Subpanel in detached garage

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hossak8

Member
Location
NY
I have an installation here in NY (2008 NEC) with a subpanel in a detached garage. I have not done a job with this architecture, and would appreciate feedback from the forum. Thanks.

This subpanel currently has a 100A breaker (busbar rated at 125A) with #2 ALU (100A) feeders.

The feeders come from a 100A breaker mounted in the a 200A service panel in the main house about 70' away.

I have 40A of inverter output (32A x 1.25).

I'd like to simply backfeed the breaker in the subpanel...downsizing the subpanel breaker to 80A will keep me to 120% regarding the subpanel busbar. One concern I have is whether or not I have to calculate the load on the #2 ALU feeder conductors at 120%, as it will still be a 100A breaker "supplying power to the conductor" from the main service panel.

It seems to me that I might have to also downsize the breaker in the main service panel in the house to 80A.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
This subpanel currently has a 100A breaker (busbar rated at 125A) with #2 ALU (100A) feeders.
...

I have 40A of inverter output (32A x 1.25).

I'd like to simply backfeed the breaker in the subpanel...downsizing the subpanel breaker to 80A will keep me to 120% regarding the subpanel busbar.

There's no need to downsize the breaker with respect to the busbar. 125A*1.2= 150A, which is greater than 100A+40A. You are required, however, to put the solar breaker at the opposite end of the busbar from the primary feed.

One concern I have is whether or not I have to calculate the load on the #2 ALU feeder conductors at 120%, as it will still be a 100A breaker "supplying power to the conductor" from the main service panel.

This really depends on how your AHJ interprets the code. In terms of physics, a conductor can't carry current in two directions at the same time, so when it is fed from opposite ends it makes no sense to apply the 120% rule. However the 2008 code is not written in a way that considers this point, so if your AHJ is 'just following the letter of the code' then they can make you do it. (The 2014 code updates this.)

There is also the issue, as mwm1752 pointed out, that the conductors may not be properly rated for their use as a feeder.

It seems to me that I might have to also downsize the breaker in the main service panel in the house to 80A.

Since overcurrent protection is required nearest the source, only the breaker at the main service panel is required. The main breaker in the subpanel, if I understand your description correctly, is not required. So if you are required to downsize a breaker because of the feeder ampacity then I would say you are required to downsize the one in the service panel and are allowed to leave the one in the subpanel alone.
 

Carultch

Senior Member
Location
Massachusetts
I have an installation here in NY (2008 NEC) with a subpanel in a detached garage. I have not done a job with this architecture, and would appreciate feedback from the forum. Thanks.

This subpanel currently has a 100A breaker (busbar rated at 125A) with #2 ALU (100A) feeders.

The feeders come from a 100A breaker mounted in the a 200A service panel in the main house about 70' away.

I have 40A of inverter output (32A x 1.25).

I'd like to simply backfeed the breaker in the subpanel...downsizing the subpanel breaker to 80A will keep me to 120% regarding the subpanel busbar. One concern I have is whether or not I have to calculate the load on the #2 ALU feeder conductors at 120%, as it will still be a 100A breaker "supplying power to the conductor" from the main service panel.

It seems to me that I might have to also downsize the breaker in the main service panel in the house to 80A.


Ask your AHJ if you would be permitted to follow the NEC2014 version of the 120% rule. They've re-worded it so that the interconnecting breaker is no longer a factor, but rather 125% of the maximum continuous current of the PV system.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Ask your AHJ if you would be permitted to follow the NEC2014 version of the 120% rule. They've re-worded it so that the interconnecting breaker is no longer a factor, but rather 125% of the maximum continuous current of the PV system.

In this case that's not any help. But the 2014 wording on feeders might be.
 

Carultch

Senior Member
Location
Massachusetts
In this case that's not any help. But the 2014 wording on feeders might be.

What i meant was, that suppose your subpanel is fed with 40A OCPD worth of solar, and it eventually feeds your 200A main panel via a 100A breaker (because it first goes thru a subpanel)

It doesn't matter that the breaker is 100A in regards to meeting 705.12(D) on the main panel. Because you can interconnect up to 32A max cont current worth of solar on the opposite end of any 200A MCB panelboard, no matter what all-pole breaker (40A and larger of course) is located at the opposite end.
 

hossak8

Member
Location
NY
The feeders to the subpanel are USE-2, which are good for 100A @ 90C. The 100A breaker in the main house panel, which supplies the #2 USE-2 subpanel feeders, is located at the opposite end of the busbar from the 200A main breaker.

I'm thinking that downsizing the breaker in the main house breaker to 80A should (just) cover all 2008 NEC compliance issues regarding the feeder ampacities.

