162.1 amps of solar output in combiner panel

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Scheffler Energy

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Hello,

I am designing a solar system where I will have 3 inverters 11.4 12.5 15 kw the total maximum output current will be 162.1 amp i have allways heard the general rule of thumb is that you don’t want to load a breaker more than 80 percent but its this actually back up by a NEC code ? I’m gonna make sure I have a 225 amp bus on the 200 amp main pv combiner panel to compile with 705.13 B 3 I can always incorporate 705.13 worst case senerio just looking for some extra advice because I’m always trying to improve my electrical wisdom ! Thanks!
 
Scheffler Energy said:
I am designing a solar system where I will have 3 inverters 11.4 12.5 15 kw the total maximum output current will be 162.1 amp
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I infer your voltage is 240V single phase.
Scheffler Energy said:
i have allways heard the general rule of thumb is that you don’t want to load a breaker more than 80 percent but its this actually back up by a NEC code ? I’m gonna make sure I have a 225 amp bus on the 200 amp main pv combiner panel to compile with 705.13 B 3
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Any breaker upstream of this combiner panel will need to be 225A, as it is required to be at least 125% of the inverter output current, and you are just over 160A. [Unless you use a 200A 100% rated breaker, which I believe is possible, but would be quite unusual and expensive.] If you need to keep your breakers at 200A or under, can you slightly downsize one inverter to get the total inverter output current under 160A?

Cheers, Wayne
 
Hello Wayne ! I already applied continuous load to the branch circuits 60 70 80 I never heard of having to do the the the combiner panels main before. I would be-able to derated one of the inverters in the settings.
 
Scheffler Energy said:
Hello,

I am designing a solar system where I will have 3 inverters 11.4 12.5 15 kw the total maximum output current will be 162.1 amp i have allways heard the general rule of thumb is that you don’t want to load a breaker more than 80 percent but its this actually back up by a NEC code ? I’m gonna make sure I have a 225 amp bus on the 200 amp main pv combiner panel to compile with 705.13 B 3 I can always incorporate 705.13 worst case senerio just looking for some extra advice because I’m always trying to improve my electrical wisdom ! Thanks!
Click to expand...
Is your interconnection to be a backfed breaker? If so, you'll need to comply with 705.12(B) (most likely subsection(2)) in your main panel. 120% of the bus rating must be equal or greater than the rating of the main breaker plus 125% of the total maximum inverter current.
 
Scheffler Energy said:
Hello Wayne ! I already applied continuous load to the branch circuits 60 70 80 I never heard of having to do the the the combiner panels main before. I would be-able to derated one of the inverters in the settings.
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If you use 705.12(B)(3) in the combiner panel, ignore the rating of the OCPD protecting the bus in the combiner. The combiner can be MLO as long as it is protected by either a breaker in the MDP or a fused disco if it is a line side interconnection, which it always is.

I think Wayne was assuming that you would use 705.12(B)(3) for the combiner, which is a good assumption for most PV systems with multiple inverters. The busbar rating must be equal to or greater than the sum of the breaker ratings.
 
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I don’t believe that rule apply because is a panel with only solar loads where you can add up all the loads as in the breakers and if your not over the bus rating your good. 705.12B3???
 
Scheffler Energy said:
Hello Wayne ! I already applied continuous load to the branch circuits 60 70 80 I never heard of having to do the the the combiner panels main before. I would be-able to derated one of the inverters in the settings.
Click to expand...
Any circuit breaker between the inverters and the service conductors that carries inverter output current needs to be sized at least 125% of the inverter output currents through that breaker, unless it is 100% rated. 705.30(B).

Cheers, Wayne
 
ggunn said:
If you use 705.12(B)(3) in the combiner panel, ignore the rating of the OCPD protecting the bus in the combiner. The combiner can be MLO as long as it is protected by either a breaker in the MDP or a fused disco if it is a line side interconnection, which it always is.

I think Wayne was assuming that you would use 705.12(B)(3) for the combiner, which is a good assumption for most PV systems with multiple inverters. The busbar rating must be equal to or greater than the sum of the breaker ratings.
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BTW, using a 100% rated breaker wouldn't help you in the MDP if it is a backfed breaker interconnection.
 
Scheffler Energy said:
I don’t believe that rule apply because is a panel with only solar loads where you can add up all the loads as in the breakers and if your not over the bus rating your good. 705.12B3???
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I'm not sure who you are replying to or which rule you are asking about.
 
ggun thank you for your input I really appreciate it !I have attached a one line I have created if you could check it out that would be amazing ! Also learning how to size and design the strings was complicated ! I think I got it right !
 

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Your diagram violates 705.30(B). You will need to change the inverter configuration so the sum of the inverter currents through your 200A OCPD is 160A or less.

No opinion on the DC side of things, I haven't studied that much.

Cheers, Wayne
 
You need to connect the PV system output to the line side of the production meter, not the load side. The disconnect, on the other hand, should be wired with the utility conductors on the line side and the PV system conductors on the load side.

Are you trying to keep your string voltage under 600V. Looks that way. If so, you have not accounted for the increase in voltage due to low temperatures. You need to calculate your highest voltage by finding the low temperature in you area and applying the TCVoc. Here is a resource for finding the low temperature. https://energyresearch.ucf.edu/solar-certification/solar-reference-map/

Example: If your TCVoc is -.275%/degC (it is, I looked), and your extreme minimum temperature is -32 C (it is for Benton, MN), then ((49.9V * .00275 * (32+25)) + 49.9) * 12 = 692.7V.

If you are needing to limit your string voltages to 600V, you might be limited to 10 modules in a string if your site is near Benton, MN.

The utilities I work with won't allow a line-side connection with the metering equipment. We ususally do that with Burndy Clear Taps in the main panel.

As others have already said, you need to reduce your total inverter output current slightly to fit on 200A equipment. PV output circuits are defined as continuous current in the NEC, and need the 1.25 factor applied. You might consider using a 15 kW inverter and two 11.4 kW inverters. that would do it. Yes, the combiner has a 225A bus, but the fuses are only 200A rated, so you can only put 160A of continuous current through them.

This looks like a line-side connected system, and I don't believe 705.13 shouldn't come into play. I am not an expert on PCS systems, but I am excited to see them coming on the market.
 
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wwhitney said:
Are the Fronius Primo Inverters able to intercommunicate to limit the total inverter current to 160A, and are so listed as a PCS? If so, sure; if not, then no, 705.13 doesn't help you.

Cheers, Wayne
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They don't really need to intercommunicate, you'd just need one of them to monitor total current and throttle back if necessary. But I don't know if Fronius supports this, can't say I've heard of anyone that does.
 
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