1

A couple of weeks we purchased a house built in 1981. First time we tried to cook something in the oven, we heard a loud pop from the wall, so I flipped the breaker and decided to take a look. What I saw after removing the outlet is this:

The outlet

The outlet is a 50A 3 prong one.

Judging by the stains on the wall, looks like something was spilled at some point and the outlet shorted.

The oven terminal was not in a great shape either:

the terminal

Judging by the corrosion, it went bad a while ago.

The questions are:

  1. The fact that the breaker didn't flip on its own -- is it a cause for concern?
  2. Is this replacement still in DIY territory? I have replaced a few outlets in my life but never had to strip the wires or do anything on a 240V outlet. I have also have never seen weaved aluminum(?) neutral. I suppose I should just try to snip it.
  3. There doesn't seem to be any ground in the box, but would it make sense to have a professional upgrade this outlet to a grounded one while we're at it? We are planning to use an induction oven with this outlet, but from what I understand the power requirements are the same. Current range "Minimum Circuit Required" is 40A, future range "Required Power Supply" is 40A. Both manuals have same sections on wiring up a 3 wire receptacle with two hots and a neutral.

Current breaker is 40A, here is the picture: enter image description here

TL;DR: based on the answers the best course of actions is to have such 3 wire outlet properly grounded and replace the twisted aluminum with copper wire, which is what I ultimately did. Spend a few bucks on a professional electrician -- save a few on a professional mortician.

4
  • I hope oven replacement is a "now" thing in this question, not a "future" thing. If not, that requires repair itself. I'd also want a serious chat (and fee clawback) with any home inspector, if you used/paid for one and this happened "the first time you tried to use the oven."
    – Ecnerwal
    Sep 1, 2023 at 4:03
  • > I hope oven replacement is a "now" thing in this question, not a "future" thing The repair is a "now" thing bound by the lead time of the range. I rather like the new house and would prefer to keep the current state of my family's limbs. Sep 1, 2023 at 5:23
  • Is this a wall oven (so oven only) or is it a stove (aka range) with a cooktop and an oven below? The problem may have been loose connection or connections on the block which caused heating and arcing. If so, there would have been no short circuit or overcurrent to trip the breaker. Sep 1, 2023 at 18:09
  • Do you have instructions for the current appliance? Some cooktops and some ovens may require only 240 V and those would not use a neutral. AFIK most older stoves (cooktop and oven in one unit) do require 120 V in addition to 240 V and so did rerquire a 4-wire receptacle for safest operation. Sep 1, 2023 at 18:50

2 Answers 2

5

Yes, this should be replaced. But first you need to figure out what you have and what you need.

A 50A receptacle is standard for both 40A and 50A range circuits. That is allowed by code. It is one of the only exceptions to "receptacle must match breaker exactly". (The other exception is the extremely common 15A receptacle on a 20A circuit.) So the first step is to find out:

  • What circuit size does your range actually specify. Typical is 30A, 40A or 50A.
  • What breaker is currently in place. This is actually the easiest part to replace.
  • What kind of wires/cable do you have.

We actually know part of the wire/cable type. You have an old 3-wire cable without ground. That has not been allowed for decades, but it was likely allowed when your house was built. But now is the time to replace it. But replacing it can be anywhere from trivially easy to supremely hard, depending on the distance between the range and the breaker panel and the access to run a new cable.

Once you know figure out how to run a new cable, the rest is relatively easy. You can either replace the cord/plug with a new 4-wire cord/plug (NEMA 14-30 for a 30A circuit, 14-50 for a 40A or 50A circuit) or, if your range allows it (most do, but check the installation manual) hardwire it with a wire whip. And, critically, you must make sure the cord or wire whip is wired up so that there is no neutral/ground bond in the range. That typically means removing a screw or wire or clip or something, but it varies by manufacturer.

As far as the actual cable to use, normally it will 10 AWG copper for 30A, 8 AWG copper for 40A or 6 AWG copper for 50A. You can use larger wire, to plan for a future range that uses more power, but the breaker size should match the existing range specifications.

