A/C wiring question

[rattle_snake]

I am investigating options for wiring a future A/C system. The electrical service to my shop is not ideal, and is currently only 40A. There is an existing, 2nd 40A (#10 wire) run to essentially the same place, just 6' from my shop. It is connected to an existing sub panel on the exterior of the garage that is mostly unused. I would like to utilize this 2nd 40A service to support an A/C for the shop. I'm guessing A/C would need 20 or 30A. I am about to finish drywall so if wiring needs to go in the wall it needs to happen now.

There are several ways I could do this:
Green) Abandon the garage subpanel and use the 2nd 40A for A/C only. Basically run PVC conduit underground from existing Jbox to A/C disconnect. The breaker for the A/C would be in house main panel.

Red) Run PVC conduit underground from existing garage sub panel to A/C disconnect.

Blue) Run PVC conduit over to the shop, then run NM inside shop wall to disconnect. This minimizes trenching. The potential issue I foresee is that this wire in the shop wall is not part of the 'shop' service and breaker labeled as such in the main panel. I think I found something in the NEC long ago stating that a building can have only one feeder.
The air handler(s) power on a mini split are feed from the outdoor unit so this issue would apply to green and red options as well, I would think.

Option 4) run A/C off shop sub panel which doesn't really have extra capacity for it but would likely work just fine. Or at least run the wire for this option and the next one.

Option $$$$) Upgrade power service to 400A, replace house panel, rewire all house breakers and install new 100A service to shop sub panel. Saving this for when I have lots of extra time and money.



WWGJD?

 

[ddawg16]

Ok...option 4 is likely not going to happen.

I think your first green option is best. Dump the garage sub....but don't run the wires to your AC from 2 different places.

If possible, pull it back to that first j-box and run everything from there to the shop sub. Then feed your AC from there.

Don't forget, you will need a separate EGC at the shop (2 ground rods)

If it was me....I'd toss in a 60A 8 Pos panel with a main breaker. Then have your AC breaker in there. You will also now have 240Vac for shop use. It will make it easier to balance out loads and prevent false trips.

 

[nehog]

Is the shop detached or attached to the house/garage?

 

[rattle_snake]

Ok...option 4 is likely not going to happen.
I think your first green option is best. Dump the garage sub....but don't run the wires to your AC from 2 different places.
If possible, pull it back to that first j-box and run everything from there to the shop sub. Then feed your AC from there.
Don't forget, you will need a separate EGC at the shop (2 ground rods)
If it was me....I'd toss in a 60A 8 Pos panel with a main breaker. Then have your AC breaker in there. You will also now have 240Vac for shop use. It will make it easier to balance out loads and prevent false trips.

The shop has it's own ground (wire in slab) and it's electrical system/building has passed final inspection. The shop panel is 100A 24 space (just needs that 100A service run to it when I come across a pile of cash)

Is the shop detached or attached to the house/garage?

Detached.

 

[ddawg16]

The shop has it's own ground (wire in slab) and it's electrical system/building has passed final inspection. The shop panel is 100A 24 space (just needs that 100A service run to it when I come across a pile of cash)


Detached.

Sounds like you're in good shape then.....love those Ufer grounds....so simple

Just remember....when you have to work on it a few years from now, how would you want it wired?

 

[wyliesdiesels]

OP- #10 is NOT rated for 40a.

Who installed this?

And how long of a run is it?

Ok...option 4 is likely not going to happen.

I think your first green option is best. Dump the garage sub....but don't run the wires to your AC from 2 different places.

If possible, pull it back to that first j-box and run everything from there to the shop sub. Then feed your AC from there.

Don't forget, you will need a separate EGC at the shop (2 ground rods)

If it was me....I'd toss in a 60A 8 Pos panel with a main breaker. Then have your AC breaker in there. You will also now have 240Vac for shop use. It will make it easier to balance out loads and prevent false trips.

An EGC and a grounding electrode(rods) are 2 completely different things and you are confusing the 2. Both are required for detached structures starting with 2008 code cycle.

An EGC is the ground wire that provides a low impedance fault current pathway and allows breakers to trip when something that shouldnt be energized becomes energized.

An electrode on the other hand is for grounding lightning limiting voltage to ground potential and shunting primary voltage when and if primary lines contact secondary lines.

The feeder to the subpanel in the garage if wired starting in 2008 should be 4-wire with ground rods or UFER.

The shop has it's own ground (wire in slab) and it's electrical system/building has passed final inspection. The shop panel is 100A 24 space (just needs that 100A service run to it when I come across a pile of cash)


Detached.

A UFER (concrete encased electrode) is different than an EGC...u need both...

Read above...

 

[teamextreme]

I would second what Wylies said and also add that most of what you are proposing would not be code compliant because you can't run multiple circuits to a detached building. You can only feed that A/C unit from your garage sub panel (opt 4), which sounds like it may be insufficient to do so. Your best and probably only real option is to run a larger sub panel circuit to the shop and feed the A/C from that.

 

[wyliesdiesels]

Yeah i forgot to comment about that.

The OP is correct in that u cant have more than 1 feed to a detached structure...

 

[nehog]

Detached.

I think the only one feed to a detached building will apply.

 

[mikegt4]

I think the only one feed to a detached building will apply.

Correct, code only allows one feed to a detached building.

 

[wyliesdiesels]

OP should just do a new feeder and feed AC off new panel.

Idea of double feeders with 40 breakers on #10 is 3 violations alone...

 

[Furd50]

I agree with the others, you need to upgrade to a SINGLE feeder to the shop.

There ARE five exceptions to the single feeder but none of them would apply in this case.

Just because you have a 100 ampere feeder to the shop does NOT necessarily mean you need to upgrade the house service. It is the total load demand that determines the size of the service, not the total connected load.

 

[rattle_snake]

OP- #10 is NOT rated for 40a.

Who installed this?

And how long of a run is it?

Sorry, it is 8/3 NM, I would guess 100'. Was installed when home was built in 1999.

Unfortunately the home has all electric appliances and it's load calc came in at 250A (not including barn or shop). Current service is 200A.

I chose not to put any additional wiring in the walls at this time. I designed the wall sheathing to come off if needed in the future. Compared to the effort of upgrading feeder, it is trivial and I'm sure I would put it in the wrong place anyhow.

The A/C will have to wait for shop service upgrade.

 

[Furd50]

Sorry, it is 8/3 NM, I would guess 100'. Was installed when home was built in 1999.

I sure hope that type NM cable is NOT buried as it is not listed for use anywhere but in a permanently dry location. Even in conduit it is not acceptable outside or in the ground.

 

[wyliesdiesels]

Sorry, it is 8/3 NM, I would guess 100'. Was installed when home was built in 1999.

Unfortunately the home has all electric appliances and it's load calc came in at 250A (not including barn or shop). Current service is 200A.

I chose not to put any additional wiring in the walls at this time. I designed the wall sheathing to come off if needed in the future. Compared to the effort of upgrading feeder, it is trivial and I'm sure I would put it in the wrong place anyhow.

The A/C will have to wait for shop service upgrade.

What is the color of the outer sheath or jacket?