Outbuilding feeder with local generator input

[mike93lx]

I am hoping to build a workshop next year and am trying to sort out the electrical. 60A will be plenty.

I want to locate my 30A/240V portable generator input at the workshop (would connect back to the main panel in the house on a proper disconnect).

the wiring would run for about 60ft inside the house and then will be underground for about 40ft to the shop.

if i ran this in the same conduit at any point, I would be subject to derating, correct? realistically, the shop would not be getting used in a power outage, which are incredibly rare anyway, but i recognize that code doesn't care about my personal situation.

Outside of derating, are there any prohibitions to this setup?

As the exterior run is so short, I'd probably run separate conduits anyway, but am curious about the options. If separate, likely 1.25" for 2-2-2-4 AL XHHW and 3/4" for 10-10-10-10 Cu THWN, if combined a single 1.5" would probably be fine. I do not need any network/cable out there.

 

[mm08822]

Yes it would be subject to derating at 80%. However for the details you provided above, the derating from 90C values still permits usage @ 60A and 30A respectively.
.
Unless the loads were mutually exclusive, all ccc have to be considered. Your loads are not.

As long as you have the proper interlock between the grid and gen sources in the main panel all is good.

 

[fitter30]

https://www.calculator.net/voltage-...ce=100&distanceunit=feet&amperes=30&x=47&y=16

 

[theoldwizard1]

I am hoping to build a workshop next year and am trying to sort out the electrical. 60A will be plenty.

I want to locate my 30A/240V portable generator input at the workshop (would connect back to the main panel in the house on a proper disconnect).

You can NOT legally and SAFELY use the same wire to both FEED your shop and then use as a generator input to your house. There is no single mechanical interlock that will prevent you from accidentally back feeding the grid.

You could run two sets of cables in the same conduit.

 

[wyliesdiesels]

You can NOT legally and SAFELY use the same wire to both FEED your shop and then use as a generator input to your house. There is no single mechanical interlock that will prevent you from accidentally back feeding the grid.

You could run two sets of cables in the same conduit.

Go back and re-read what he wrote. He never said anything about using the existing feeder to the garage as a generator backfeed to the house.

 

[mike93lx]

You can NOT legally and SAFELY use the same wire to both FEED your shop and then use as a generator input to your house. There is no single mechanical interlock that will prevent you from accidentally back feeding the grid.

You could run two sets of cables in the same conduit.

At no point did i even suggest that

 

[mike93lx]

Yes it would be subject to derating at 80%. However for the details you provided above, the derating from 95C values still permits usage @ 60A and 30A respectively.
.
Unless the loads were mutually exclusive, all ccc have to be considered. Your loads are not.

As long as you have the proper interlock between the grid and gen sources in the main panel all is good.

Ah OK, so derating is off the 95 degree column. Thanks


Would you recommend separate conduits for any reason?

 

[mike93lx]

https://www.calculator.net/voltage-...ce=100&distanceunit=feet&amperes=30&x=47&y=16

Did you link the right thing? I didn't ask about voltage drop calcs

 

[wyliesdiesels]

Ah OK, so derating is off the 95 degree column. Thanks


Would you recommend separate conduits for any reason?

Its 90* c column. There is no 95* c column

Yes i would do separate conduit

 

[Bretny]

Do you really want to limit your self to 30a for the generator? Thats only a 7,200w generator. The time to put bigger wire in the ground is before you dig the hole.

 

[ard]

You can NOT legally and SAFELY use the same wire to both FEED your shop and then use as a generator input to your house. There is no single mechanical interlock that will prevent you from accidentally back feeding the grid.

Out of curiosity...

is there ***ANY*** switchgear that would (legally) accomplish this?

 

[mm08822]

Its 90* c column. There is no 95* c column

Yes i would do separate conduit

Yep, that's what I meant!

 

[mm08822]

Out of curiosity...

is there ***ANY*** switchgear that would (legally) accomplish this?

A Kiirkkey interlock system could do it using a captive key between two seperate interlock systems.

 

[mm08822]

Ah OK, so derating is off the 95 degree column. Thanks

Would you recommend separate conduits for any reason?

