NEC 2017 705.12(B)(2)(3)(c)

Twisremo

Member
Location
florida
Occupation
electrician
Hey everyone, I have a question regarding the title code and am looking for input

Lets say a home has a 200amp meter/main, with 200 amp bus, with no branch circuit breakers in it and feed thru lugs to a secondary 200amp main breaker panel. and in this sub panel are taps before the 200 amp that go to a 60 amp fused PV disconnect. Would this be considered code compliant, or would the sum of the 200 amp sub panel breaker(load) and the 60 amp PV fuses(supply) be taken into consideration and that it exceeds the 200 amp bus rating of the main?


Thanks
 

Joe.B

Senior Member
Location
Arcata Ca
Occupation
Building Inspector
Hopefully some of the experts will weigh in, but to help them help you maybe I can ask to clarify a few points here. 200amp meter/main has the service disconnect and this is where your grounded and grounding conductors (neutrals and ground) are tied, correct? Then from there it goes to another panel, also with a 200amp disconnect, and this panel is a sub panel with grounded and grounding conductors isolated, still correct? Somewhere between those two points is your solar "tap"? I'm not certain but I think this might be what is sometimes called a "Hawaiian Tie-in", again I hope the experts will weigh in because I haven't seen this setup and I would also like to know how to calculate the ratings for this. Thanks for sharing!
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Hey everyone, I have a question regarding the title code and am looking for input

Lets say a home has a 200amp meter/main, with 200 amp bus, with no branch circuit breakers in it and feed thru lugs to a secondary 200amp main breaker panel. and in this sub panel are taps before the 200 amp that go to a 60 amp fused PV disconnect. Would this be considered code compliant, or would the sum of the 200 amp sub panel breaker(load) and the 60 amp PV fuses(supply) be taken into consideration and that it exceeds the 200 amp bus rating of the main?


Thanks
You've tapped the feeder between the main and the sub, and the sub has a main breaker, correct? Is the 200A main feedthrough panel a Cutler-Hammer, by any chance? If it is, it has a 225A bus (we have it in writing from Eaton), so even if there are loads in the MDP you are OK with the 120% rule (1.2 X 225A - 200A = 70A). Otherwise, I know of AHJ's that will allow it as long as you have signage on the MDP that says "NO LOAD BREAKERS". The MDP can be just a service disconnect.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
As I read the description, the tap itself is write ahead of a main breaker in the sub. That main breaker protects the sub bus and there's no real length of conductor in between it and the tap (debateable if this even matters) so that part is all completely okay.

Now for the main panel. This arrangement meets the letter of the code for NEC 2017 705.12(B)(2)(3)(c). However the use of the feed through lugs is really a loophole, as you avoid counting the 260A of downstream breakers that would count if they were breakers in the panelboard. The 2020 NEC has a new section addressing feed through lugs. It's a little vague on the precise question, but it would see to require another 200A OCPD at or very near the feed-through lugs (i.e utlity side of the tap) to allow the use of the above-mentioned rule.

Another option, presuming the feed through lugs are at the opposite end of the main panel, would be to downsize the 200A main breaker in the meter main to 175A. (You didn't actually mention that breake's rating but I'm assuming it.)
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
In the context of the OP, what do people think of riveting a solid plate over all the breaker locations in the dead front other than the feed-thru lugs to justify treating the service panel as a service disconnect without distribution? Because that is how it's being used. And the 200A feeder and 60A feeder tap (can easily) comply with 705.12(B)(2)(1) and (2).

Of course, if the feed thru lugs are a 200A breaker, the installation clearly qualifies under the section quoted in the title of the thread. And using feed thru lugs instead of a breaker doesn't change anything electrically or safety wise; it just means that the feeder subject to 705.12(B)(2)(1) and (2) is partially a busbar (without other devices on it) and partially a wire type feeder. So this whole thread is all about the inefficiency of the language in 705.12(B)(2).

Cheers, Wayne
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
In the context of the OP, what do people think of riveting a solid plate over all the breaker locations in the dead front other than the feed-thru lugs to justify treating the service panel as a service disconnect without distribution? Because that is how it's being used. And the 200A feeder and 60A feeder tap (can easily) comply with 705.12(B)(2)(1) and (2).

