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Not an electrician. Just wondering how much my subpanel can safely power.

I have a 125-amp subpanel in my attached garage powered by two separate 6/2 NM-B cables with ground, run in the attic. Distance to main panel is about 65 ft. Breaker in main panel is 50 amp dual-pole. The subpanel has room for 8 slim breakers (4 spaces?). It appears spaces 1 & 3 are powered by the first hot wire and spaces 2 & 4 are powered by the second hot wire. Both 6 gauge neutrals are connected to the same bus that is isolated from ground in the subpanel. The two ground wires are connected to a ground bar which is mounted directly inside the subpanel.

Questions:

  • The two ground wires from each of the 6/2 cable runs appear to be 12 gauge. Everywhere I've read states these need to be at least 10 gauge. Are two 12 gauge wires OK?
  • Can this subpanel draw 85-90 amps as wired?

Thanks!

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    What exactly is it you are trying to power? By the way the short answer is "no, not even close", but if you can say your plans we may be able to figure a way to salvage it. What is the cable type and is it run outdoors or underground? May 17, 2022 at 6:41
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    If I'm reading the question correctly, you have a MAJOR code violation going on. You can't feed a sub-panel with 2 cables (I assume you meant cables and not wires). On the surface it may seem OK, like I've got 2 cables supplying 50 amps each, but it's NOT OK, reason being is that if one of the 50 amp supply breakers trips or you have a circuit failure , the entire load will be on the other breaker....not good. But to rescue this, like @Harper-ReinstateUkraine asked, "what are you trying to power"? May 17, 2022 at 9:04
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    How hard would it be to replace the existing cables feeding the subpanel? May 17, 2022 at 11:42
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    Pictures of the panel, with the cover carefully removed, and of all the labels inside the panel (on the inside of the swinging door and inside the panel itself) will help tremendously.
    – FreeMan
    May 17, 2022 at 12:10
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    It sounds like you have single 50A Dual-Pole breaker (fine) feeding the black wires of a pair of 6/2 w/ground cables (not fine) and the white wires are connected to the neutral bus, grounds to ground. That's a 50A (@240V) feed, on inappropriate cabling. It's not a 100A feed, though you can run 100A@120V from it. But it's improperly cabled.
    – Ecnerwal
    May 17, 2022 at 12:11

2 Answers 2

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Your panel was wired improperly in the first place. But we can save it.

What they did was combine two cables to get enough wires to do the job. That's not allowed. I would call it "cheap" but they actually spent a fortune compared to what they could have spent.

So let's fix it on the cheap, using the existing wires.

You will need two subpanels. You already have one, so you'll need one more. One panel can be 120V and the other one 240V-only (no neutral). Or both 120V. Or both 240V-only. Your call.

Each cable goes to a different subpanel.

The wires are only good to 55A and you can only plan to put 55A of load on them. However, since 55A breakers are not sold, you can use a 60A breaker. This is not permission to plan to use 60A of ampacity.

To wire a 120V-only panel

On the 120V panel(s), the white wire goes to neutral both in the subpanel and on the main panel. On the main panel, the black wire lands on the breaker - the other half of the breaker is empty.*

At the subpanel, the hot wire is split to feed both hot lugs on the panel. Lugs are only rated for 1 wire, so both lugs will need to be pigtailed and tied with a 3-wire splice. 120V loads will now work. Do not use MWBCs.

A 120V panel can support 6600 VA of calculated load. (120V x 55A).

To wire a 240V-only panel

The white wire at both ends gets taped with black or red tape to indicate its use as a hot. It goes to both poles of the 240V circuit breaker, and to the two hot terminals on the subpanel. At the subpanel, keep the neutral bar completely empty (this panel does not have neutral) and buy an accessory ground bar - this is to make clear to "the next guy" that there is no neutral here.

It'll need to be a nice big panel, because you'll be using spaces 2 at a time. Spaces are cheap.

You then wire up 240V-only (no neutral) loads, such as a heater, air conditioner, water heater, welder, compressor, or EVSE that is hardwired or uses a NEMA 6 family socket. You cannot install a NEMA 14 socket, but EV's don't need neutral.

