Warning to check outlets before plugging in high powered EVSE

A friend recently purchased a house, built in the early 00s, one thing I noticed was a nice 6-30 outlet in the garage along with a 6kw electric heater mounted to the ceiling with a corresponding 6-30 plug. I measured the outlet early on and noted it seemed to have power, 120v on each leg, 240v between the hots. It had a 30a breaker feeding the outlet and I thought it would be a nice way to charge my Leaf at up to 24a should I take it there. I've been taking the Prius because it's around 60+ miles one way and on a crazy freeway where if you're not doing 75mph you'll get blown off the road, it's only two lanes each way and almost always bumper to bumper in both lanes. IMO the particular stretch of I94 is the busiest and fastest stretch of freeway in our state, a major artery for people going E/W, both for in-state travel and people just passing through. Because of this, I figured I'd need all of my 11 bar 24kw Leaf battery, I wouldn't even attempt it with my 8 bar Leaf. Once to his house, I'd need to be plugged in the whole time I was there and the fastest charge rate the better, at 24a it would take around 5h for a full charge.

Anyway, I was there yesterday again with my Prius and this time he asked if I could look at his heater and why it wasn't working. I popped off the bottom of the heater and noted I was only getting one hot. I unplugged the heater and like before measured 120v to each hot, 240 between them. I figured it had to be either something in the plug or the 4' of flexible conduit between the two. I first took apart the plug and immediately saw the problem, the wire going to one of the prongs was burnt completely through, a chard mess, I could also see the prong had a bluish tint, probably from getting so hot. I couldn't figure how or why it had gotten so hot but when I looked inside the slot in the outlet I could see one of the hot sides had more space(not as tight) as the other side. I shut the 30a breaker off, measured for any voltage to the outlet and when I verified there was none I took the melted prong out of the plug and holding on to the back end with a needle nose pliers I manually inserted the plug into each side and found one side slid in and out much easier than the other, probably resulting in some resistance which after a long time could have easily caused heat build-up, possibly resulting in the melted plug.

Next, I removed the cover plate to the outlet and to me the wiring didn't look like the minimum 10g wire I would have expected but lucky to be 12g. Whoever ran this outlet, and I totally believe it was done after initial construction, probably by the homeowner or a clueless friend, had used Romex wiring inside a narrow plastic conduit(1/2"?) I don't know how they managed to pull it through but they did. Initially, I couldn't see enough of the Romex jacket to see the gauge listed on the jacket but I was sure it wasn't 10g. Well as long as I had this much time invested and was planning on replacing the outlet and plug I pulled the outlet out and then I could see the 12g on the side of the jacket :shock: So whoever ran the wiring used 12g inside a tight conduit, used a 30a breaker and a 30a plug, all hooked to a 25a heater than probably ran 24x7 or damn near!

I can't believe that the wiring never melted or burned down the house but for the most part, the Romex looked fine, it was just the plug that failed, melted into a globby mess. Needless to say, I'm going to rework the outlet for my friend, take out the 30a breaker, inspect the wiring going to it, and hopefully, it will look OK. Next will be to get rid of the junky 6-30 outlet and plug and instead install a quality L6-20 twist-lock plug outlet to his heater and lastly rewire the heater to only draw 13.7a which according to the schematic on the bottom of the heater is possible by adding/removing some jumpers. Currently, the heater looks to be configured for the max 6kw, which again I can't believe didn't cause their 12g wiring to fail.

Long story short, I'm sure glad I didn't plug my Leaf into that outlet and charge much, kinda a bummer I'll be restricted to 16a in the future as he will for his ceiling heater but the outlet is quite a ways from his basement breaker box and has several areas with sheetrock ceilings so running new 10g or if I were to go through that much trouble I'd probably want to run 8 or even 6g so I could put a 14-50 RV plug for him. Boy I can't believe how someone would have done that, as I told my friend, whoever wired that outlet "knew enough to be dangerous" :shock:

12g Romex is rated at 20A at 140 degrees F. If the run was indoors even in conduit, at 25A draw, it would probably never overheat. There's a lot of "fudge factor" in the electrical tables just for safety. The real problem was the loose connector arcing.. Code does call for 10g for a 30A plug and 6g for that 50A circuit ( you could get away with 8g THHN in conduit by code but most inspectors won't pass it).

jjeff said:

I can't believe that the wiring never melted or burned down the house but for the most part, the Romex looked fine, it was just the plug that failed, melted into a globby mess.

