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Just bought a house built in 1979 and am working on renovations. Today we had a new range delivered and ran into an issue trying to install it. Essentially, the old range was connected to a 30A "dryer socket" in the kitchen while the new one requires a 40A "range socket". The delivery guys were adamant that all we had to do was pop over to a store and swap out the receptacle in order to get it working.

Based on Dryer Outlet in the kitchen?, I'm skeptical that it will be so simple. Checking the breaker box: breaker box

and wiring description: wiring description

shows that the old circuit has a 40A breaker. Given that, is it ok to assume that the wiring for this circuit is also safe up to 40A and I truly can just swap the receptacles and call it a day?

30 amp dryer plug is hooked to a 40 amp double breaker, is this ok? indicates that the current setup maybe shouldn't have existed in the first place so I'm trying to be cautious about how I fix this.

Edit: here are some pictures of the wiring open socket close up of the back

Edit 2: full electrical panel and back of receptacle full panel back of receptacle

Wire Gauge:

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    What's going to determine if you can simply swap outlets or if you actually need to upgrade the whole circuit is going to be the size of the wire. See if you can see any markings on the cable jacket that indicate what size it is. Or, if you can't find anything, with the power turned off, you can use a wire gauge to check (example: amazon.com/SE-JT47WG-C-Dual-Sided-Non-Ferrous-American/dp/…) – Nate S. Feb 10 at 17:23
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    Also, since you're renovating anyway, is the current circuit 3-wire or 4-wire? If it's an old 3-wire, it would be a good idea to take this opportunity to retrofit a ground wire. – Nate S. Feb 10 at 17:25
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    @JimStewart, it's not an answer yet; it's asking clarifying questions. Once OP provides that info I will. – Nate S. Feb 10 at 17:26
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    Just a clarification, NEMA does not designate a 40A receptacle, the odd code specified receptacle for a 40A breaker is a 50A receptacle (NEMA 10-50 or 14-50). The cord is spec'd 40A due to wire size. Please do update wire size connected to breaker, but really it is unlikely that somebody ran wire smaller than size capable of 40A for a 240V range. – NoSparksPlease Feb 10 at 18:10
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    We need to know the size, metal (copper vs aluminum) and insulation type of the existing wires, so we can assure the wire is ready for 40A. We also need to know if there's a metal conduit, metal jacket on the wire, or ground wire. – Harper - Reinstate Monica Feb 10 at 19:36
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It appears to me that this cable has two insulated conductors for the two hot legs plus an uninsulated conductor to be used as a ground, i.e., it does not have a third insulated cable to be used as a neutral. The cable is wired to the receptacle using the uninsulated conductor as a neutral. This is not allowed by code.

This cable can legally supply 240 V, not both 120 V and 240 V. It is not suitable for any appliance which requires both 120 and 240 V. What are the requirements of your new range?

Unless your new range is a special design which requires only 240 V, you must replace this cable all the way back to the panel or subpanel if there is one. Look at the installation instructions for the range. It should tell you whether it requires a neutral. If it does, then you will need a new (4 wire) cable.

You could use a 3-wire appliance cord and plug it into this receptacle and the range would work perfectly, but if the range requires a neutral, then current will regularly be flowing in the uninsulated conductor. This is not allowed by code.

If the cable had 3 inslated conductors, you could use it as is or you could retrofit a ground, but you cannot retrofit a neutral.

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    The OP's install falls under the "grandfathering" clause in the Exception to NEC 250.140, and they also could retrofit a grounding wire to this setup if they wanted to move to 4-wire, especially if this cable is 4-4-4 instead of 6-6-6 – ThreePhaseEel Feb 11 at 2:05
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    Thanks for the help, everyone. It sounds like I just need to bite the bullet and have a professional upgrade this to a 4 prong setup. A couple more days of microwaving food is a small price to pay for having a safe range :) – 5E4ME Feb 11 at 2:14
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    If the cable for the range is wrong (and this has yet to be confirmed), then one must suspect the wiring for the electric dryer. Modern wiring for electric dryers is 4-wire just like wiring for a range. If the OP has an electrician in for the range he should have the dryer wiring upgraded to 4-wire at the same time, if his dryer requires that. In our 1970 house, the range was the was 4-wire, but the dryer 3-wire. I have yet to rectify the dryer situation, but I intend to retrofit a ground and change to a 4-wire cord – Jim Stewart Feb 11 at 3:11
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    This answer misses the key points about how range plugs have changed over the years and why and it misses the clear and unambiguous options OP has to deal with it. – J... Feb 11 at 14:34
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    @JimStewart, what you're missing is that this type SE cable has an uninsulated neutral and no ground. It's probably the only type of cable that does not insulate the neutral, but it is designated for use as a neutral by code nonetheless. – Nate S. Feb 11 at 17:44
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Go for it, for now at least...

