Garage subpanel, splicing wire

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Taylorjm

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So I'm adding a subpanel for my attached garage. 100 amp panel, so I was looking for 2-2-2-4 ser cable. Well There's nothing in the state available except at electrical supply stores and it's over twice as much as the big box stores, but you can't find it in big box stores. I have 60' of 4/0-4/0-4/0-2/0 ser cable that I got really really cheap. I also about 25' of 2-2-2-4. This stuff is like wrestling an anaconda but I can make it work, but I'd need to spice each end with the 2-2-2-4 to run into each of the service panels because there's no way I can get that 4/0 into the service and subpanel. Of course the breaker would only be for the 100amp. I've seen barrel splices that would work, but do you just leave them in the open air like the overhead service? How do you contain the spiced wire after insulating it? One spice would be in the floor joists above the panel in the basement and the other would be in the attic above the service panel, so not just hanging out in a room.
 

wwhitney

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- Splices in cables methods need to be contained in a junction box, which needs to remain accessible. Box sizing for conductors #4 and larger is a bit complicated, it involves determining the smallest size conduit that could carry the same conductors, and then some of the dimensions have to be 6 times that conduit size.

- #2 Aluminum can't be protected at 100A unless it's on a 100A residential service. For all other cases you are limited to 90A, the 75C ampacity of #2 Al.

Cheers, Wayne
 

Taylorjm

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- Splices in cables methods need to be contained in a junction box, which needs to remain accessible. Box sizing for conductors #4 and larger is a bit complicated, it involves determining the smallest size conduit that could carry the same conductors, and then some of the dimensions have to be 6 times that conduit size.

- #2 Aluminum can't be protected at 100A unless it's on a 100A residential service. For all other cases you are limited to 90A, the 75C ampacity of #2 Al.

Cheers, Wayn
Thanks. It is a residential service. So I understand using a junction box and I agree, but why where my service comes into the house, the overhead lines are spliced and wrapped in tape and in open air? I could never understand that one.
 

wwhitney

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OK, it's a residential service, but is the service rating 100A? I.e. is there a single breaker marked 100A that kills power to the whole property? If that service disconnect breaker is marked anything above 100A (with 125A and 200A being likely candidates), then your #2 Al needs to be protected at 90A or less.

For an overhead service, the splices at the weather head are the responsibility of the power company and subject to their rules. Everything downstream of there is subject to the NEC.

Cheers, Wayne
 

Taylorjm

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OK, it's a residential service, but is the service rating 100A? I.e. is there a single breaker marked 100A that kills power to the whole property? If that service disconnect breaker is marked anything above 100A (with 125A and 200A being likely candidates), then your #2 Al needs to be protected at 90A or less.

For an overhead service, the splices at the weather head are the responsibility of the power company and subject to their rules. Everything downstream of there is subject to the NEC.

Cheers, Wayne
The main panel is rated for 200A. Ahhhh, so the power company gets a pass.
 

wwhitney

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OK, so your #2 Al has to be protected at 90A or less.

The POCO sort of gets a pass, but really it's a different set of rules for different circumstances. Their conductors are pretty much all outdoors. And there are rules on weatherhead location which means those splices are pretty much inaccessible without a big ladder.

Cheers, Wayne
 

Taylorjm

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OK, so your #2 Al has to be protected at 90A or less.

The POCO sort of gets a pass, but really it's a different set of rules for different circumstances. Their conductors are pretty much all outdoors. And there are rules on weatherhead location which means those splices are pretty much inaccessible without a big ladder.

Cheers, Wayne

Thank You! I guess I will just make it easier and pony up the big bucks for some 2-2-2-4 ser. These supply issues are a killer. The closest place I can get it reasonable is 2 hours away. I'm not even in the boondocks, I'm a couple hours north of detroit and there's nothing in the state!
 

wwhitney

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Thank You! I guess I will just make it easier and pony up the big bucks for some 2-2-2-4 ser. These supply issues are a killer. The closest place I can get it reasonable is 2 hours away. I'm not even in the boondocks, I'm a couple hours north of detroit and there's nothing in the state!
Definitely the right call, any professional would get the correct cable to avoid the splices, and the cost of the boxes, splice connectors, and the labor to install them would negate the cost savings of using the cable on hand.

There are places on-line that will cut it to length and ship it to you. Might normally be a bit pricey, but if all the other options are pricey, I bet they would be competitive. You might be able to get 1-1-1-3 Al SER, which would let you use a 100A breaker.