Of course, if the feed thru lugs are a 200A breaker, the installation clearly qualifies under the section quoted in the title of the thread. And using feed thru lugs instead of a breaker doesn't change anything electrically or safety wise; it just means that the feeder subject to 705.12(B)(2)(1) and (2) is partially a busbar (without other devices on it) and partially a wire type feeder. So this whole thread is all about the inefficiency of the language in 705.12(B)(2).

Cheers, Wayne
I some products (particularly Eaton) I could imagine removing the panelboard component and landing the conductors directly on the load side main breaker lugs, too. Probably someone will raise 110.3(B) but in some cases there are products that are sold that way that are for all practical purposes identical to the result.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I some products (particularly Eaton) I could imagine removing the panelboard component and landing the conductors directly on the load side main breaker lugs, too. Probably someone will raise 110.3(B) but in some cases there are products that are sold that way that are for all practical purposes identical to the result.
110.3(B) would not be an issue if the main breaker that was part of the panelboard is one that can be configured with load side lugs per the manufacturer. Then you just have a main breaker mounted in a cabinet compartment.

So obviously doing that would get rid of the panelboard and turn the equipment into a meter/disconnect. But I would say that the less invasive option of leaving the busbars in there and just eliminating the ability to add breakers via the dead front modification I proposed would suffice. The equipment is then no longer "capable of supplying multiple branch circuits or feeders, or both." So the internal busbars are immaterial.

Cheers, Wayne
 

Carultch

Senior Member
Location
Massachusetts
Hey everyone, I have a question regarding the title code and am looking for input

Lets say a home has a 200amp meter/main, with 200 amp bus, with no branch circuit breakers in it and feed thru lugs to a secondary 200amp main breaker panel. and in this sub panel are taps before the 200 amp that go to a 60 amp fused PV disconnect. Would this be considered code compliant, or would the sum of the 200 amp sub panel breaker(load) and the 60 amp PV fuses(supply) be taken into consideration and that it exceeds the 200 amp bus rating of the main?


Thanks

What you are describing, is a feeder interconnection, rather than a busbar interconnection. I've sketched the sequence of equipment below. Currents A, B, and C are the currents that are in question. I've marked an arrow on each, indicating assignment of positive values to each one. From Kirchhoff's current law, you can see that Ib = Ia + Ic. You can also see that both Ia and Ib are limited to not exceed 200A, due to them both flowing through a 200A breaker on their side of the tap point. Ic is of course limited to 60A, and is based on inverter(s) with a current that adds up to a maximum of 48A.

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Because we have a 200A main on the panelboard, the panelboard cannot draw more than 200A without tripping this breaker. The panelboard bus is protected regardless of what mixture of Ia and Ic supply current to it. Protecting the busbar is "out of the picture" of your interconnection method. The section of conductor for current C would be treated as a 240.21(B) feeder tap. The section of conductor for currents A and B, would be the feeder, and would be able to remain sized according to the rules that govern its sizing, if the interconnection weren't there.

I'd consider the conductor carrying current C as a feeder tap, though there are people who will disagree with calling it a tap by strict definition. The same working principles of conductor protection that would apply to a feeder tap with a load, would also apply feeder tap with a source. It is a conductor that is protected in excess of its ampacity, and relies on a downstream OCPD to protect it from overload.

If instead of an MCB in the panelboard, you had an MLO (main lug only), then you'd need to introduce a new 200A OCPD in some form or another between the feeder tap point and the panelboard. When you have an MLO panel fed from an externalized OCPD like a main breaker in a meter socket, that extermal OCPD acts as if it were the main breaker of your panelboard, for purposes of satisfying 705 rules on busbar protection.

Had you connected at the opposite end of the bus, rather than at a feeder tap prior to the panelboard, then this would be a situation where the 120% rule would have to be applied, and you'd actually have to consider the 200A busbar rating. If you had desired to connect a 40A system instead of a 60A system, that is where I'd recommend making your connection instead. Given a 60A system, it is reasonable to connect on that feeder tap.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I've sketched the sequence of equipment below. [. . .] Protecting the busbar is "out of the picture" of your interconnection method.
It's not quite that simple--what you've drawn is a meter/disconnect as the service equipment, with no distribution capability. Everyone should agree that the feeder interconnection you've drawn is fine if done in compliance with 705.12(B)(2)(1) and (2), and that the meter/disconnect is not restricted by 705.12(B) because it is not "capable of supplying multiple branch circuits or feeders, or both".