I meant 90c column.

I would run separate conduits. Also depends on ability to get 2 conduits vs. one larger conduit routed and entering panel.

 

[mike93lx]

Thanks for the corrections on the temp.

To Bretny, yes, 30a is plenty. We never lose power and my current 7kw generator will run everything I need. If I ever need something bigger, something significant would have to change, which I can't foresee in this house. Already have a well, three fridges, lighting, outlets and furnace covered

 

[mike93lx]

I meant 90c column.

I would run separate conduits. Also depends on ability to get 2 conduits vs. one larger conduit routed and entering panel.

That made me realize I don't need the generator wiring running through the subpanel and I'll likely have the input on the exterior of the building anyway.

Separate conduits would make that easier to manage

 

[theoldwizard1]

A Kiirkkey interlock system could do it using a captive key between two seperate interlock systems.

WOW ! Talk about an expensive and complex system, BUT it does meet the requirement that one system is locked out BEFORE the other one is energized !

 

[theoldwizard1]

Separate conduits would make that easier to manage

Apologies for my earlier comment that I made before COMPLETELY reading your original post !

According to this calculator, 100' of 6 AWG Al @ 60A is only a 3% VD (no derating). There is 15¢/ft/conductor difference between 2 and 6 AWG XHHW.

100' of 10 AWG Cu (THWN) at 30A is 2.5% VD.

 

[mm08822]

That made me realize I don't need the generator wiring running through the subpanel and I'll likely have the input on the exterior of the building anyway.

Separate conduits would make that easier to manage

Since there would not be any connection in the subpanel, the generator wires could not enter/pass through it.

It is was all one conduit panel - panel, then the gen wires would have to "T" off before the conduit entered the subpanel. Only at the main panel could they enter together.

 

[mm08822]

WOW ! Talk about an expensive and complex system, BUT it does meet the requirement that one system is locked out BEFORE the other one is energized !

Expensive yes. complex no. There is a way!

There are other mfrs who do this also....Rockwell, Schmersal, ......I'm sure ebay has os/used available.

 

[mike93lx]

Apologies for my earlier comment that I made before COMPLETELY reading your original post !

According to this calculator, 100' of 6 AWG Al @ 60A is only a 3% VD (no derating). There is 15¢/ft/conductor difference between 2 and 6 AWG XHHW.

100' of 10 AWG Cu (THWN) at 30A is 2.5% VD.

No worries.

It is hard to pass up the headroom for so little money, but realistically, there is only so much power one person can draw in a small shop. I bet that 95% of the time, I would have at most a 8' 240v baseboard heater, a 2hp dust collector an a 15a table saw running at the same time.

 

[mike93lx]

on the #2 AL XHHW, does the ground wire need to be green or can it be labeled with green tape? the neutral is ok to mark with white tape, correct?

 

[mm08822]

on the #2 AL XHHW, does the ground wire need to be green or can it be labeled with green tape? the neutral is ok to mark with white tape, correct?

Conductor sizes #4 and larger can be marked with tape to re-identify.

 

[mike93lx]

Conductor sizes #4 and larger can be marked with tape to re-identify.

 

[Bert_]

Even if it's all in the same conduit it's only four current carrying conductors, you use 80% of the 90* ampacity.

If they are in the same conduit you would only need to run two hots for the generator. Neutral and ground could be the same as the sub panel.

 

[mm08822]

Even if it's all in the same conduit it's only four current carrying conductors, you use 80% of the 90* ampacity.

If they are in the same conduit you would only need to run two hots for the generator. Neutral and ground could be the same as the sub panel.

Neutral and grd could be the same but where do you splice them to breakout for subpanel and pib? Cant use the subpanel for that. Just seems like an unnecessary complication vs. conductors dedicated for each purpose.

 

[Bert_]

Neutral and grd could be the same but where do you splice them to breakout for subpanel and pib? Cant use the subpanel for that. Just seems like an unnecessary complication vs. conductors dedicated for each purpose.

I'd splice it right on the neutral and ground bar just like the rest of the circuits. Why do you say "can't use the sub panel for that"?