Don't forget to get a torque wrench/screwdriver. All the cable terminations will have a specified torque.

Cheers, Wayne
 

Taylorjm

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Definitely the right call, any professional would get the correct cable to avoid the splices, and the cost of the boxes, splice connectors, and the labor to install them would negate the cost savings of using the cable on hand.

There are places on-line that will cut it to length and ship it to you. Might normally be a bit pricey, but if all the other options are pricey, I bet they would be competitive. You might be able to get 1-1-1-3 Al SER, which would let you use a 100A breaker.

Don't forget to get a torque wrench/screwdriver. All the cable terminations will have a specified torque.

Cheers, Wayne
So one last question. What if I were to run the 4/0 cable into the panels, then butt splice it down to say a piece of #4 copper to make the connections to the breaker, ground and neutral in the panel. Assuming I could use a 100a breaker that way. Are the butt splices made inside the panel ok? Is using the butt splice with the set screws with aluminum and copper ok or I should I just stick with a piece of #1 aluminum to splice.
 

wwhitney

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So one last question. What if I were to run the 4/0 cable into the panels, then butt splice it down to say a piece of #4 copper to make the connections to the breaker, ground and neutral in the panel. Assuming I could use a 100a breaker that way.
#4 Cu has a 75C ampacity of 85A, so you'd need #3 Cu for a 100A breaker. You're either using the wrong table (the one that only applies to residential services) or the wrong column (90C, which can only be used as a basis for derating, as equipment terminations are in practice all limited to 75C). Or you could use #1 Cu or #2/0 Al with a 125A breaker. Or a 200A subfeed kit for your panel (if it exists, it would take 4 spaces instead of 2) and just land the 4/0 Al on it directly (since on a 200A residential service, 4/0 Al is fine for a 200A feeder).

The only downside to that plan is the space the splice would take up in the panel, it's allowed but could be a bit congested. You could use something like these:


In the garage, it would be simplest to use a 200A MLO panel, as the lugs would be large enough to land the 4/0 Al on directly.

Cheers, Wayne
 

Taylorjm

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#4 Cu has a 75C ampacity of 85A, so you'd need #3 Cu for a 100A breaker. You're either using the wrong table (the one that only applies to residential services) or the wrong column (90C, which can only be used as a basis for derating, as equipment terminations are in practice all limited to 75C). Or you could use #1 Cu or #2/0 Al with a 125A breaker. Or a 200A subfeed kit for your panel (if it exists, it would take 4 spaces instead of 2) and just land the 4/0 Al on it directly (since on a 200A residential service, 4/0 Al is fine for a 200A feeder).

The only downside to that plan is the space the splice would take up in the panel, it's allowed but could be a bit congested. You could use something like these:


In the garage, it would be simplest to use a 200A MLO panel, as the lugs would be large enough to land the 4/0 Al on directly.

Cheers, Wayne

Thanks. So I have a couple options depending on what's available. I could do everything and wire in the lights like it was before on one previously wired breaker and leave it be until wire becomes available again to connect the subpanel. I won't be moving in any tools for a couple years when we move up to that house anyway, I just wanted to get the remodel done and check one thing off my list. I can't believe it's been over 2 years and we still aren't back to where we were prior to the pandemic. The electric supply store I checked with last week had over 1000' of the wire I wanted available, this week it's gone and they are out of stock. It's crazy. Even the online stores stock is all over the place, and it's going for a premium.
 

WorthFlorida

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Have you considered going down in size? 100 amp sub panel for a garage is extremely large. Most sub panels are in the 30-60 amp range and it definitely reduces the wire size. If you are making this a workshop with good size machines with large motors it might warrant 100 amp. If you are going with a welder, check the specs for input current at 120 or 240 volt. If you add up all the power needs, not all of the equipment will be on at the same time. If you are going to add electric space heaters, that can take some serious power. I do not know if you did do a load calculation, but this site has an article on how the size up a sub-panel or just google "what size sub panel do I need". I'll give you at least a base line to go for. https://www.thespruce.com/calculating-subpanel-loads-1152758
 

wwhitney

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For a counter perspective, the labor costs for a 100A panel or a 60A panel are very close, and the material costs shouldn't be much higher for 100A, particularly if you are using aluminum wire, which is the cost efficient choice.

Also, with the shift to EVs over the next 10-30 years, more power in the garage will be appreciated.

Cheers, Wayne
 
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