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I have read some of the NEC code but I am no professional. I am planning to run a 100amp sub panel in my detached garage with no more than 80' of wire, most likely URD 1/0 or so, from my main breaker in the house. My house was built in the 70s and may have been wired with 3 wire service wire. I will checkout the panel soon to confirm. I have quite a few questions.

  1. Can I run the wire straight from the 100amp breaker into the garage sub panel, or do I have to put a junction box somewhere?

  2. If the house was run with 3 wire service, can I run 3 wire to the garage and just run a new grounding rod and ground cable from the sub to the rod, or do I have to run 4 wire?

The plan is to run the cable direct burial at 24" and of course use conduit schedule 80 on both exit of house and entrance of garage. Thank you.

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    Before you do any of this: 1 - Do a load calculation on your existing service/loads to determine if you have capacity for 100A, or if not then how much you can add; 2 - figure out how much you really need for the garage. You can put in a 100A panel and feed it with a 30/40/50/60/70A breaker depending on what is available, and you could always upgrade in the future if you put in large wire (or conduit) now and later do a service upgrade to the main panel. If your garage use includes EV charging, keep in mind that while most chargers can handle 50A or 60A, they don't have to use that much. Jun 29 at 2:01
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    Might be good to compare cost of 90A 2-2-2-6 aluminum. (Assuming load calc compliance.) Jun 29 at 2:05
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    Your 70's house service was probably 3 wire, even current services are 3 wire. Jun 29 at 2:08
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    Houses are always supplied with 3-wire service (well, unless you only have 120V or some full three-phase services). Ground does not come from the utility, because it must be local to be useful. New 240V feeders to sub panels must be 4-wire. As a detached building, your garage will likely require its own local ground rods in addition to being connected to the house main panel ground.
    – nobody
    Jun 29 at 2:28
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    NEC is not designed for training. It says so in 90.1, so technically it's a code violation to try to learn code from it :) Jun 29 at 2:53

1 Answer 1

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Subpanel rating >= feeder wire rating >= supply circuit breaker

Tires come in several speed ratings. 85 MPH, 112 MPH and 130 MPH. Subpanels come in 70A, 100A, 125A and 225A. It's only a "never exceed" red-line. You don't have to drive 130 MPH or 100A :)

I for one am not a fan; the truly valuable thing in a panel is breaker spaces, and we recommend plenty of them. You're more likely to max out spaces than amps in most cases. 100A panels are notorious for not having very many breaker spaces.

On subpanel feeder, we like spending safety money wisely. Since the lugs at both ends are aluminum, and since large aluminum wire is proven safe, we like stuff like 2-2-2-4 aluminum, which is an inexpensive commodity and 90A.

Since your distance is only 80 feet, voltage drop won't be a concern and there is no need to upsize wires for that.

One of my favorite money-savers is 100-200A panel (for the spaces) fed by 90A 2-2-2-4 (for the cheapness) fed by a 60A breaker (for the cheapness). The 60A breaker can be upped to 90A at any time. This will cover almost anything folks do in a garage, especially, charging EVs. Most people new to EVs learn a ton of stuff that's dead wrong, but we can put you right and save you a ton of money vs that wrong stuff. And also not burn your house down :) So if EVs are in your plan, ask us for advice/review before you buy any wires. I'm talking to you, passive reader!

No need for junction boxes

The 4-wire feeder from house to garage does not need any intermediate boxes or splices - except - that to the extent it's routed in conduit, and single wires must be routed in conduit, the conduit needs access points enroute to facilitate pulling. With rare exception, conduit must be built empty and the wires pulled in only after the conduit is complete, anchored and tamped. Access points must remain accessible forever. They can be above drop ceilings but cannot be plastered or built over.

4 wires for real. + ground rods.

A service is not the same thing as a feeder.

The electric service from your utility is 3-wire because the utility has no business supplying ground. You need to originate that locally, and make ONE bond between neutral and ground in your entire system.

The feeder to the garage needs to be 4-wire. It needs to be able to return enough fault current on ground to trip a breaker, and dirt can't do that. (that's why we bother to mine copper and aluminum.) Neutral and ground can't be combined - we tried it for many decades, but the reality is, when you combine them you don't have ground anymore - you have neutral. And attaching metal chassis of things to neutral is a recipe for fatality.

The ground rod is also required. They do different jobs. The rod is more about dissipating ESD and lightning, and assuring that the dirt near the outbuilding is near the same voltage as electrical grounds.

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    Out of curiosity - do I understand this correctly, in the US ground and neutral are bonded on the customer side of the meter? Jun 29 at 15:40
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    @VladimirCravero yes they are- on the customer side.
    – Sam Rueby
    Jun 29 at 19:47
  • @Vladimir More a case that the utility is not permitted to supply ground at all. Ground doesn't exist until the customer-side main disconnect, and must be established there. That way we don't get into the TN-S and TN-C-S nonsense that is vexing Britain. youtube.com/watch?v=gZVx7GbAwlg The closest "T" designation you could call it is TT + local bonding. Jun 29 at 21:50
  • Here in Italy the utility does not supply ground as well, but the N-GND tie happens at the utility transformers, not at the customer. I am trying to think what are the benefits/drawbacks of each approach - perhaps it's worth a separate question. Jun 30 at 8:19
  • I mean at the end of the day, the side of the transformer that you tie to ground is N by definition - it's not that simple in a three phase system ofc but you get the idea @Harper-ReinstateMonica Jun 30 at 8:20

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