120% and derating a main service breaker

[philmickelson]

Hi All. I've learned much from the forum discussions, but I would like some feedback on code regarding the 120% rule (705.12(D)(2)) and minimum service amperage to a residential home (230.79(c))

We are proposing to install a 30A backfed breaker on a 100A rated service panel with 100A rated bussing. We thought that a workaround to the 120% rule would be to derate the main service breaker from 100A to a 90A breaker. However, we are getting denied from the AHJ citing the aforementioned code that a minimum of 100A service needs to be available to a residential home.

My questions
1) Does the 100A minimum service NEC code only apply to a new construction?
2) Can an existing service be modified such as our de-rating of the main breaker to 90A?

Thanks in advance for any feedback

 

[Carultch]

Hi All. I've learned much from the forum discussions, but I would like some feedback on code regarding the 120% rule (705.12(D)(2)) and minimum service amperage to a residential home (230.79(c))

We are proposing to install a 30A backfed breaker on a 100A rated service panel with 100A rated bussing. We thought that a workaround to the 120% rule would be to derate the main service breaker from 100A to a 90A breaker. However, we are getting denied from the AHJ citing the aforementioned code that a minimum of 100A service needs to be available to a residential home.

My questions
1) Does the 100A minimum service NEC code only apply to a new construction?
2) Can an existing service be modified such as our de-rating of the main breaker to 90A?

Thanks in advance for any feedback

If a main breaker is going to be reduced, you either need a load calculation or 15-minute demand load data (utility metered or customer metered) to justify it.

 

[don_resqcapt19]

Is the 100 amp breaker the service disconnect and is this in a one family dwelling? If so take a look at 230.79(C).

 

[philmickelson]

Carultch. Thanks for the expedient reply. We can perform that analysis and have done that in the past when derating a main breaker in a 200A rated panel.

However, the AHJ will not accept that load calc given his code citation that a residential service cannot be less than the minimum 100A, irrespective of the load.

My question is whether that is a valid interpretation of NEC 230.79(c) or is that code only applicable to new service feeding a new residence.

P

 

[mwm1752]

line side tap may be your option to use

 

[jaggedben]

Yeah, you can't downsize an existing 100A breaker on a single family home. I've gotten away with the type of configuration in question only when the existing main breaker was already less than 100A and grandfathered. Load calcs are irrelevant once you get down to 100A because of the code section Don mentioned.

If the panel label doesn't specify the size, then next time just label the 90A breaker as 'existing' on your planset, and let it somehow be existing before the AHJ ever shows up to inspect.

 

[Smart $]

...My question is whether that is a valid interpretation of NEC 230.79(c) or is that code only applicable to new service feeding a new residence.
...

Valid, yes... by consensus. Absolutely correct, no.

Look at the diagram in 230.1. It shows the disconnecting means (as a switch) separate from the overcurrent protection. In the case of using a fusible safety switch, there is no question which is which, but when you use a breaker as the overcurrent device, understandings get blurred.

When using a panelboard with a listed main breaker as the service disconnecting means, the rating of service disconnecting means is same as the panelboard rating, i.e. a listed SUSE 100A panelboard is rated as a 100A service disconnecting means no matter what rating
[listed] main breaker you put in it. Good luck convincing an AHJ this is the case.

If you scour the 'net you may be able to find some literature that explains the concept. I don't know of any offhand so I wish you good luck that endeavor. If you do find such literature, I'm sure there are many here which will be mighty interested in having a link...

 

[Smart $]

Valid, yes... by consensus. Absolutely correct, no.

...

Here's a simple question to ask yourself or the AHJ when considering for my earlier comment...

If I use a 100A-rated heavy duty SUSE fusible safety switch feeding a 100A MLO panelboard with 100A service and feeder conductors then install 60A fuses, do I have a 100A service or a 60A service?

 

[jaggedben]

Here's a simple question to ask yourself or the AHJ when considering for my earlier comment...

If I use a 100A-rated heavy duty SUSE fusible safety switch feeding a 100A MLO panelboard with 100A service and feeder conductors then install 60A fuses, do I have a 100A service or a 60A service?

If there's a solar system that backfeeds 30A in the panelboard, then arguably you have an 90A service, since it would be a code violation to change the fuses to anything greater.

 

[Smart $]

If there's a solar system that backfeeds 30A in the panelboard, then arguably you have an 90A service, since it would be a code violation to change the fuses to anything greater.

Where does it say a service rating is based on the OCPD rating?

230.79(C) is in Part VI Service Equipment — Disconnecting Means.

Then there's complete separate stipulations and requirements under Part VII Service Equipment — Overcurrent Protection.

Now if you had multiple disconnecting means and one was a breaker, 230.80 would apply. Other than that, I don't see where a panelboard main breaker rating determines the service rating.

 

[philmickelson]

Thanks for the feedback.

So I guess I'll give more color on our current dilemma. We incorrectly identified the panel as having 125A rated bussing. When the AHJ wanted documentation, we found out that this panel is only rated at 100A, so now our 30A solar breaker violates the 120% rule. I would guess the options would be...

1) Load tap - unfortunately not possible given it invalidates the UL and there isn't much room to access the conductors anyways.
2) Derate the main service breaker to 90A - which as I stated isn't allowed because of the requirement for 100A minimum service breaker size.
3) Upgrade the service and main panel - which is costly

I was hoping that some variant of option 2 would work, but I don't have enough technical knowledge to make that argument.

