NEC 2017,705.12(B)(2)(3)(C)

Designer101

Member
Location
California
Occupation
Solar Designer
How the NEC 2017,705.12(B)(2)(3)(C) doesnot violate the NEC 2017,705.12(B)(2)(3)(b).
The Situation Looks like:
6 Tesla battery connected to 400 ampere load center. 6 Tesla battery has each 30 ampere breaker.. at the end the 400 ampere load center is fed by 200 A breaker in the center fed panel.. According to 705.12(B)(2)(3)(C) is this code complaint?
The Center fed panel has only the 200 ampere breaker for that 400 ampere Load center. each Tesla requires 30A OF OCPD
if we follow 2017,705.12(B)(2)(3)(b)
6*30=180A Backfeed, is connected to 400A Load center ,
but the main panel only has 1.2*200-200=40A Backfeed ( which is straight no)
can somebody explain me how it works
 

Carultch

Senior Member
Location
Massachusetts
You don't need to satisfy all of the subsections of 705.12(B)(2)(3) at once. You get to pick the subsection of it, that works in your situation. This rule mentions "one of the methods that follows", which confirms that once one of the methods works for you, the other rules need not apply to the same situation.

Subsection (a) is a "100% rule". Add up the main supply breaker and 125% of the inverter current. Busbar must meet or exceed this, and the breakers can be anywhere on the busbar. Prior to 2014, it specified the interconnecting breaker. As of 2014 and onward, it uses a bit of round-about language to tell you that you don't need to look at the actual breaker you are using for interconnecting the system, just the calculation that determines the minimum value of it. I.e. 125% of the power source(s) output current.

Subsection (b) is the famous 120% rule, that requires the interconnection on the opposite end of the main supply. Again, same change happened in 2014, as I mentioned earlier. This is the rule commonly applies to on load-side interconnections, because most existing panels you'll find in practice, already have their main supply matched to the busbar, which would otherwise give you no headroom at all for an interconnection. This rule that allows you to take partial credit for the fact that current flowing in opposite directions subtracts, rather than adds. The reason you don't get to take full credit and have this be a 200% rule, has to do with the concern of the total heat accumulated among the branch breakers, exceeding what the enclosure is listed to withstand.

Subsection (c) is the option that allows you to add up the breakers regardless of application, and exclude the main supply. You apply this rule on a phase-by-phase basis, if the panelboard has breakers in it that don't use all phases. Whether it is a load or a source, the branch breaker counts. The only one that doesn't count, is the main supply breaker.

Subsection (d) gives us a solution on centerfed panelboards. It says that you can apply the 120% rule to either side of a centerfed panelboard, but not both. It also limits the scope to dwellings, so it doesn't apply to commercial/industrial applications. For anything beyond the scope of dwellings, you'd have to use subsection (e).

Subsection (e) is the engineering supervision section, that with a licensed engineer, you can examine more complicated situations not covered by the above rules.
 
Last edited:

jaggedben

Senior Member
How the NEC 2017,705.12(B)(2)(3)(C) doesnot violate the NEC 2017,705.12(B)(2)(3)(b).
2017,705.12(B)(2)(3)(C) and 2017,705.12(B)(2)(3)(b) are separate ways to qualify a panel for backfeed. The code states that the panel must comply with one of the rules in 2017,705.12(B)(2)(3), not all of the rules. So you can use one or the other to qualify one panel for backfeed. One panel on a site can be qualified under one rule while another could be qualified under another rule.

The Situation Looks like:
6 Tesla battery connected to 400 ampere load center. 6 Tesla battery has each 30 ampere breaker.. at the end the 400 ampere load center is fed by 200 A breaker in the center fed panel.. According to 705.12(B)(2)(3)(C) is this code complaint?
Yes. In fact it could be a 200A panel. It is compliant according to any of the rules.

The Center fed panel has only the 200 ampere breaker for that 400 ampere Load center. each Tesla requires 30A OF OCPD
if we follow 2017,705.12(B)(2)(3)(b)
6*30=180A Backfeed, is connected to 400A Load center ,
but the main panel only has 1.2*200-200=40A Backfeed ( which is straight no)
can somebody explain me how it works
You are evaluating it according to 2017,705.12(B)(2)(3)(b), and you are correct it does not qualify under that rule.

