Solar PV 120% 690.64(B)(2)

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davedottcom

davedottcom

Senior Member
I understand the sum of all breakers feeding the busbar can't exceed 120% of the busbar's rating in a single phase situation, but shouldn't it be different in a 3-phase situation? If you have (3) 2-pole breakers back feeding a 3-phase panel. Let's say (3) 2-pole 40's on a 200 amp 3-phase panel w/ 150 amp main breaker.

200 amp x 120% = 240 Amps
150+40+40+40= 270 Amps (Too much)

But since the 2-pole breakers only land on each phase twice shouldn't it be:

150+40+40= 230 Amps

:confused:
 
T

ty

Senior Member
Location
3rd Rock from the Sun
I'm going to assume you are on the 2008.
In 2005, there was no exception for commercial to have the 120% value.


For starters, where did you get the (3) 2-pole 40's rating?
Is it actual, or were they the next size up? (is this a hypothetical question?)

You already lowered the Main to 150A. So, it would be 270A.


In a real world scenario, you could possibly take the 3 circuits into a small panel and then feed the Main panel with one larger circuit that would be based on the Actual total inverter output.
 
davedottcom

davedottcom

Senior Member
Ty,

Let's say 2008 NEC.

This is not the actual situation just an easier version!

I don't follow your 270 answer. The busbar is still rated at 200 amps even though the main breaker is reduced to a 150.

So the max on the the busbar is always 200 x 120% = 240

240-150(Main) = 90 amps Maximum allowed to be back fed

My question was; why do you add all the 2-pole PV breakers when they are in a 3-phase panel? They will land as follows A+B C+A B+C That is only 80 amps per phase, not 120 amps per phase.
 
T

ty

Senior Member
Location
3rd Rock from the Sun
yes it would under 2008 NEC.

There are other things to concider.
By reducing the Main down from 200 to 150, does the 150A violate the original panel load calculation?

This leads certain scenarios to be better off with a Supply-Side Connection.
 
davedottcom

davedottcom

Senior Member
ty said:
yes it would under 2008 NEC.

There are other things to concider.
By reducing the Main down from 200 to 150, does the 150A violate the original panel load calculation?

This leads certain scenarios to be better off with a Supply-Side Connection.
Click to expand...


That's not an issue at all, it will be a brand new panel specifically for the 3 PV inverters.

I'm just trying to understand why the "Sum of all breakers" formula is used for single phase and 3-phase. I don't see any exceptions saying there is a different formula when the breakers are in 3-phase panels so I guess it is ALWAYS the "the sum of all breakers" period.

It just doesn't make sense to me.
 
davedottcom

davedottcom

Senior Member
I JUST FIGURED IT OUT! :grin:

The formula is for only 1 busbar at a time.

Therefore, no need to specify single phase or 3-phase.

:roll:
 
George Stolz

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Dave, you've got three separate inverters serving the same panel?

I've only done one of these, but in my case, the inverters were combined into one feeder before tying into the panel.

Do you have a one-line for us to look at? :)
 
davedottcom

davedottcom

Senior Member
"Looks good" meaning it's correct, or just nice to look at!?

Our brothers should get together... maybe 2 wrongs would make a right!
 
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davedottcom

davedottcom

Senior Member
I don't have any cause this is just an example.

Assume the 40 amp breakers are correctly sized according to the Inv. specs.

so now......Thumbs Up!? :)
 
davedottcom

davedottcom

Senior Member
George,
Do you see my confusion?

690.64(B)(2) says the sum of the breakers... which sounds like you add up 40+40+40+150. Which would be correct in single phase but not in 3-phase.

I just think the NEC wording is lacking some clarity... imagine that!:roll:
 
S

SeanD

Member
davedottcom said:
It will be a brand new panel specifically for the 3 PV inverters.
Click to expand...

Does this mean the panel will be just for the solar system?

Just for the purpose of starting an argument. If this 200 amp subpanel is for solar only. Would this really be considered a "Connection to other source" (Title of NEC section VII)? Or if the panel is marked "Solar Only" would this be considered part of the solar system?

If this is considered part of the solar system, 690.64 wouldn't apply. You could then size the sub-panel to the appropriate size for combining the AC outputs of the inverters (100 amp panel w/ 100 amp main, I think. I don't work with 3-phase). You would then apply 690.64 to your interconnection at the main.
 
davedottcom

davedottcom

Senior Member
SeanD said:
Does this mean the panel will be just for the solar system?

Just for the purpose of starting an argument. If this 200 amp subpanel is for solar only. Would this really be considered a "Connection to other source" (Title of NEC section VII)? Or if the panel is marked "Solar Only" would this be considered part of the solar system?

If this is considered part of the solar system, 690.64 wouldn't apply. You could then size the sub-panel to the appropriate size for combining the AC outputs of the inverters (100 amp panel w/ 100 amp main, I think. I don't work with 3-phase). You would then apply 690.64 to your interconnection at the main.
Click to expand...

SeanD,

It's hard not to start an argument around here!

Excellent point, 690.64(B) goes into describing which panels fall under this rule...

"...where this distribution equipment is capable of supplying multiple branch circuits or feeders or both the interconnecting provisionsfor the utility-interactive inverter(s) shall comply with (B)(1) through (B)(7)."

So... what makes a panel "capable of supplying circuits"? Available spaces?

I don't see any thing that mentions that labeling the panel "Solar Equipment Only" is even acceptable or by doing so would make the panel Not capable of supplying other circuits.

:confused:
 
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