2
Existing:

I have a 100 Amp panel in my garage. It is supplied by 3-2/0 Aluminum cables (Hot-Hot-Neutral) in 2" PVC conduit, from my 200 Amp Main Service Panel on the house about 50' away.

Goal:

Convert the garage to a shop and upgrade to 200 Amp new Service Panel.

Question:

If I run two additional cables (Hot & Hot) from the Main Service Panel to the garage, what size would they need to be to give me 200 Amps at the new panel? I am assuming that I would only need the existing 2/0 cable as ground.

My plan is to supplement the two hot leads with two more cables so that, combined, they would be sufficient for 200 Amps. The third existing 2/0 cable should be sufficient for the neutral/ground. The supplemental cables do not need to run in the existing conduit as most of the run is in the crawl space of the house.

HERE'S A NEW IDEA:

Can I gang two of my 2/0 al cables together to equal a single 4/0 hot lead, and then run one 4/0 cable for the other hot and use the remaining 2/0 as the neutral? Right now I have 3 2/0 al cables to the garage panel.

8
  • 1
    Is this a garage to park your car and do some work or a three/four bay mechanics business. 100 amps should be enough for most garages, unless you want electric heat and on demand large water heater, there are cheaper ways to supply them.
    – crip659
    Feb 17, 2023 at 0:31
  • I'm turning it into a shop including hvac.
    – Kennydeee
    Feb 17, 2023 at 1:15
  • I'm OK with splitting a 200A service to two 200A feeders, one 200A to house one 200A to garage. That means only one Load Calculation in which all loads are considered. You don't need a Load Calc per panel since they are all 200A. Feb 17, 2023 at 2:17
  • How many square feet are a) the house and b) the garage, and can you post photos of the existing panels for that matter? Feb 17, 2023 at 3:24
  • Wood shop with lots of big stationary tools? Automotive shop with a big compressor, lift(s) and other big electric tools? There are lots of definitions for "shop".
    – FreeMan
    Feb 17, 2023 at 15:13

2 Answers 2

2

Your original subpanel was installed on the pre-2008 rules where no ground wire is run to the subpanel, and grounds in the subpanel bootleg off neutral. This is a bad idea (the Brits are moving to this arrangement and it's killing people)... especially if EV charging is involved. (the Brits are forced into absurdity to avoid electrifying the chassis of the car when PE+N breaks). Your new feeder is not grandfathered, and thus must be 4-wire. That's OK, I think you can reuse 2 of your wires.

Note that 4/0 aluminum is only allowed if your entire service is 200A, per 310.15(B)(7). If the service is larger than 200A, then 250 kcmil aluminum is required and you might have a fit problem in that conduit.

What 310.15(B)(7) is saying is that feeder wires never need to be larger than the service wires, because, that would be dumb lol. 4/0 service wires are allowed since your residential service is 200A, and therefore 4/0 feeders are also allowed. And now that I think about it, that applies to neutral too. Therefore if your Load Calculation supports a 2/0 neutral in your service wires, it's gotta be allowed for any feeder off that service.

HERE'S A NEW IDEA: Can I gang two of my 2/0 al wires together to equal a single 4/0 hot lead, and then run one 4/0 wire for the other hot and use the remaining 2/0 as the neutral? Right now I have 3 2/0 al wires to the garage panel.

Nope, you still need a ground so it doesn't save you a wire. And doubling up wires like that is paralleling, and that's not really practical. Particularly, you need protective equipment that is UL-Listed for paralleling at the origin, and that stuff is industrial-priced.

