carrier electric furnace keeps blowing fuse

[hvacMartinez]

I am trying to figure out why a furnace keeps blowing 40 amp time delay fuses. It has 2 heating elements at 14 and 14.5 ohms each, I also pulled them out and visually inspected. The blower spins freely and the capacitor has 300 + volts. Each leg is pulling 35 amps when both elements are energized at 248 volts. It will run for about 15 to 30 minutes before it blows the fuse. The transformer was humming really loud so I replaced it. I don't know what else to check.

[mofotech]

Shouldn't you have a 60 amp on there? How many KW are the heating elements in total?

[beenthere]

Probably cause they are a bit under sized for the continuous load they are on. If you didn't include the blower in your amp check. Check it, and you'll see your close to 40 amps. If you included the blower motor amp draw in your reading, then you need a 45 amp fuse, 50 if blower not included(based on 8.6KW of strip heat).

[hvacmontana]

Code requires the heating circuits to be fused at 125% of load, this calls for 30amp protection for each 5 KW of heating element.

[hvacMartinez]

Thanks for the replies, there are two elements at 3.5 kw each. I got the amp reading(35 amps) from each of the lines coming in. I checked the current through one element and it was around 15 amps and the fan was pulling 1.5 amps. The name plate calls for 40 amp over current protection and it is on a 40 amp breaker.

[timebuilder]

Five posts and no one has asked this....???????

ARE YOU A TECH?

[hvacMartinez]

Yes sir, brand new though. Don't know what else to check.

[dandyme]

are all the line voltage connections tight?

is there anything flopping around near the elements?

are the fuse clips tight?

[dgussler]

Sounds like loose connection somewhere. I would get someone more experienced to check it with you ,no use in taking chance of burning house down due to wire getting too hot

[timebuilder]

Yes sir, brand new though. Don't know what else to check.

Okay, then.

Work on getting your post count up to 15 so you can apply for Pro membership. It's free, and it will allow you access to the educational forums and the Pro Technical forums, where we really dig into technical problems. We stay pretty vague out here in the open, as the search engines index these pages.

On the equipment label, you should see a listing for the maximum overcurrent device. Did you see that value?

[hvacMartinez]

I checked all the connections and everything is tight. While I was doing that I noticed that the fuses were really hot. The fuses fit tightly, is it possible the disconnect is loose and that is what is causing it to get hot and causing the fuse to blow? Should I replace the disconnect?

[Sleuth]

.

Is the cooling coil very clean?
..

[timebuilder]

I checked all the connections and everything is tight. While I was doing that I noticed that the fuses were really hot. The fuses fit tightly, is it possible the disconnect is loose and that is what is causing it to get hot and causing the fuse to blow? Should I replace the disconnect?

The test for the disconnect is that when the connections are together, and the unit is under its normal operating load (in this case, making heat) you look for a voltage drop across the disconnect.

Once again, this is not a forum for in-depth technical discussions. Your service manager should be able to walk you through testing the drop across the disconnect. A high resistance in one of those connections will act like a little heater.