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I recently bought a house that has a subpanel in the garage served by a 50 amp circuit from the main breaker. The breaker has 4 wires coming in: red, black, green, white. Currently, the subpanel feeds a 30 amp and a 20 amp circuit Each served be a 3-wire connection where the third wire goes to green for one, white for the other.

subpanel photo breaker in main panel

I want to rewire to install a NEMA 14-50 with a single 50 amp breaker in lieu of the 20 amp service (bottom right exit), and leave the second circuit unused. That would require a 4 wire connection, do I just use green and white? What about the second circuit? Disconnect it? Leave it in place?

I'm looking to connect an EV (Model 3) that draws under 45 amps. Based on the cerrowire application chart, 8 AWG would be sufficient? => https://www.cerrowire.com/products/resources/tables-calculators/applications-charts/

Did I forget to ask anything else I should?

  • I started my answer before your edit. Are you going to use metallic conduit, pvc? EV chargers require 125% or the wire size identified by the manufacturer. – Ed Beal Apr 15 at 1:56
  • My plan is to replace the right outlet right under the subpanel. It has a preexisting 2 inch conduit. The subpanel is supplied with 8AWG from the panel in an underground metallic conduit. I don't understand the comment "EV chargers require 125% or the wire size identified by the manufacturer." Can you please explain? – NYMike Apr 15 at 3:42
  • The subpanel is also Murry. Where can I find out what breakers are listed for what panel? I want to do the right thing for things I touch. On the other hand, I'm super hesitant about touching the main panel, not least because it can never be completely disconnected from the grid. – NYMike Apr 15 at 3:56
  • 2017 NEC 625.42 says EV chargers are to be treated as continuous loads, so are subject to 125% requirement of 210.20(A), so a 45A charger requires a 60A Circuit breakers and 60A wire.. – NoSparksPlease Apr 15 at 9:03
  • Also the NEMA 10-50 receptacle on the lower left of the subpanel is only legal for use by ranges and clothes dryers and only on existing circuits where the Equipment Ground Conductor is not present, and at very least the metallic box assembly provides an EGC, so is not legal, and the other receptacle is a NEMA 6-15, which needs a 15A breaker. – NoSparksPlease Apr 15 at 9:21
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Looking at your sub it was done correctly , the white neutral bond was not connected so you can connect the 14-50 , you will need to remove one of the double pole breakers and yes you use all 4 wires 2 from your new 50 amp breaker 1 ground and 1 neutral. , great question and good photos. Make sure to get the correct brand and type of breaker for the sub I can’t quite make out the brand but the main is a Murray , but the tandem breaker is Bryant this is something I have seen many times and usually not code compliant for 2 reasons , Bryant breakers were not listed for a Murray panel even though they fit. Item 2 is early panels were limited on the total number of spaces so manufacturers came up with “cheaters” early tandem breakers that did not have a rejection feature allowing the buss to be overloaded, these breakers usually have a sticker stating they are Non Current limiting. (Just want you to know). If you can find the proper double pole breaker for the sub all may be fine but I would want the main panel updated so there is not a problem if there is a fire later even though everything on the sub is correct there could be problems because of the alien breakers in the main.

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    If run in metallic conduit as the other items are, there's no need for a ground WIRE. i.e. that "black white red" circuit is actually a "black white red AND metallic conduit ground" circuit. 4 "wires" even if you only see 3 of them. – Ecnerwal Apr 15 at 1:38
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The last person did a poor job.

Change the left socket to NEMA 14-50

You cannot keep the old left socket. If you have something that needs it, you'll need to change its plug to NEMA 14-50.

The highest priority is removing the left one. That should be smashed to bits. It's the obsolete/dangerous NEMA 10 type which provides neutral but not ground. Those were not legal to install at the time this one was installed; I cannot imagine what they were connecting it to. Further, it's a NEMA 10-50 type, and it's on a 30A breaker: the breaker must match the socket.

  • So, obtain a NEMA 14-50 and a 4x4" steel junction box domed cover that will fit it. (that same cover might work). Install it.
  • Get a 50A breaker. Install it to replace the 30A.
  • Get three wires in 6 AWG THHN (8 AWG THHN allowed if the socket is stamped that it is rated for 75 C). White and 2 black. You do not need a green.
  • Use the 3 wires to cable up the NEMA 14-50's 2 hots and neutral. 2 hots to the 50A breaker. Neutral to the neutral bar.
  • Ground is handled via the mounting screws, box cover, box and conduit nipple to the subpanel. You can wire a #10 ground if you really want to.

The right socket can't stay that way.

Just look at its little face. "NOPE!"

That is a NEMA 6-15 single receptacle on a 20A breaker. Remember? Socket sizes must match breaker sizes exactly. 15 is not 20.

So change it to a 15A breaker.

Or change the single socket to a NEMA 6-20 (easy enough). Anything that plugs into a 6-15 will plug into a 6-20.

Or... there is an exception that allows 15A sockets on 20A circuits if there are 2 or more sockets. They make a "duplex" 6-15 recep, it looks exactly like a normal recep except it's saying "Nope". Just get a domed cover for a duplex recep (very common, under $2) and you can fit a duplex 6-15.

Check that this panel actually is a Siemens/Murray

The last guy loved to do things wrong. This panel has all Siemens Murray breakers in them. Make sure it's a Siemens Murray panel. You must use the same brand breaker in the same brand panel, UNLESS you are dealing with a UL-Classified breaker specifically made by a competitor for that panel.

Change that red and blue breaker to a Siemens

That blue/red breaker is a BRyant breaker. It does not belong in a Siemens/Murray panel. It is unsafe and you have a good chance of burning up the bus stabs, because the stabs don't fit properly and they'll arc under heavy load. You need the Murray equivalent of that thing, called a Murray MP25020. (under $30).

Note where that BR breaker says "Non common trip". That's not allowed for a subpanel feeder. It requires common trip! The linked breaker specifically says "Inner poles have common trip".

Get much bigger subpanels

You have a main panel and at least 2 subpanels in that house - I know that tiny 12-space "main" can't be the only other panel in the house. All of them are amazingly overstuffed or just way too small. I would not disturb the actual main panel. But definitely change out the subpanels to much larger units. Spaces are cheap. Having to wrestle with problems because you're out of spaces is expensive.

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  • BTW: in order to use the 8AWG wire, the OP will need to make sure their NEMA 14-50R is 75degC rated – ThreePhaseEel Apr 16 at 11:42

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