6
  • I should edit my question to say, there are three 2/0 aluminum cables existing - hot, hot, neutral. My plan is to supplement the two hot leads with two more cables so that, combined, they would be sufficient for 200 Amps. The third existing 2/0 cable should be sufficient for the neutral/ground. The supplemental cables do not need to run in the existing conduit as most of the run is in the crawl space of the house.
    – Kennydeee
    Feb 17, 2023 at 1:02
  • Paralleling like you're proposing is not trivial. First, you must use listed paralleling equipment (you can't just splice them together). Second, while you don't necessarily have to run all the conductors in the one conduit, if you do split up the routing, the conduits must have the same characteristics. All told, for 50', it's probably simpler to avoid paralleling and use single wires of the right size.
    – nobody
    Feb 17, 2023 at 1:23
  • Well, let's say that he could get away with one 2/0 AL grounded conductor based on load calculations and the other objections about paralleling are satisfied, he'd need a #1 ECG jumper. So five 2/0 AL conductors and a #1 AL conductor. My first thought is that would be very hard to pull in 2 inch pvc, especially sch 80.
    – Edwin
    Feb 17, 2023 at 2:07
  • 1
    @Edwin OP is not proposing to parallel. OP wants to downsize the neutral and not install a ground. However it dawns on me that you can't downsize neutral in a feeder but you can in a service wire. And 310.15(B)(7) say the feeders never have to be larger than an allowable service wire. Feb 17, 2023 at 2:09
  • @Harper-ReinstateMonica - Thanks - didn't realize that - so he'd need to take one 2/0 conductor out and add two 4/0 conductors? And make sure his service grounded conductor is 2/0 AL. Nice. :)
    – Edwin
    Feb 17, 2023 at 2:20
0

@Harper covered the wire specifics quite well, as usual. But two more things to consider:

  • Load Calculation

Presumably you want to upgrade the garage subpanel in order to use more power. That is fine, except that if you are actually using that much (presumably > 100A) then you have to consider whether your utility service and main panel are up to the task. Typical service is 200A. Even if your utility provides more, and the feed wires from the utility are large enough, your 200A main panel can't handle using 150A itself and sending 150A to the subpanel (300A altogether). A load calculation gives a formal way to determine whether your expected usage is OK with each panel and with the utility service.

There are 5 main reasons I know of to make a garage subpanel upgrade like this one:

  • Adding electric on-demand water heating - Don't. Add a tank (or upgrade from a small tank to a large tank) and you will have a much lower peak demand, even though the total energy used will be about the same.
  • Adding HVAC - turning an unconditioned space into a conditioned space. Done wrong (i.e., lazily) with electric resistance heat in a leaky building, this uses a lot of power. Done right, with a modern heat pump and properly insulating the building, this doesn't have to use huge amounts of electricity.
  • Turning the garage, or a space above the garage, into an accessory dwelling unit (a.k.a., an apartment for relatives or to rent out). This actually can use quite a bit - water heating, HVAC, kitchen, bathroom - though 100A is often enough.
  • Workshop - welding, table saw, drill press, dust collector, etc. These tools can all use a lot of power, but if you only have one or two people working then generally not all that much power at any one time.
  • EV charging

My hunch is EV charging. There is a lot of confusion about EV charging, and a lot of people seem to think that 50A (40A continuous) or even 60A (48A continuous) is necessary. That is absolutely not the case for most people. Most people drive 100 miles a day or less and can easily charge up every night using 240V @ 30A or even 240V @ 20A. So if this is your reason for the upgrade, it is worth doing a load calculation on the existing panel and seeing how much headroom there is. You may not have 50A, but you might find you have 20A or 30A, in which case you don't have to upgrade your subpanel at all. Most EV service equipment can be configured for a variety of charging rates, so it is just a matter of figuring out how much power your subpanel (and the main panel feeding your subpanel) can spare.

10
  • You're right, the motive for the upgrade is to make the garage a workshop and also for a future ADA. I am concerned that the existing 200 amp service which is supplied via 3 4/0 aluminum cables in below ground conduit.
    – Kennydeee
    Feb 17, 2023 at 1:07
  • The question is, will it be possible to upgrade the garage to 200 amps while keeping the service drop the same - basically it would be running two 200 amp circuits from a single 200 amp service drop (A 2bed/3bath house and a garage/shop - table saw etc. and hvac).
    – Kennydeee
    Feb 17, 2023 at 1:14
  • And the answer to that (2 x200A from 1 x200A service) is load calculation. All electric, not so likely. Gas ovens, gas water heaters and there is a good chance it will all be OK. Feb 17, 2023 at 1:18
  • Right. Can you recommend a good load calculator. No gas available. Thinking mini-split hvac.
    – Kennydeee
    Feb 17, 2023 at 1:25
  • 1
    Great ponts on EV charging. I may have flubbed with the neutral wire sizing. If OP is allowed to use a 2/0 neutral on the service I think it'll fly on the feeders. Feb 17, 2023 at 2:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.