1

My non-attached garage is currently fed with a 10-2 w/ground wire from my main service panel via a single pole breaker buried in conduit. When the wire enters the garage it enters a junction box were 2 lights, 3 outlets are fed off of this. I haven't had any issues tripping the circuit breaker feeding the garage, but want to add a sub-panel for a more local disconnect/protection.

If I am understanding correctly I can install a 6 slot panel and not bond the neutral with ground, install a separate ground rod and then feed these 3 circuits with breakers?

I also could "jumper" the other side of the panel (to use all 6 slots), but this might not pass via inspector (making it a 120v only panel)?

I am aware of the main feed capacity which is currently a 15a breaker.

Thanks!

  • Hello, and welcome to Home Improvement. Good question; keep 'em coming. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Jun 26 at 17:02
  • Is the existing wire NM (with a paper separator in it) or UF (no paper separator)? Also, is the conduit a continuous run? (i.e. does the cable move when tugged on?) – ThreePhaseEel Jun 26 at 22:38
  • It is NM wire, but I do not know if it's a continuous run I see metal pipe on each end and it's fairly tight. If it was a continuous run I bet it wasn't fun to fish. – legger99 Jun 27 at 1:55
  • Is replacing the wire run an option? Also, running cables in conduits is a bad idea for the reason you mention (it's never fun to fish, unless the conduit is way oversized, or you're talking about something like a comms or tray cable that's intended to go into conduit) – ThreePhaseEel Jun 29 at 2:11
  • In the future I might end up replacing it with something larger and running 240v out to the garage too. I did find out when putting in ground rods that the wire is direct buried and seems to be a UV resistant cable, but prob not supposed to be buried. For now, today actually I installed a small panel with a backfed breaker as a disconnect (w/ retainer) and put in a couple breakers for the outlet and lights. I know how fun fishing ethernet drops through conduit is as I have done several structured cabling jobs. ~ Thanks for the help though! – legger99 Jun 30 at 3:15
3

Hold on. Why is it 10 AWG? You're always allowed to use bigger wire than is required, but why did they do it? It's important.

By the way, a local breaker will not trip first. In most cases, it's at least 50/50 that that the breaker in the house will trip first, closer to 100% if it's raining :)

Factoring for distance

For a long cable run, you need to examine your actual loads (not breaker trip rating) and the distance, and consider the voltage drop you would suffer on that circuit given those actual loads. If you're a cable salesman, you want to keep it under 3%. If you're a cable buyer, you want to keep it under 6%, and if there are other segments of wiring, make sure they all together don't exceed 8%. 8% isn't going to break anything.

Most people do not realize the above, and take the cable salesman's default of breaker trip value / 3%. That calls for a bump in wire size every 58 feet @ 120V, or every 115' @ 240V, give or take 10 feet or so.

If your garage is >60' from the house, you need to consider your loads and recompute voltage drop for your loads and percent preference.

One option: change the circuit to 240V

Right off the bat, doing so cuts your voltage drop in half, and your power drop by 3/4. After this refactoring, you could definitely fuse the circuit for 20A -- and just like 120V, 20A is the cap for "normal" branch circuits. You would need to use 240V lighting (which is easy, many lights are 120-240-277V multi-voltage these days). And you would need to change all your receptacles to NEMA 6, and use 240V appliances obviously. You could use an isolation transformer for 120V appliances.

Since this is still one circuit, you don't need to install local grounding rods.

Another option: Multi-wire branch circuit

If the existing wire is conduit, this will be easy, but you will need to replace the 10/2 cable with individual wires. You get to do your voltage drop calculations based on 240V if both legs are loaded at least halfway - so if you have 6A of lighting on one leg, and 12A of tool on the other, you're golden.

This is still a single circuit, so you still don't need grounding rods.

120V Subpanel

This is the solution you are thinking of. Without running an additional neutral, you can wire the subpanel as a 120V subpanel. Pick a tiny panel (e.g. 18 spaces)* that has a main breaker (mandatory), and just split the 120V hot wire to both sides of the main breaker.

Technically, you don't need a main breaker only a shutoff switch, but the cheapest way to get large-amperage shutoff switches is main breakers in panels. Also, it's morally important that the shutoff switch be readily obvious, and a larger "main breaker" set above the other breakers does the job. Code technically allows a back-fed breaker with a label, but I think that's too confusing to someone in a panic, in a hurry, in the dark.