What i meant was, that suppose your subpanel is fed with 40A OCPD worth of solar, and it eventually feeds your 200A main panel via a 100A breaker (because it first goes thru a subpanel)

It doesn't matter that the breaker is 100A in regards to meeting 705.12(D) on the main panel. Because you can interconnect up to 32A max cont current worth of solar on the opposite end of any 200A MCB panelboard, no matter what all-pole breaker (40A and larger of course) is located at the opposite end.

But isn't the feeder ampacity still calculated by the sum of the 100A OCPD (supplying the #2 USE-2 feeders to the subpanel) and the 40A inverter output?
 

Carultch

Senior Member
Location
Massachusetts
But isn't the feeder ampacity still calculated by the sum of the 100A OCPD (supplying the #2 USE-2 feeders to the subpanel) and the 40A inverter output?

No. Because the current of the PV system through that feeder is opposite of the current due to the subpanel loads.

In the event that there are only subpanel loads active, the maximum current will be 100A consumption.
In the event that there is only solar, and all subpanel loads are inactive, the maximum current will be 40A production (or rather 32A with the continuous load factor).
In the event that both the subpanel loads are active, and solar is at maximum capacity, the solar will first feed the subpanel loads, and the remainder of the loads will be sourced by the feeder.

You could have 140A of subpanel loads, but that is because you feed the subpanel from opposite ends. At no point will the feeder or subpanel busbar see the combined 140A.

Solar will never be additive with the loads in the subpanel, in terms of its current on the feeder. Except for the negilgible standby consumption of the inverters, but that is milliamps.

The feeder ampacity shouldn't need to exceed the OCPD protecting the feed to the subpanel, unless you upsize for voltage drop.

Do make sure that you comply with 110.14(C), and size all wires for the termination requirements. Formally, the 60C column of the NEC for 100A and less, unless listed and labeled otherwise. Practically, equipment is usually listed and labeled otherwise for 75C.
 

hossak8

Member
Location
NY
No. Because the current of the PV system through that feeder is opposite of the current due to the subpanel loads.

In the event that there are only subpanel loads active, the maximum current will be 100A consumption.
In the event that there is only solar, and all subpanel loads are inactive, the maximum current will be 40A production (or rather 32A with the continuous load factor).
In the event that both the subpanel loads are active, and solar is at maximum capacity, the solar will first feed the subpanel loads, and the remainder of the loads will be sourced by the feeder.

You could have 140A of subpanel loads, but that is because you feed the subpanel from opposite ends. At no point will the feeder or subpanel busbar see the combined 140A.

Solar will never be additive with the loads in the subpanel, in terms of its current on the feeder. Except for the negilgible standby consumption of the inverters, but that is milliamps.

The feeder ampacity shouldn't need to exceed the OCPD protecting the feed to the subpanel, unless you upsize for voltage drop.

Do make sure that you comply with 110.14(C), and size all wires for the termination requirements. Formally, the 60C column of the NEC for 100A and less, unless listed and labeled otherwise. Practically, equipment is usually listed and labeled otherwise for 75C.

Thanks...your description is the way I had originally been thinking regarding the direction of current flow and the load on the feeders, but didn't want to get into a sticky situation regarding the interpretation of the wording "busbars and conductors".
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Thanks...your description is the way I had originally been thinking regarding the direction of current flow and the load on the feeders, but didn't want to get into a sticky situation regarding the interpretation of the wording "busbars and conductors".

Hopefully your AHJ doesn't want to get you into a sticky situation regarding that either.

I feel that we can assure you that a feeder that is only rated for the load is safe. We just can't assure you that your AHJ won't interpret code as requiring something else. My strategy would be to submit plans that don't apply the 120% rule to the feeder. If the AHJ objects, you can downsize the breaker and/or appeal to them to look at the 2014 code.
 

hossak8

Member
Location
NY
Hopefully your AHJ doesn't want to get you into a sticky situation regarding that either.

I feel that we can assure you that a feeder that is only rated for the load is safe. We just can't assure you that your AHJ won't interpret code as requiring something else. My strategy would be to submit plans that don't apply the 120% rule to the feeder. If the AHJ objects, you can downsize the breaker and/or appeal to them to look at the 2014 code.

Can you assure me that interpreting the NEC will always be tons of fun? Thanks again!
 

Carultch

Senior Member
Location
Massachusetts
Thanks...your description is the way I had originally been thinking regarding the direction of current flow and the load on the feeders, but didn't want to get into a sticky situation regarding the interpretation of the wording "busbars and conductors".

A busbar fed from opposite ends will never see more than the maximum current feeding either of the two ends.

The interpretation of the famous 120% rule, is to prevent breakers within a panelboard from overheating and nuisance tripping one another. Suppose you had a 200A panelboard fed by 200A on both ends. Theoretically, Kirchhoff's laws would tell you that there would never be more than 200A at any point on the busbar, because the current will diminish to zero at some point along the busbar. However, you will have a total of 400A worth of heat dissipation across the small resistances internal to all the breakers in the panelboard. And this is what the 120% rule is intended to prevent. It limits you to a total of 240A worth of heat dissipation among all breakers (excluding the main supply, and PV interconnection breaker). And this is considered a reasonable limit to not have problematic heating among the breakers.

