Water Heater blows AC fuse

[druid]

I just installed a West Marine 6 gal water heater that's supposed to draw 12.5 amps. After initial installation a few months ago, it worked perfectly: hot water in about 1/2 hr. NOW, it blows the 15 A AC fuse after only a few minutes (not immediately, so it's not a direct short).
- Has anyone else experienced this with West Marine water heaters? I had a SeaWard heater in my last boat and I had it on all the time, and it lasted for years.
- Anyone guess the possible cause? Am I going to have to rip the thing out again and take it back?

It's not a defective fuse: I've tried several. And it's not something else on the cct - it blows the fuse even if it's the only thing on the cct.

Any ideas...?
druid

 

[plenny7]

If you have a volt-ohm meter, disconnect the power and measure the resistance across the heating element.

A water heater is purely resistive so there's no overloading that can happen. I'm wondering if perhaps there's a short across a few of the windings.

 

[kloudie1]

Check the connections in/out of the fuse and holder.. a weak connection will generate heat which will cause the fuse to open at a lower amperage.. The parts that hold the fuse and the connections on the fuse unit. You can measure the current draw pretty cheaply: http://www.harborfreight.com/digital-clamp-meter-96308.html
It won't be absolutely accurate, but it will tell ya if you are drawing around 13 amperes versus 16..
Check the water heater connections at the power and common buss .. tight and clean..

 

[druid]

Good thought about the connections at the fuse. But I can run a 1500W heater on the same cct (not at the same time, obviously!) so I doubt that's the problem.
I have a multimeter - I'll check the resistance (easy!) but since it's not a clamp-on I'd have to make a jig to measure the actual current.
Trouble is, the boat is a 3-hr drive and $60 ferryride away, so it may be a week or so before I get to it.

 

[Terry Cox]

Hi Druid, 12.5 amps seems a bit high for your hot water heater compared to our new six gallon Seaward that draws ten. But it is what it is; probably a higher draw element than ours. An alternative would be to replace your fuse with one a bit higher, say 18 or 20. That is as long as everything else mentioned above checks out okay.

 

[druid]

Hi Druid, 12.5 amps seems a bit high for your hot water heater compared to our new six gallon Seaward that draws ten. But it is what it is; probably a higher draw element than ours. An alternative would be to replace your fuse with one a bit higher, say 18 or 20. That is as long as everything else mentioned above checks out okay.

Depending on what Ampacity chart you look at, my wiring is good for 20A. BUT, since the unit is supposed to be drawing 12.5 A, I'd hesitate to cover up what's obviously some kind of problem with a bigger fuse.

druid

 

[plenny7]

If this is the model installed, then 12.5A is correct. You should expect the measured resistance of the heater to be about 9.6 Ohms.

While you're at it, measure the circuit voltage. 12.5A is the rating at 120VAC. That's a nominal voltage...the actual voltage can be higher or lower than that. Our house voltage is typically 125V. On a heater, higher voltage = higher current, so on a 125V circuit the current would be just a tad over 13A, which is getting pretty close to that 15A fuse.

I don't know what Code governs boats in Canada but in a house in the US, the National Electrical Code would required the fuse be 20A for that heater as it only allows you to use 80% of overcurrent protection device rating for continuous loads

 

[Benny17441]

If the heating element had failed it would be an instant tripping of the fuse. Apparently the problem is heat build up as it takes a few minutes to blow. Check all the connections in the circuit as one could be loose or dirty.

 

[Ron20324]

As the usual, the fuse is sized to protect the wire. The appliance should take care of itself, which it seems to have been doing. The 15 is too close to the 12.5 and can blow easily as you experience- whatever the cause.. Mine is a 20 amp breaker, and it works well that way. I'd change it instead of fighting it- a very simple first step. If still a problem, then pursue that.