P

 

[jaggedben]

Where does it say a service rating is based on the OCPD rating?

230.79(C) is in Part VI Service Equipment — Disconnecting Means.

Then there's complete separate stipulations and requirements under Part VII Service Equipment — Overcurrent Protection.

Seems like an unintended loophole, and one which is irrelevant to the OP because the two are the same device (circuit breaker). I suppose I could try to use this argument with an AHJ but I don't know how it would go over with a customer. :lol:

 

[GoldDigger]

Seems like an unintended loophole, and one which is irrelevant to the OP because the two are the same device (circuit breaker). I suppose I could try to use this argument with an AHJ but I don't know how it would go over with a customer. :lol:

And then there is the ever popular LADWP. They do not allow supply side connections and as recently as about a year ago had some strange ideas about the 120% rule.
If, to meet the 120% rule, you replaced an existing 100A service panel with a 200A rated panel with a 100A main installed they required you to pay for upgrade to a 200A service.

 

[Smart $]

Seems like an unintended loophole, and one which is irrelevant to the OP because the two are the same device (circuit breaker). I suppose I could try to use this argument with an AHJ but I don't know how it would go over with a customer. :lol:

And that is exactly where things get blurred when using a panelboard with a down-rated main breaker. Technically the service disconnecting means is the entire panelboard, and that is rated 100A in the OP's case. The circuit breaker is simply the switch and the overcurrent connection combined into one integral device... and Code does not say you have to use the breaker rating.

Note just the circuit breaker by itself is not listed as SUSE. Only the entire panelboard (with enclosure in most cases) is listed as SUSE.

 

[electrofelon]

Thanks for the feedback.

So I guess I'll give more color on our current dilemma. We incorrectly identified the panel as having 125A rated bussing. When the AHJ wanted documentation, we found out that this panel is only rated at 100A, so now our 30A solar breaker violates the 120% rule. I would guess the options would be...

1) Load tap - unfortunately not possible given it invalidates the UL and there isn't much room to access the conductors anyways.
2) Derate the main service breaker to 90A - which as I stated isn't allowed because of the requirement for 100A minimum service breaker size.
3) Upgrade the service and main panel - which is costly

I was hoping that some variant of option 2 would work, but I don't have enough technical knowledge to make that argument.

P

By "load tap" I assume you mean "line side tap" or " supply side tap". I think often times we get obsessed with trying to do a load side connections via a breaker and make it work even it involves getting expensive odd downsized main breakers, doing a load study, etc. Its really pretty quick and easy to put in a trough or box under the meter and do a line side connection. I often wonder why this seems to be avoided at all cost. I understand there are some circumstances where a supply side connection is difficult or not possible due to say having a meter/main, or funky poco rules....

 

[don_resqcapt19]

And that is exactly where things get blurred when using a panelboard with a down-rated main breaker. Technically the service disconnecting means is the entire panelboard, and that is rated 100A in the OP's case. The circuit breaker is simply the switch and the overcurrent connection combined into one integral device... and Code does not say you have to use the breaker rating.

Note just the circuit breaker by itself is not listed as SUSE. Only the entire panelboard (with enclosure in most cases) is listed as SUSE.

I don't agree that the service disconnecting means is the panel board. In my opinion the main breaker itself is the service disconnecting means. That is the device that does the "disconnecting".

230.76 Manually or Power Operable
The service disconnecting means for ungrounded service conductors shall consist of one of the following:
(1) A manually operable switch or circuit breaker equipped with a handle or other suitable operating means ...

 

[Smart $]

I don't agree that the service disconnecting means is the panel board. In my opinion the main breaker itself is the service disconnecting means. That is the device that does the "disconnecting".

So you can take that breaker out of that panelboard and put it in a separate can to use as the service disconnecting means?

230.66 Marking. Service equipment rated at 1000 volts or
less shall be marked to identify it as being suitable for use
as service equipment. All service equipment shall be listed.
Individual meter socket enclosures shall not be considered
service equipment.

 

[jaggedben]

By "load tap" I assume you mean "line side tap" or " supply side tap". I think often times we get obsessed with trying to do a load side connections via a breaker and make it work even it involves getting expensive odd downsized main breakers, doing a load study, etc. Its really pretty quick and easy to put in a trough or box under the meter and do a line side connection. I often wonder why this seems to be avoided at all cost. I understand there are some circumstances where a supply side connection is difficult or not possible due to say having a meter/main, or funky poco rules....

I'd say 90% or more of my projects involve meter main combos with no place to tap.

 

[jaggedben]

So you can take that breaker out of that panelboard and put it in a separate can to use as the service disconnecting means?

That's not what he said. But even if one could, if that means were a circuit breaker then its rating would still have to meet 230.79. I think you've really jumped the shark here. 230.79 applies to the service disconnecting means (not the equipment) and 230.76 says what that must consist of. The only loophole is the one you pointed out about fuses.

 

[don_resqcapt19]

So you can take that breaker out of that panelboard and put it in a separate can to use as the service disconnecting means?

I don't see that as changing anything...yes the complete system must be listed as service equipment, but the service disconnect is the breaker and the rating of the service disconnect is the rating of the breaker.