It might qualify under 2017,705.12(B)(2)(3)(c), but only if the 200A breaker you mentioned is the only breaker left in that panel, and the label or placard required by that code section is also applied.

I was typing at the same time as Carultch, hope both responses help.
 

Designer101

Member
Location
California
Occupation
Solar Designer
Thanks for the reply,
Now My question is can we add more loads to that 400 A sub panel.
like 60 Ampere PV circuits.
Will it be still code compliant as the current now will be 30*6+60=240 A and the breaker in Main panel is limited to 200 A Only.
 

jaggedben

Senior Member
Thanks for the reply,
Now My question is can we add more loads to that 400 A sub panel.
like 60 Ampere PV circuits.
Will it be still code compliant as the current now will be 30*6+60=240 A and the breaker in Main panel is limited to 200 A Only.
A PV circuit is not a load, and it makes a bit of difference.

Under 2017,705.12(B)(2)(3)(C) one could at 220A of load breakers to the 180A of battery inverter breakers, and that panel would still be compliant. 705.12(B)(2)(3)(C) also does not care if you add 220A of PV to the 400A panel, so long as the breakers for batteries, PV, and loads add up to no more than 400A.

However, 240A of battery and PV inverters would require a 240A output to the service, so the main panel could no longer be compliant if rated 200A. So that would probably be the problem here.

(There is something called Power Control Systems which could allow more than 200A of PV and batteries because it would prevent all of them from fully outputting at the same time. However it is a new standard and to my knowledge Tesla is not yet listed for it, and it is also not referenced in the 2017 NEC. So I doubt it's relevant for you at the present time, though it might be in the future.)
 

Designer101

Member
Location
California
Occupation
Solar Designer
Now I am confused a little bit,
Originally the there was just 60 Ampere breaker for Solaredge 11.4 kw, then they added 3 Tesla batteries so,
60 ampere Of Source and 3*30=90 A batteries, total of 150 A, Which was less than 200A breaker .

Now if we add 3 more Tesla batteries, the Loads will be 6*30=180A, but the source current is still the same 60A only,
Cant we still use same 2000/200 Main Service panel without upgrade?
Do you have link to some reading that I can read and make my knowledge more wider?
Thank you for the reply
 

Designer101

Member
Location
California
Occupation
Solar Designer
You don't need to satisfy all of the subsections of 705.12(B)(2)(3) at once. You get to pick the subsection of it, that works in your situation. This rule mentions "one of the methods that follows", which confirms that once one of the methods works for you, the other rules need not apply to the same situation.

Subsection (a) is a "100% rule". Add up the main supply breaker and 125% of the inverter current. Busbar must meet or exceed this, and the breakers can be anywhere on the busbar. Prior to 2014, it specified the interconnecting breaker. As of 2014 and onward, it uses a bit of round-about language to tell you that you don't need to look at the actual breaker you are using for interconnecting the system, just the calculation that determines the minimum value of it. I.e. 125% of the power source(s) output current.

Subsection (b) is the famous 120% rule, that requires the interconnection on the opposite end of the main supply. Again, same change happened in 2014, as I mentioned earlier. This is the rule commonly applies to on load-side interconnections, because most existing panels you'll find in practice, already have their main supply matched to the busbar, which would otherwise give you no headroom at all for an interconnection. This rule that allows you to take partial credit for the fact that current flowing in opposite directions subtracts, rather than adds. The reason you don't get to take full credit and have this be a 200% rule, has to do with the concern of the total heat accumulated among the branch breakers, exceeding what the enclosure is listed to withstand.

Subsection (c) is the option that allows you to add up the breakers regardless of application, and exclude the main supply. You apply this rule on a phase-by-phase basis, if the panelboard has breakers in it that don't use all phases. Whether it is a load or a source, the branch breaker counts. The only one that doesn't count, is the main supply breaker.

Subsection (d) gives us a solution on centerfed panelboards. It says that you can apply the 120% rule to either side of a centerfed panelboard, but not both. It also limits the scope to dwellings, so it doesn't apply to commercial/industrial applications. For anything beyond the scope of dwellings, you'd have to use subsection (e).

Subsection (e) is the engineering supervision section, that with a licensed engineer, you can examine more complicated situations not covered by the above rules.
 
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