For a conductor protected from both ends, with no circuits connected ("tapped") in the middle, it will never see more current than either end can individually supply.
 

Smart $

Esteemed Member
Location
Ohio
...

For a conductor protected from both ends, with no circuits connected ("tapped") in the middle, it will never see more current than either end can individually supply.
That is true even for conductors with tapped load circuits, even under an overcurrent condition on the taps.
 

kwired

Electron manager
Location
NE Nebraska
Since overcurrent protection is required nearest the source, only the breaker at the main service panel is required. The main breaker in the subpanel, if I understand your description correctly, is not required. So if you are required to downsize a breaker because of the feeder ampacity then I would say you are required to downsize the one in the service panel and are allowed to leave the one in the subpanel alone.

Since the feeder has a 100 amp breaker at each end - it will never carry more then a 100 amp device allows.

The feeder is supplying a separate building - there needs to be a main disconnect on the load side of the feeder in the separate building, main breaker in the panel is probably simplest way to do so in most cases.

The feeder conductor is only a 90 amp conductor, one could have a 100 at the supply end and that makes the conductor a feeder tap - but if outside there is no limit to length, but a 90 amp breaker would be needed on the load end. Or a 90 amp is acceptable on the supply end and it really wouldn't matter what setting is on the load end when it comes to protecting the feeder conductor.
 

mwm1752

Senior Member
Location
Aspen, Colo
The feeders to the subpanel are USE-2, which are good for 100A @ 90C. The 100A breaker in the main house panel, which supplies the #2 USE-2 subpanel feeders, is located at the opposite end of the busbar from the 200A main breaker.

Your feeder is for a Garage -- If the terminations are rated @ 90C yes 100 amps, otherwise the 100 Amp rating is only good for derating purposes.
 

kwired

Electron manager
Location
NE Nebraska
The feeders to the subpanel are USE-2, which are good for 100A @ 90C.

That 90C rating can only be used for ampacity adjustments to the conductor in almost all instances, the breaker teminations are still only rated 75C and if you are going to have an 100 amp overcurrent device you must use at least a 100 amp conductor @ 75C as absolute minimum which is 1 AWG for aluminum. If you were supplying a dwelling unit service or feeder then the allowances of 310.15(B)(7) would let you protect a 2AWG aluminum with a 100 amp overcurrent device, but this feeder is not supplying a dwelling unit. It is part of a dwelling/dwelling accessory building but it needs to be supplying the entire dwelling before one can apply 310.15(B)(7).
 

Carultch

Senior Member
Location
Massachusetts
That 90C rating can only be used for ampacity adjustments to the conductor in almost all instances, the breaker teminations are still only rated 75C and if you are going to have an 100 amp overcurrent device you must use at least a 100 amp conductor @ 75C as absolute minimum which is 1 AWG for aluminum. If you were supplying a dwelling unit service or feeder then the allowances of 310.15(B)(7) would let you protect a 2AWG aluminum with a 100 amp overcurrent device, but this feeder is not supplying a dwelling unit. It is part of a dwelling/dwelling accessory building but it needs to be supplying the entire dwelling before one can apply 310.15(B)(7).

Unless listed and marked otherwise for 75C, your equipment at 100A and less is only good for 60 C. This rule is more academic than practical (as most equipment is rated otherwise for 75C), but in some circumstances you may need 1/0 AL wire.
 

kwired

Electron manager
Location
NE Nebraska
Unless listed and marked otherwise for 75C, your equipment at 100A and less is only good for 60 C. This rule is more academic than practical (as most equipment is rated otherwise for 75C), but in some circumstances you may need 1/0 AL wire.
Unless you have equipment of this type (panelboards and circuit breakers in particular) that was made before 1985-ish, they will be rated 75C. If it is new equipment I don't check the temp rating, unless something unusual I haven't seen before - then I am checking everything you can think of.
 

hossak8

Member
Location
NY
My apologies...this building is a garage, and a dwelling. The structure has an apartment on top of the garage. We are still on the 2008 NEC, which only goes to 310.15(B)(6).
 

kwired

Electron manager
Location
NE Nebraska
My apologies...this building is a garage, and a dwelling. The structure has an apartment on top of the garage. We are still on the 2008 NEC, which only goes to 310.15(B)(6).
If it is a dwelling then the 2AWG aluminum can be on the 100 amp breaker. Sort of doesn't make much sense doesn't it? (same size building but with nothing to designate it as a dwelling unit likely has less load, but needs a larger supply conductor)
 
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