120% rule on stand alone systems

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Meko

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Location
Fresno, Ca
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PM
We have a stand alone system we are installing, the Solar is tied into the gear direct to the utility pole along with CT cab and meter. With this being a stand alone system are we required to implement the 120% rule since there will be no more loads added to the gear/panel.
 

Carultch

Senior Member
Location
Massachusetts
Posting a diagram would help us understand this better.

The 120% rule is one of your options for busbar interconnections. You have other options that you can follow as well, such as sum of branch breakers excluding main supply, not to exceed busbar ampacity. The rules in 705.12 for busbar interconnections are separate options, and you don't necessarily need to comply with any one of them in particular.

The rationale behind these rules, is that panelboards are routinely populated with load breakers in excess of the busbar rating. When fed from just the utility, the main breaker will trip before the busbar overloads or the heating among the breakers becomes detrimental. If fed from multiple sources such that the main breaker isn't the only path for current to enter the busbar, you could cause the breakers to draw current in excess of the busbar rating. The 705.12 rules are there to mitigate this possibility. The 120% rule takes advantage of the fact that current feeding the busbar from opposite sides, diminishes to zero rather than accumulates. In theory it could be a 200% rule by Kirchhoff's laws alone. But because of heating among all the branch breakers is a possibility, the NEC settled on 120% as the limit.
 

Meko

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Location
Fresno, Ca
Occupation
PM
Thank you all for the feedback.
My mistake on the stand alone This is actually a panel with direct tie in into the utility power pole not the customers main panel. So the only loads on our panel are for PV only with no branch circuits or (E) circuits tied to this. 120%.png
 

Carultch

Senior Member
Location
Massachusetts
Now it makes more sense about what you mean by "stand-alone". This term has multiple meanings in the industry, since it means a different thing to the utility, than it does to equipment manufacturers.

You mean it is a stand-alone service from the utility for the PV, in contrast from a behind-the-meter system on a service for both site loads and PV.
As opposed to "stand-alone" as in "island system", where the PV+battery+site loads are off-grid entirely, or have the potential to be off-grid.
 

Meko

Member
Location
Fresno, Ca
Occupation
PM
carultch I appreciate the response , Would you know if the 120% rule applies if this is not connected to other panels/loads?
 

Carultch

Senior Member
Location
Massachusetts
You could apply the 120% rule, but you don't necessarily need to do so. The rule regarding the sum of the breakers excluding the main supply, to size the busbar, is a much more realistic rule to apply for busbar sizing in this situation.

I think it is overkill to require 705.12 rules to apply to busbars in panelboards dedicated to the PV system, as the NEC wording also requires you to accumulate rounding errors (e.g. 17x 18A inverters on 25A breakers), and also account for a 15A breaker for insignificant loads like power monitoring systems. If there are no significant loads, and it is dedicated to the PV system, I see no issue with sizing the panelboard to 125% of the total inverter current, as you would size its main OCPD. The NEC doesn't have a rule that agrees with this, so it would be up to an AHJ to agree with the reasoning.

The intent of the breaker summing rule is for mixed use panelboards, where some are loads and some are sources. They could all be loads, or all be sources, and no matter how the current adds up, it's not going to add up to more than the sum of the branch breakers connected to any given busbar. It makes sense to apply 705.12 rules to busbars that contain both loads and sources, to cover the physical consequences of bidirectional current in a panelboard.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
In the diagram posted, isn't the 400A panelboard just after the meter completely redundant? I.e. couldn't the 400A AC disconnect with 300A fuses be the service disconnect?

As for (2017) 705.12, if everything after the meter is dedicated to the PV, couldn't we just say this is a 705.12(A) supply side connection? : - )

Cheers, Wayne
 

Meko

Member
Location
Fresno, Ca
Occupation
PM
$00A panel board is located on the structure away from the POI. we are required to have disconnecting means at the car port in a combiner panel with Brach circuits to protect the Inverters. .
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
$00A panel board is located on the structure away from the POI. we are required to have disconnecting means at the car port in a combiner panel with Brach circuits to protect the Inverters. .
So the fused disconnect is next to the load center on the carport? And someone is requiring a bladed disconnect, rather than just the main breaker on the combiner panel next to it?

Cheers, Wayne
 

Meko

Member
Location
Fresno, Ca
Occupation
PM
So the fused disconnect is next to the load center on the carport? And someone is requiring a bladed disconnect, rather than just the main breaker on the combiner panel next to it?

Cheers, Wayne
POI is not located on the structure unfortunately and local fire requires a visible disconnect with appropriate labels to identify ON-OFF switch location, Basically make it dummy proof.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Sorry, I'm not following, because when you refer to POI, I'm not clear on which physical location you mean. I understand the PV is on the carport, as is the combiner panel, and that the meter is not on the carport. And that you apparently wanted OCPD at the meter, rather than run service conductors to the carport.

So if the fire department's disconnect is on the carport, it makes sense: you have OCPD at the meter by choice, you're required to have the fire department disconnect, and you chose to use a main breaker combiner panel instead of an MLO panel. But if the disconnect is next to the meter, the panel between the disconnect and the meter is completely redundant, I would think.

I don't mean to make a big deal out of this, it just seems weird to have three OCPD in a row that are all doing the same thing (with the possible exception of the first one after the meter, which is shown as 400A, but that one could be 300A, also.)

Cheers, Wayne
 

Meko

Member
Location
Fresno, Ca
Occupation
PM
No worries I can always improve and love the feed back, Helps me out tremendously. I will run this by my engineer and see what he thinks.
 
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