Since its' a subpanel, it will need 2 ground rods set at least 6' apart - yes, in addition to the ground wire going back to the house. It only needs one ground rod if it passes the ground impedance test, but that test is not DIYable, and ground rods are.

120/240V subpanel

If it's in conduit, and you can run 3 wires + ground, you can make it a 120/240V subpanel, in the normal way subpanels are done. For least voltage drop, try to have loads balanced (or at least 50% balanced). Other than that, see above.

Transformer-fed panel

Lastly I'll mention the option of using the /2 cable to feed a 240/480--120/240 transformer. You get to keep the same cable, but you are able to deliver 120V/240V, and voltage drop on 120V loads is reduced by 3/4.

In this case you obtain a 5 KVA transformer (Craigslist sometimes has them in the $100-ish range) and feed the primary with 240V from the house. The secondary feeds the garage panel, which is a "main panel" (because it's transformer-derived) so we need ground rods again. The transformer automagically converts your 120V draws into 240V at half the amperage, which means 1/4 the voltage drop. You can also jumper the transformer to give you a few volts "bump" to compensate for voltage drop.

  • Wow that's a lot of info, my head might explode. I really like the part about the conscientious thing to do: "too confusing to someone in a panic, in a hurry, in the dark". Easy to not think about such things. Excellent work, thanks. – DonBoitnott Jun 26 at 19:33
  • I also like the info. When I originally purchased the house it had a 20a breaker feeding the garage and when I seen all the wiring off the junction I dropped this down to 15a feed, just for a piece of mind. If I am understanding you right. I could buy a 6 slot panel, switch feed back to 20a (under 60' to garage), purchase a 20a 2-pole for the sub-panel, back feed to this, label "MAIN" cover off rest of breakers so only main is showing (emergency shutoff) and then install two ground rods off this? and connect nothing to the "lugs". Looking to be cost effective, but still safe. Thanks!! – legger99 Jun 26 at 20:10
  • If it's under 60' to the garage, there is no reason to derate the #10 wire -- use a 30A breaker to the subpanel. 30A/120V breakers have no practical use except for TT30 circuits to small campers, so I would buy a 30A/240V breaker and use half of it. – Harper Jun 26 at 20:32
  • 1
    Yes, that plan sounds fine, but label the breaker "MAIN" and make it the one on top. Backfed breakers must be bolted down so don't buy a panel unless the seller can give you a boltdown kit also. It's classic Home Depot to sell you a panel and send you to the electrical supply house for the extra bit you need, then you find out it doesn't actually exist... (because big box staff consistently know nothing). Better buy the kaboodle from the guy that knows. – Harper Jun 26 at 20:35
  • I like your idea of buying a 30A/240v breaker and using half. I seen that they need to be bolted down (when backfed), now to find a panel that accepts or has retainers and isn't too big. I might end up getting a panel with a main, but some panels require special main breakers that get costly it seems. I guess I could put a separate disconnect, but then that probably requires grounding too :) – legger99 Jun 27 at 2:09
0

10-2 w/ground wire

main feed capacity which is currently a 15a breaker.

With 10 AWG, you should be able to move from 15A to 30A. Even with only 10/2 == 120V, that would let you put in a subpanel that includes multiple circuits so that you can have a circuit for tools separate from the circuit for lights and actually be able to use 15A to do "stuff" while still having the lights on. But the more interesting part is:

buried in conduit

Is it buried or in conduit or both?

If it is buried cable then you are stuck (max. being 120V 30A instead of 120V 15A). But if it is in conduit then you should be able (depending on conduit size, of course) to replace 10/2 with larger wire for higher capacity and go from 2 wires + ground (== 120V) to 3 wires + ground (== 240V = double your capacity with one more wire). In fact, if your current wiring is 2 individual conductors + ground (and not a 10/2 cable stuffed through conduit), you can just add a single 10 AWG wire to move up to 240V x 30A.

  • Good to know I can increase the main breaker size. It is currently 10-2 romex? running through metal conduit on each end about the size of the wire. I do not know if it is direct buried or in pipe the whole distance. There would be no way to increase or add to the existing wiring. My main concern was using the sub-panel like I described any thoughts on that (120v / earth ground)? – legger99 Jun 26 at 15:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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