The Tank › Fuse box overheating on 60 gallon electric water heater
- February 6, 2010 at 6:47 pm #12815
I have a two-fuse fuse box supplying power to a 60 gallon Giant electric water heater. Yesterday, the right fuse (30amp) became so hot it partially blackened the socket into which it is screwed along with producing a burnt Bakelite type of smell. I unscrewed the fuse and it was extremely hot (although surprisingly it did not blow). I checked the elements for continuity and shorts and both appear good.
What could be overheating the fuse so badly?
Thanks, Valtec.February 6, 2010 at 11:44 pm #12822
Hello: This is unsafe 😯 All connections in the fuse-box need to be looked at as there is likely a loose one that’s shorting. I’d replace the fuses as well and better yet think about breakers. There are breakers made to fit the screw in type fuse holders. Sorry to say it but you need to turn off the heater until the box is looked at.
Yours, LarryFebruary 8, 2010 at 9:14 am #12837
I checked the fuse box (it’s only two years old) and everything is OK. The white wire from the entry panel leading to the hot fuse on the right side of the fuse box is blackened where it is secured at the terminal. Interestingly, a time delay 30 amp fuse does not blow, just gets very hot. A normal 30 amp fuse does blow after 3-4 minutes. I’ve checked both elements for shorts and they appear fine. There is some small continuity between the two left terminals of the upper thermostat. Could this be causing the right side (white wire) to be drawing more current?
Gord.February 8, 2010 at 10:07 am #12838
Hello: Is the heater running at about 240 volts? If so, there should be only two black (hot) wires and no white (neutral) involved. If the fuse/breaker overheated that much, replace it. Check for continuity/leaks to ground in the heater. That’s done by disconnecting wires and checking at each screw terminal and at ground. Put the meter in ohms times 1000 or some similar sensitive range.
I’d treat this like you were going to a job to troubleshoot and assume nothing about the condition of the fuse box or wiring. If it were right, there could not be overheating in the box. Do let us know what you find 😎
Yours, LarryFebruary 8, 2010 at 6:34 pm #12841
If WH was wired using 2 wire (with ground) nonmetalic sheathed cable, there very well could be a white wire. Electrician should have taped the white wire black but I see a lot of cases where it was not.
DavidFebruary 9, 2010 at 12:11 am #12844
Hello David: I once ran across a job where the green wire was hot and the red was neutral 😯 It’s sort of like breaking the speed limit while driving on the wrong side of the road, backwards. Carrying a meter is good.
Yours, LarryFebruary 9, 2010 at 8:30 am #12849
I tested both elements and there are no shorts – but they are 10 years old and maybe calcium build up is making them run hot. I figure I’ll change both elements and maybe the two thermostats to boot and whatever caused the fuse box to overheat should go away.
ValtecFebruary 9, 2010 at 9:01 am #12850
A 4500 watt WH element is 12.8 ohms whether it is new or 20 years old, whether it is clean or scaled up. The scale will act as an insulating layer and cause the centerline temperature to increase in order to drive the heat flux through the scale. But nothing about the WH will cause the fuse box to over heat.
You did not specify your element size. Normally WHs operate one element at a time. If your upper thermostat malfunctioned and you had both elements energized simultaneously and you have 3800 watt elements and voltage was 230 instead of 240 then both elements operating together would draw 30.34 amps.
If you change anything, the top thermostat is most suspect. But I’d prove it with a meter before I started changing parts wholesale.
DavidFebruary 9, 2010 at 9:49 am #12851
My tank has two 4500W elements and measured voltage is 234 so you could have a point. I tested the upper thermostat with an ohm meter by placing a lead on each of the two left-hand terminals as instructed by someone on this site. There was some continuity which according to the instructions would indicate the thermostat should be changed out. Do you think the thermostat could be malfunctioning intermittently? I tested it yesterday and it seemed to be functioning properly (sending 230 volts to the upper element and about 48 volts to the lower, then switching to the lower with about 230 volts once upper tank water temp reached set level).
Valtec.February 9, 2010 at 10:13 am #12852
When thermostats misbehave they usually continue to misbehave. Your results seem to indicate a good thermostat. The top thermostat sends power to either the top element or the bottom thermostat. Of course the other (non-thermostat) side of each element is always hot.
DavidFebruary 10, 2010 at 12:56 am #12859sky_techParticipant
You say a normal fast-acting 30 amp fuse blows in 3-4 minutes. If it’s really a 30 amp fuse, that would argue that something is indeed drawing a too much, as a single 4,500w element at 230 volts should only draw about 20 amps. So either you have another load also running off that fuse (trace out the wiring) or somehow the water heater is energizing both elements at once. Any chance you tested with a 20 amp fuse by mistake? (Somebody help out – any chance those element draw a lot until the get hot? Some heating materials have what is called high temperature coefficients, meaning they have a different resistance when cold (simple tungsten light bulbs are a great example). Seems unlikely to fully explain your problem, though, as the localized damage to the fuse-box tells a different story. See next paragraph.
Assuming the heater wiring checks out, it is quite possible that the problem is not the water heater and it is not that something is drawing too much current (e.g. no shorts). The super hot fuse and blackening sounds like there is a poor connection in the fuse-box. That is to say, something is (or was) not tightened properly. What happens is that the poor connection doesn’t conduct electricity very well and gets warm due to the current flowing through it to the heater. Your heater draws about 20amps. If the connection to the fuse had a 0.5 ohm resistance, the current would cause 10 watts to generated at the fuse (Think a small light bulb’s worth). That would cause the connection to get very warm. The heat causes local oxidation of the connection which makes it worse and you get more resistance and more heat. Eventually you end up with the wire, the fuse socket, etc getting extremely hot (but not enough to blow the fuse). While not common, it is certainly not rare. Note, the heat damages whatever is loosely connected as well the socket/wire/etc nearby. It might have started with a fuse that wasn’t screwed in tight, or a wire that wasn’t securely connected to the fuse box. Any good electrician will know what to do. If you want to handle it yourself, you will need to kill the power, partially disassemble the fuse-box, and replace anything that shows signs of heat stress. A little bit of darkened insulation of a wire is not the end of the world, but charred insulation needs addressing (replace the wire or reinsulate in a code-approved manner) AND any damaged fuse socket needs replacing. The spring contact loses its springiness and contact plating get damaged, etc.
Hope this helpsFebruary 10, 2010 at 8:46 am #12860
I`ll check the fuse box this weekend (the water heater is in a country home that we use only on weekends).
Valtec.March 10, 2010 at 4:14 pm #13098
I checked the fuse box and found the overheating problem was caused by what must have been a malfunctioning TD type 30A fuse. It was carrying current to the elements but getting extremely hot in the process (hot enough to scorch surrounding wiring). Naturally I thought it was something to do with the heater but after changing the TD fuse out for a standard 30A fuse, everything is now working perfectly. Thank you all for your considered advice.
- You must be logged in to reply to this topic.