As far as "the power requirements are the same", not necessarily. There is actually quite a range (pun intended) of range power requirements. Check the specs of old and new. But if you go with a wire whip instead of cord/receptacle and use cable large enough for 50A then even if it is 40A now you can upgrade to 50A later just by swapping the breaker.

7
  • Thanks! Both ranges specify 40A as the requirement, so looks like the breaker can stay for now and in theory complete like for like outlet replacement is possible. Can you clarify the "replace" part of your answer -- does it mean removing and replacing the aluminum twisted wire or only adding a ground to this receptacle? Sep 1, 2023 at 5:40
  • 2
    In theory you could run a separate ground to this box, insulate the bare neutral so it does not contact the box or the ground wire and turn this into a 4-wire connection. Also known as "retrofit ground". But since a 40A circuit needs a bigger ground than you likely have elsewhere in the kitchen, that likely means running a new ground wire from the panel, which is often as hard as running a whole new cable. FYI, if the new range has both "3 wire" and "4 wire" instructions then you really, really, really should do a proper 4-wire (hot/hot/neutral/ground) connection. Sep 1, 2023 at 5:45
  • At least in a 1981 build you're way more likely to be dealing with drywall (easy to repair) than plaster and expanded metal lath (PITA to repair)!
    – Huesmann
    Sep 1, 2023 at 13:18
  • What do you mean by "both ranges"? Sep 1, 2023 at 18:57
  • 1
    I personally don't have full confidence in the quality of connections of stranded aluminum conductor in some of the NEMA 14-50 receptacles, even though they state Cu or AL. I prefer connections that confine the stranded conductor and prevent it from splaying out under the pressure and twisting action of the screw. Still it would be a lot cheaper to retrofit a ground and use the exisiting 3-wire cable. Tempting, but the best thing would be to replace the cable with 4-wire copper of the appropriate size. Sep 1, 2023 at 19:14
2

It takes a great deal to trip a 50A breaker. Not tripping it with a minor fault is not unusual. When my oven element arced to death, my 50A breaker said, "hey, not over 12KW, not my problem, buddy!"

If you intend to connect a device that requires neutral, I would strongly urge rewiring with a ground and a 4-wire outlet. NEMA-10 ungrounded receptacles have been banned other than where grandfathered since the 1996 code cycle, for the usual code reason i.e. dead people. Code is written in blood.

If you intend to connect a device that only needs two hots and ground, I believe you can repurpose that particular type of neutral as ground.

If the experience of what a 50A breaker will tolerate without tripping has made you more willing to open your wallet, you might consider a rather expensive and probably not presently required GFCI or AFCI two-pole breaker as well. But don't buy a 50A one if your replacement appliance only needs 40A, or 30A, or 20A. That will trip faster on a fault without requiring that the fault be more than full rated power, as a standard breaker does.

There are certain breakers out in the world that may not trip at rated current (Federal Pacific and Zinsco being two examples - you can post a picture of your breaker panel for advice on if that might be contributing) but in my case it was a good old reliable SquareD-QO, and the fault just was not big enough to bother it, so it merrily arced away. I shut it off rather than waiting for it to get worse.

2
  • Thanks! I updated the question with a picture of the breaker. With regards to the new range -- it does not explicitly require neutral and has a section in the installation manual dedicated to "3-wire connection: power cord". So complete like for like replacement is possible. I will be replacing the cord as well. Quote from the manual: "The white middle (neutral or ground) wire ... has to be connected to the middle post of the terminal". Sep 1, 2023 at 5:26
  • 2
    That 3-wire connection with neutral, not ground, is the same thing that was removed (finally, after sufficient dead bodies) from code in 1996. Your house is grandfathered, so you CAN do that, but it's a dubious choice. Your Homeline breaker is not one that would fail to trip on actual overcurrent, so your "pop" was presumably less than 9.6 KW worth of "pop." Or not so much over for so long as to engage the trip curve.
    – Ecnerwal
    Sep 1, 2023 at 13:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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