You appear to have at least 6AWG if not 4AWG aluminum present there, so you will have no trouble with a full-sized range circuit using the existing 40A breaker and a NEMA 10-50R receptacle. Since this is aluminum wire, though, you'll need a Cu/Al rated receptacle, and also to make sure you use anti-oxidant grease on the connection and a torque screwdriver to accurately tighten the lug screws to the specification torque.

You'll want to find a way to run a separate grounding wire back to the panel or the grounding electrode system in the future, though

In the future, though, you'll want to run a 10AWG copper wire from this box back to the panel, or any other point on the existing grounding electrode system, so that you can replace this (hazardous) NEMA 10 with a (much safer) NEMA 14 receptacle and convert your range over to the matching 4-wire (NEMA 14) plug.

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    It is aluminum wire and OP also needs to be careful about making those connections on the new outlet. They will need to ensure the 10-50R outlet is rated for #4AWG aluminum and they will need to refresh the anti-ox paste and torque the lugs correctly. Workmanship is very much more important when fitting aluminum. – J... Feb 11 at 14:39
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Aside from what else is going on, this is a goobed up mess.

The #1 problem is that a junction box can't just have a hole bashed in it by smashing a rock against it. The cable entering it needs to have a proper cable clamp, which means the box needs a proper knockout.

The cable is #4 SE cable, and that cable can do 2 interesting things: First, that bare wire actually does get to be a neutral wire. And second, it can run at 75 degrees C - we can actually get 65A out of this wire!

However, it has a nearly fatal flaw: It is definitely aluminum wire, and that means it cannot splice to a receptacle rated for copper only. (remember; that's what caused all the trouble in the 1970s!) What's more, I would be very surprised if a 30A receptacle was rated for #4 wire, since it only needs #10 copper (or #8 aluminum if it accepted aluminum). So I am suspicious as to how the wires were attached to the NEMA 10-30 recep.

If the receptacle is not rated for aluminum, or not rated for #4 wire, then you will need to pigtail it with #4 Polaris connectors. This will require significant room in the box.

The size is #4. The size of the individual wire strands is utterly irrelevant, as you cannot use the wires individually. At all. The only purpose of stranding is to increase the flexibility of the entire wire. You must not remove some strands to get a wire to fit a too-small connector.

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  • I don't think you can run the SE at 75C here -- as of the 2008 NEC, when it's indoors used as a branch circuit or feeder, rather than as a service conductor, it's limited to 60C. There's a discussion of that here: forums.mikeholt.com/threads/ser-cable-temperature-rating.47880 – Nate S. Feb 11 at 17:16
  • Wait, after researching it a little more, apparently that section changed again in 2014, 2017, and 2020 code updates, so the exact rules will depend on what OP's jurisdiction has adopted. Possibly, depending on the year that applies, whether the walls it runs through are insulated will make a difference. If it's 2017 or 2020, most likely the 75C rating can be used, so I was (mostly) wrong. – Nate S. Feb 11 at 17:28
  • What are the proper uses of SE cable? – Jim Stewart Feb 11 at 17:34
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    @JimStewart Yes, sometime in the early 90s the 3-wire connections were outlawed. Til then, you COULD use SE cable since the bare wire was neutral. You could also use /3NM without ground (white neutral) which is what you have, but that was nonexistent once the old stock ran out. Lots of people just switched to using /2NM w/gnd, but that was never legal. /2 w/g NM's ground wire is NOT allowed to be neutral. – Harper - Reinstate Monica Feb 11 at 19:53
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    @NoSparksPlease -- that's a reasonable interpretation, at least. TBF: old SE-cable range runs like the OPs don't really have a particularly good upgrade path short of lobbing a transformer-and-subpanel setup or a brand-new cable run at the problem – ThreePhaseEel Feb 12 at 0:37
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That cable appears to be type SE cable, and so it would be allowed by the exception to NEC 250.140 to replace the 30A receptacle with a 3-wire NEMA 10-50R. Below is the code section you can read. It seems to me your existing installation fits the conditions, even condition (3) about the uninsulated cable.

It would be better to upgrade to a 4-wire, but not required.

Reading the section below can be confusing, understanding of Code terminology is critical. "Grounded Circuit Conductor" is what is commonly called the neutral, and the "Equipment Grounding Conductor" is the ground.

250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.

Exception: For existing branch-circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met. (1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system. (2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum. (3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment. (4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

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You are right to be skeptical. The components in your circuit are the breaker, the outlet, and the wiring in between, which is the unknown in this case. If the wire is the right gauge (8 I think), it's condition is good and has been run correctly you could change the outlet, if it's too small a gauge you could overload the wiring.

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  • #6 aluminum is also rated for 40 A. – Jim Stewart Feb 10 at 17:34
  • Good point @JimStewart, I figured given the age of the setup aluminum was unlikely, but it's good to be complete. – GdD Feb 10 at 17:35

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