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It was luck on their part. The short run was probably just low enough resistance to be functional for the heater without catching fire, though I imagine the wire probably did get too hot to touch many times, just not enough to burn down the house. There is a lot of fudge factor on wiring just for that reason, if it was a 60' run, it would have probably gotten hot enough to catch fire then.

knightmb said:

jjeff said:

I can't believe that the wiring never melted or burned down the house but for the most part, the Romex looked fine, it was just the plug that failed, melted into a globby mess.

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It was luck on their part. The short run was probably just low enough resistance to be functional for the heater without catching fire, though I imagine the wire probably did get too hot to touch many times, just not enough to burn down the house. There is a lot of fudge factor on wiring just for that reason, if it was a 60' run, it would have probably gotten hot enough to catch fire then.

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Yes I'd guess the run to be half that, around 30 feet. It's amazing the electrical "issues" I've run into in this house. My friend wanted almost all the ceiling lights replaced. Two of the ceiling fixtures had the insulation on the wiring burnt to a crisp but the bare wiring survived, very close to touching the metal fixture though, I'd guess they had a 100w or two in fixtures with a maximum rating of 60w max. Then there was another fixture he kept but when I put in a LED bulb it no longer worked......put the incandescent back in and it lit :? I tried another brand LED and it lit :? I discovered someone at one point had wired the bulb socket backward, the hot ran to the screw part while the natural was wired to the center! Obviously, this is a safety issue as of someone was unscrewing the lightbulb and had their hand too close to the base of the bulb and touched the screw part of the bulb(and the light switch was on) they'd be basically touching 120v! Apparently, one brand of LED light was OK with this while the other must have expected the hot to be the center and neutral to be the screw. The bulb that didn't work in this fixture worked fine in another normally wired socket. Needless to say I reversed the wiring and now he should be able to purchase any light and have it work.
I rarely run into this type of thing in the commercial buildings I work in but I'm guessing it's more common in residential settings and that's a little scary. It's one of the reasons whenever I'm plugging in my high powered EVSE into an outlet I haven't used before I always like to unplug it and feel the plugs a few minutes after the charging starts and then again 15 min or so after charging, if I feel more than a little bit of warmth I either stop or dial down the setting on my adjustable EVSE and repeat the process. Of course I don't know whats going on near the breaker or any junction boxes but my adjustable EVSE also has a voltage meter so if I see voltages lower than expected that's kind of a warning sign too.

I saw a similar situation to what Jeff described in an RV park. I had to replace the plug on my friend's RV as well as the receptacle (it was on a Sunday so there was no way to get the park to fix the receptacle) in order to restore power for the refrigerator and lights. It was a 120-volt TT30 receptacle at the pedestal and it had been wired with No. 12 wire for one terminal and No. 10 wire for the other terminal. The terminal with No. 12 wire got hot and that heat transferred to the plug on the RV cord. Both the receptacle and plug were melted near one terminal. The other terminal (with No. 10 wire) was fine. I replaced the receptacle and plug so my friend would have the refrigerator and lights (could not run the air conditioner until park maintenance personnel replaced the No. 12 wire).

In the mid Nineties I wanted to move our computer from a downstairs sun porch to my bedroom. The outlet where I wanted to located it wasn't grounded, so I went up to the attic to see about running 10' of new cable for a new, grounded outlet. I encountered a knob & tube setup, with the insulation completely gone from both wires.

A year later I was still replacing runs of wire. I had to "retire" one whole circuit because it was convoluted, and at some unknown point inside a wall was fed by the same knob & tube wiring. My housemate's Father had installed a new 200 amp service, with me helping and then taking over for things like a dryer circuit, but he had balked at replacing the seeming miles of old romex and pre-romex cloth-covered cable, instead running a new cable to the old fuse boxes from the new panel. After 40 years I'm still replacing outlets and pulling romex.