If you have an EVSE with a NEMA 14-50 plug, either send it back, or throw away the plug and install a NEMA 6-50 plug (notice when disassembling the 14-50 how there is no neutral wire in the cord, and the neutral pin connects to nothing?) EV's have no use whatsoever for neutral, so the 14-50 socket is pointless and stupid. I have no idea why EV suppliers love that plug so much.

A 240V panel can support 13200 VA of load. (55A x 240V).

Two of them can support 26400 VA of load, more than the 21600 VA you were asking for.

"They are 4-wire 120/240V loads, or I can't say."

Then you have no choice but to replace the entire cable run.

I would use 2-2-2-4 aluminum wire of a type such as SER which is rated for the same places NM cable is rated. There is nothing wrong with aluminum at these large sizes, and the panel lugs are aluminum anyway.

Now here's the kicker. Price 2-2-2-4. Now price 6/2 copper NM. See how the 2-2-2-4 is about 1/5 the price of what they did put in? This is the trouble with using wire "you have laying around". They could have sold the 6/2 on Craiglist for more than the 2-2-2-4 cost, and had plenty left over for a larger subpanel.

2-2-2-4 aluminum is good for 90A. Some people believe it is 100A wire - there's a reason they think that, but it's wrong for your application.

This will support 21600 VA.




* They don't make 1-pole breakers larger than 30A, so if you need a 120V breaker that is 50A or 60A, you will need to buy a 2-pole breaker and use only one pole. If your setup is dual 120V subpanels, you can use the same breaker (but they'll trip together if you do).

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  • This is the answer I didn't know I needed. Thank you.
    – Ryan Haney
    May 18, 2022 at 17:35
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No, it is not safe for 100A. Your 6Ga wire is rated for 55A maximum (if the cables are NM-B or UF-B, 65A in the unlikely even that they're SE or USE). It would be a code violation to attempt to parallel those wires to "double up" the current-carrying capacity (and anyway if you did parallel the two pairs of insulated wires in your two /2 cables, you'd be left with no insulated wires for neutral).

Furthermore, your current setup is a code violation. All wires in a run must be in the same conduit or cable. You may not have one of the hots in one cable and the other in another. You must use /3 cable or wires in a conduit to make a 4-wire feed to a subpanel. Additionally, it is code violation to parallel conductors, as you have with the neutrals. One of the 6Ga wires alone is sufficiently large so you could just disconnect one, but you'd still have the multiple cables problem.

To address the current violations, you could:

  • Replace the feed with a 6/3 cable
  • Replace the feed with individual wires in conduit (3 6Ga copper in EMT / 4 wires in non-metallic conduit, or larger, cheaper aluminum)
  • Disconnect one cable entirely if you can get by with only 50A @ 120V (no 240V loads).

To actually increase the amount of power available at the subpanel, you must replace the feed with larger wires (the specific size will depend on what type of wire/cable you choose, copper or aluminum, how much capacity you want, etc).

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  • Does a 6/3 cable safely provide two 50A@120V circuits connected to the same 50A dual-pole breaker? All the breakers in the subpanel so far are single-pole 20A.
    – Ryan Haney
    May 17, 2022 at 16:31
  • @RyanHaney from the first paragraph of the answer, "Your 6Ga wire is rated for 55A maximum", so yes, you can support 50A on #6 wire. HOWEVER, you're going to have a hard time fitting #6 wire onto a 15a or 20a socket. ("Hard" as in "impossible" - the wires are just too big to fit.) There are ways around this, and they've been addressed in other questions here if you'll search.
    – FreeMan
    May 17, 2022 at 17:24
  • @FreeMan Nowhere did I even remotely suggest connecting #6 wire to 15a or 20a sockets.
    – Ryan Haney
    May 17, 2022 at 17:46
  • @RyanHaney A load being "50A@120V" is highly improbable. Typo? May 17, 2022 at 18:16
  • I'm going to accept this answer as there are obviously code violations here and I need to get some general answers about what is allowed.
    – Ryan Haney
    May 17, 2022 at 18:57

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