Hot Topics: Furnace Fuse Issues

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Original Post: Carrier packaged heat pump: cannot locate the 3amp fuse

dogdodge - Member


I am a new homeowner and was trying to change my thermostat. I flipped my breaker labeled AC but when I went to disconnect the wires it sparked. I shut off the main and rewired everything exactly the same (according to pictures) and now have no power.

My AC unit is an older carrier model: 50yq030300 ser: w115200. I changed the disconnect fuses already but am not able to find a circuit main board or the little 3amp fuse to check if it has blown. My thermostat is battery powered but is not receiving any power and will not turn on my ac like it used to.

Any help would be much appreciated.

Additional Details:

Heat Pump

Rooftop Unit

Changed both disconnects for the heatpump and ac unit with 30 amps.

I have pictures of where the wiring connects to the rooftop unit and additional pictures of the main plate and the interior of electrical components.

PJmax - Group Moderator

Welcome to the forums.

Unfortunately there is not usually a fuse in an old packaged unit. Go to the area where the low voltage controls are. Locate the transformer. That may have a fuse in one of it's secondary wires. If not..... more than likely the transformer is blown. There should be a wiring diagram on the service door. A fuse.... if there was one.... should be shown on it.

Carrier 50YQ manual (pdf)

dogdodge - Thread Starter

Hi Pete,

Thank you so much for your reply! I had been looking for that manual for hours but had no luck.

I have attached the photos as well. Do you think changing the transformer is something that a DIY'er can do?

I have found this website that says it has the transformer.

I have also found this website that seems to list out the parts but has no transformer but is for the exact model of the package.

thermostat wiring

old thermostat wiring

thermostat wiring panel with arrow pointing to piece

furnace model information

electric furnace model information

roughneck77 - Member

The transformers in the last photo by the timer.

Doesn’t look like it has current protection. You can test it with your meter to see if it’s working or not.

PJmax - Group Moderator

You don't need to use the same exact transformer. You need a 24VAC 40VA - 240VAC in.

Most replacement transformers are going to be 120/208/240VAC in. That's fine. You'll just use 240v.

Replacement transformer

I put an arrow in the picture. There will be two wires to each side of the transformer. Two wires will be the 240v IN and two wires will be the 24v OUT. More than likely you'll be missing the 24v side.

dogdodge - Thread Starter

Thank you Pete and Roughneck. I will take a closer look at the part you highlighted and order the transformer and attempt a replace myself. I will report back once I have done so.

Thank you guys so much for your help on this!! I would be truly lost without this.

roughneck77 - Member

Get a transformer with a built in circuit breaker, or install an inline fuse for future protection.

PJmax - Group Moderator

I think the circuit breaker only comes in the mega transformers.... like the 80va.

A fuse and fuseholder from an automotive store would be a good choice.

Use 3A or 4A ATC type fuses..... same as a car.

a small fuse with wires

roughneck77 - Member

You can get them in the smaller versions. I have a 40 vA with an internal breaker with me now.

dogdodge - Thread Starter

Hi All,

It was the transformer and I did install an inline fuse. Thank you guys so much for your help on this!! Really saved me. Especially PJMaxx!

PJmax - Group Moderator

Good job. Thanks for letting us know how you made out.