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I'm posting this question to determine if there is a safe manner to do this that meets code and isn't a redneck solution.

The sub panel in my garage is run off a 50 amp breaker that is in my service box/panel outside. The wire appears to be 4 awg (possibly 3, but at least 4) aluminum wire. I couldn't seem to find any labeling on the wire or sheathing, so that has been annoying. It seems I could maybe upgrade this to at least 55A if it is 4 awg aluminum, but that's another question. The sub panel wire runs through roof just above the ceiling.

I need to add a 50A/240V circuit on the other side of the garage from the sub panel. One option is to run it from the existing sub panel, and I can do that but something to note is that the sub panel wire runs directly above where I need to put this 50A outlet.

So, I'm wondering if there is an approved way to splice the sub panel wire in a junction box above the ceiling, and then I'd only have to buy wire to go directly down the wall from there (or in conduit, if need be). This would save me all kinds of snaking and about 25ft of 6awg copper.

If the answer is yes, do I have to run it to a sub panel first, or can I run it directly to something like this: https://www.walmart.com/ip/Parallax-PGU055C-60A-120-240V-Outdoor-RV-Power-Outlet-with-20A-1-50A-2-Pole-Breaker-14-50R-5-20R2-GFl-Receptacle/100815504

I guess that kind of negates the cost savings on wire, but it might be easier still.

As I write this I feel like I'm maybe asking how to cobble something, and I'd really rather not cobble electrical work :).

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    A 55A breaker has the number "60" on the handle, and is marketed as a 60A breaker ;) Also, the wire which is correct for a 50A circuit is also correct for a 60A circuit, in either Cu (#6) or Al (#4). – Harper Nov 13 '18 at 20:54
  • What loads are on this subpanel already, and is there spare space in the subpanel? – ThreePhaseEel Nov 13 '18 at 23:56
  • There is space in the subpanel for an additional 240 breaker. Currently has normal stuff, lights, outlets, etc. The only heavy circuit is for my welder outlet, which has a 60A breaker on it and at least 6awg wire. The welder itself would actually be fine on a 30A breaker as it's max draw is 21amp. I found it funny that they had installed a 60A breaker in a 50A sub panel, but I assume they were using what was already laying around. – slambeth Nov 14 '18 at 16:43
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I'd use a flush mount NEMA 1 pull box and mechanical setscrew/splice connectors

While an ordinary junction box is simply too small for splicing 4AWG conductors, larger boxes are available. These are called pull boxes or NEMA enclosures, and in particular, you need a flush mount (into the ceiling), NEMA 1 (indoor) box. Size-wise, you are looking at a minimum dimension of 6" across as a 4/4/4/6 or 4/4/4/4 SER cable will fit into a 1" conduit of the appropriate type (nobody uses Type A PVC conduit, but it provides the most fill area/diameter of any conduit type in the Code, so its what's used when transposing cable sizes into conduit sizes for this) and NEC 314.28(A)(2) requires the box to be at least six times as long as the largest conduit entry into it, more or less:

(2) Angle or U Pulls, or Splices. Where splices or where angle or U pulls are made, the distance between each raceway entry inside the box or conduit body and the opposite wall of the box or conduit body shall not be less than six times the metric designator (trade size) of the largest raceway in a row. This distance shall be increased for additional entries by the amount of the sum of the diameters of all other raceway entries in the same row on the same wall of the box. Each row shall be calculated individually, and the single row that provides the maximum distance shall be used.

Exception: Where a raceway or cable entry is in the wall of a box or conduit body opposite a removable cover, the distance from that wall to the cover shall be permitted to comply with the distance required for one wire per terminal in Table 312.6(A).

The distance between raceway entries enclosing the same conductor shall not be less than six times the metric designator (trade size) of the larger raceway.

When transposing cable size into raceway size in 314.28(A)(1) and (A)(2), the minimum metric designator (trade size) raceway required for the number and size of conductors in the cable shall be used.

Inside this box, the actual wire splicing is performed using appropriately sized mechanical splice connectors ("Polaris" or "Unitap" are common trade names for them) for the hots and neutral, and a small equipment grounding bar for the ground wires -- this ground bar screws into a pre-tapped hole for such on the back of the box using a 10-32 machine screw. All of these connections will need to be torqued to manufacturer specification using an inch-pound torque wrench or torque screwdriver, by the way.

As to mounting the box, you'll want to use a piece or two of scrap 1x or electrical strut ("unistrut") as crossmembers between the joists to mount the box to, given that your average NEMA 1 box is designed to be mounted through the back, not a side. You'll need to screw it to the crossmembers with the appropriate screw for the crossmembers you're using, too -- NEMA boxes are not designed to be nailed to things.

The RV outlet is a subpanel

The good news is that the RV outlet assembly is a subpanel, so provided you use appropriately sized cable (6/3 copper NM is fine here provided physical damage is of no concern), it's no different than any other situation where you have two subpanels on the same feeder. The "feeder tap" rules in NEC 240.24 do not apply here, by the way, since the conductors from the splice point to the RV outlet assembly are sized for the full 50A the breaker in the main panel will allow.

(As a sidenote, this logic does not apply to a simple receptacle -- the use of a plain receptacle here would result in a mix of utilization devices and subpanels on the same circuit section, which is something that the NEC does not appear to contemplate.)

  • Thank you for the effort you put into this response, excellent information. – slambeth Nov 14 '18 at 16:48

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