Burnt 50A 3 prong range outlet: DIY replace, have professional do it or rewire to 4?

Question

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 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:

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:

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.

Answer 1

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.

Answer 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.