1.25= 326A max inverter output (idesign) into each 400 amp service? Assuming (5) 70 amp OCPD? Would each 70 amp OCPD act as it's own service disconnect as previously stated by @Carultch ?

Therefore, not needing an upstream main OCPD besides the 400 amp switch?

Thank you.

 

[SolarNinja]

this is an interesting pickle. If you were not subject to the 6 disconnect rule, then you MAY be ok as 62.5*9=563 and the panelboard bus is not subject to the 125% continuous load adder (unless you consider the busing service entrance conductors which an argument can be made for). Your service entrance conductors would still need 125% of that so then you would be at the mercy of how the poco defines the service size with no single main - but anyway your 9 disconnects are still a deal breaker unless they will let you have two seperate groups of 6 or less, you could maybe try that out

A few other options come to mind. One is to use a 100% rated main breaker. Id have to cogitate on that a little more as to whether that would get you where you need to be. The other is to go 277V to lower your current - I think there are some single phase inverters that do that. The poco may no want to give you a "non standard" 277V single phase service however.

Excellent point(s)

Looks like the other option from Utility is (2) 400 amp service drops. The option would then be to use (10) 12.5 kW inverters because the 15k inverters max output power is only 13.5kW and the 12.5kW inverter max output is 12.5kW. So maximizing the output on the 12.5kW seems like a better option. Each 12.5kw inverter can handle upto 15kWdc max, therefore my total PV system would be 150kw w/ (10) 12.5kW inverters.

If this is the case, then how to go about feeding 5*52.1*1.25= 326A max inverter output (idesign) into each 400 amp service? Assuming (5) 70 amp OCPD? Would each 70 amp OCPD act as it's own service disconnect as previously stated by @Carultch ?

Therefore, not needing an upstream main OCPD besides the 400 amp switch?

Thank you.

 

[electrofelon]

FYI I forgot about 705.12(A) in my previous reply. I was thinking for a supply side connection, you could use the inverter output current but it is the sum of the breakers So even though you may be within the rating of the panelboard for a supply side connection your breakers' sum will exceed the rating of a 600 amp service.

But I think you are all set if they will give you 2 sets of 400. Just get 2 400A MLO service rated panelboards each with 5 70's and you are all set.

 

[jaggedben]

Excellent point(s)

Looks like the other option from Utility is (2) 400 amp service drops. ...

If this is the case, then how to go about feeding 5*52.1*1.25= 326A max inverter output (idesign) into each 400 amp service? Assuming (5) 70 amp OCPD? Would each 70 amp OCPD act as it's own service disconnect as previously stated by @Carultch ?

Therefore, not needing an upstream main OCPD besides the 400 amp switch?

You don't need the 400A switch. 5 70A breakers on an MLO service panel, times two services, and if I were your AHJ, this would be approved. :thumbsup:

 

[SolarNinja]

Which option is more viable, safer and would receive less pushback from utility & ahj

Which option is more viable, safer and would receive less pushback from utility & ahj

Here's what the service planner is asking for :



I'm getting ready to order a MLO 400 amp service that meets utility requirements and will suffice AHJ requirements.

Here are a few options:

Option 1:



Option 2:

Option 3:


Which looks like the best option?

Again, thank you in advance!

 

[ggunn]

Here's what the service planner is asking for :

View attachment 14303

I'm getting ready to order a MLO 400 amp service that meets utility requirements and will suffice AHJ requirements.

Here are a few options:

Option 1:

View attachment 14304

Option 2:
View attachment 14305
Option 3:
View attachment 14306

Which looks like the best option?

Again, thank you in advance!

Option 2 would be my choice.

 

[SolarNinja]

Fuses & 120% rule

Fuses & 120% rule

Option 2 would be my choice.

That would be my option as well, given the convenience of a throw switch and "utility disconnecting means". But if fuses are applied to that switch, would't those play into the 120% Rule along with the rest of the OCPDs?

Here's my concern :

400(fuses)+70+70+70+70+70+50 = 800 amps which very much exceeds 120% of the bus bar rating.

After all, a fuse is an OCPD right?

 

[GoldDigger]

That would be my option as well, given the convenience of a throw switch and "utility disconnecting means". But if fuses are applied to that switch, would't those play into the 120% Rule along with the rest of the OCPDs?

Here's my concern :

400(fuses)+70+70+70+70+70+50 = 800 amps which very much exceeds 120% of the bus bar rating.

After all, a fuse is an OCPD right?

A fuse is OCPD, yes.
If the OP is under [2014] NEC then it is possible to apply a different rule, the 100% rule for loads, avoiding the 120% rule entirely. Otherwise a picky AHJ will essentially require an 800A bus panel OR a wired combination of standalone breakers and fuses with large wiring instead of a breaker pane with a bus.