 

[mm08822]

I'd splice it right on the neutral and ground bar just like the rest of the circuits. Why do you say "can't use the sub panel for that"?

The pib is connecting back into the main panel where the interlock is. The generator input feeds a cb in the main panel not the sub. Running the pib hot conductors thru the sub would make the sub a junction box. Connecting onto the grd and neutral bar in the sub would make the sub a jbox.

 

[rlitman]

Out of curiosity...



is there ***ANY*** switchgear that would (legally) accomplish this?

Residential, no. Commercial, yes, it could be done with a stamped engineer’s plan and a Kirk Key Interlock.

BTW, all this discussion of detrating has me thinking. With the generator off, those are NOT current carrying conductors, so without generator, there is no derating. With the generator on, you’re limited to the generator’s power output, so derating may not matter.

 

[mm08822]

Residential, no. Commercial, yes, it could be done with a stamped engineer’s plan and a Kirk Key Interlock.

BTW, all this discussion of detrating has me thinking. With the generator off, those are NOT current carrying conductors, so without generator, there is no derating. With the generator on, you’re limited to the generator’s power output, so derating may not matter.

The same would be required for residential. It is a combination of hardware for either use and since none of the required components are specifically listed together, the inspector would most likely require stamped plans to cover himself. Not saying it is cost effective, just answering how it could be accomplished.

Derating would still apply as the generator connection occurs in the main panel and that does not preclude the use of the subpanel simultaneously..

 

[rlitman]

...Derating would still apply as the generator connection occurs in the main panel and that does not preclude the use of the subpanel simultaneously..

I agree that it does not, however, depending on the relative size of the subpanel feed and generator output power, it may be likely that the generator doesn't have enough power to supply even the derated sub-panel's limit.

 

[mike93lx]

I'd splice it right on the neutral and ground bar just like the rest of the circuits. Why do you say "can't use the sub panel for that"?

My main panel has a neutral disconnect as well, so I would definitely keep it separate.

Not worth cutting out wire at 25 cents a foot to add complexity, IMO.

 

[Bert_]

The pib is connecting back into the main panel where the interlock is. The generator input feeds a cb in the main panel not the sub. Running the pib hot conductors thru the sub would make the sub a junction box. Connecting onto the grd and neutral bar in the sub would make the sub a jbox.

And?

My main panel has a neutral disconnect as well, so I would definitely keep it separate.

Not worth cutting out wire at 25 cents a foot to add complexity, IMO.

If you're disconnecting the neutral then yes it would need to be separate. Most interlocks or transfer switches do not switch the neutral.

 

[mike93lx]

If you're disconnecting the neutral then yes it would need to be separate. Most interlocks or transfer switches do not switch the neutral.

Mine is integrated in a Reliance Controls main panel. Three pole disconnect that is interlocked to the main. Lights up the whole panel, plus disconnects the neutral and it has integrated watt meters to help with balancing.

 

[mm08822]

I agree that it does not, however, depending on the relative size of the subpanel feed and generator output power, it may be likely that the generator doesn't have enough power to supply even the derated sub-panel's limit.

In the op's case, the gen does not have enough output to supply full feeder rating to the subpanel. From a practical standpoint it is not an issue.

However, derating doesn't account for that detail - only exceptions are for mutually exclusive loads and signal/control wiring. Derating is based upon the # of ccc. Since there are 4 ccc in that conduit, OP is at 80% of the 90C ampacities. In his case the derating at 90c has no impact to the 75C values.

 

[mm08822]

And?

And.... 312.8 and 702.10 indicate what you suggested can be done per code.

Although permitted in the same panel, conduits, etc., the splicing in the subpanel just seems like a hack practice. That pib circuit has no business in the subpanel since it connects back at the main panel, IMO.

Apparently my understanding of feed-thru conductors (e.g. - riser panel type applications) is not what code limits the purpose to in 312.8. In this thread's application, these conductors are merely passing through.

IMO, pass-through conductors should be an included description in 312.8. That would harmonize with the intent of 702.10. And of course, there is no definition for feed-through conductors.