However, a very common situation in California, at least, is that the service equipment is a meter/main/distribution with a panelboard inside. So now you empty the panelboard of all breakers except for a 200A breaker. The service panel has a busbar and must comply with 705.12(B), but it complies with 705.12(B)(2)(3)(c), no problem.

Now suppose that instead of a 200A breaker on the busbar, you use 200A feed-thru lugs, no OCPD. That is what the OP is about. The configuration is logically and electrically equivalent to what you drew, but the service equipment is clearly subject to 705.12(B) because it has the capability of accepting other breakers. So the question is how does the service panel comply with 705.12(B)?

Since 2017 705.12(B) is silent on the topic of feed-thru lugs, it certainly complies with a literal reading of 705.12(B)(2)(3)(c). However, that literal reading is also open to the potentially abusive allowance for up to 200A of additional breakers in the service panelboard(*), so I can see some AHJs being concerned about that. I haven't looked closely at the confusing-to-me 2020 language to see how it could be interpreted to apply to this configuration, but the OP specified the 2017 NEC.

Cheers, Wayne

(*) For that to actually overload the bus you'd have to interconnect a 3rd power source using one breaker, along with a load using another breaker.
 

Twisremo

Member
Location
florida
Occupation
electrician
Thanks everyone, I'm just reading it as 705.12(B)(2)(3) One of the methods shall be used. And this set up does not comply with any of the a thru e options. I see it as you have a 200 amp potential load and a 60 amp potential supply, exceeding the busbar rating of the main panel that this feeder is tapped onto. Am I wrong here?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Carultch explained well how the 200A service panel busbar can't be overloaded. And the setup complies with 2017 705.12(B)(2)(3)(c) as written: the sum of the breakers in the service panel other than the main breaker is 0, which is less than the busbar rating.

Cheers, Wayne
 

L. Price

New User
Location
Huntington, WV
Occupation
Electrical Supervisor
You've tapped the feeder between the main and the sub, and the sub has a main breaker, correct? Is the 200A main feedthrough panel a Cutler-Hammer, by any chance? If it is, it has a 225A bus (we have it in writing from Eaton), so even if there are loads in the MDP you are OK with the 120% rule (1.2 X 225A - 200A = 70A). Otherwise, I know of AHJ's that will allow it as long as you have signage on the MDP that says "NO LOAD BREAKERS". The MDP can be just a service disconnect.
Could you share that letter from Eaton? That is good information.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
That letter may only apply to CH panelboards. I mean, I haven't seen it. But BR meter mains come in different flavors that explicitly say that the panelboard busbar is rated 200 or 225, depending on the one you get.
 

pv_n00b

Senior Member
Location
CA, USA
I'll see if I can get it for you. It says that all their panels from 150A to 225A all use a 225A busbar.
This is a reasonable cost-saving measure, just use the same size busbar over a range of panels. You don't save enough in material to overcome the cost of manufacturing and stocking many different busbar ratings. That being known, a panelboard is only rated for what the label says. So depending on the AHJ they may care less about the components in the panelboard having a higher rating if the panelboard stated rating is less.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
This is a reasonable cost-saving measure, just use the same size busbar over a range of panels. You don't save enough in material to overcome the cost of manufacturing and stocking many different busbar ratings. That being known, a panelboard is only rated for what the label says. So depending on the AHJ they may care less about the components in the panelboard having a higher rating if the panelboard stated rating is less.
I agree, and it would not be surprising to hear that they all do it, but Eaton CH is the only case we have in writing. We have it in our back pocket should we ever need it, but we use the knowledge quite frequently and we have never been challenged on it.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
That letter may only apply to CH panelboards. I mean, I haven't seen it. But BR meter mains come in different flavors that explicitly say that the panelboard busbar is rated 200 or 225, depending on the one you get.
You could be correct about that; I missed that the case in point was a meter main, not a 200A feedthrough panel.
 
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