Rural USA. We just had a gas furnace installed. The new furnace has a 1-gang junction box with a switch and a fuse.

It's the 21st century, here, I swear. And not England.

The furnace circuit is tip-top. It's punched down into a modern service panel with perfectly typical 15A breaker, and if it needed something special, that really wouldn't be a problem.

So why does the furnace also need a fuse?

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  • What make/model is this furnace of yours? Commented Oct 6, 2017 at 2:16
  • Is the fuse value something smaller than 15 amps? In the case where the system power requirements are less than 15 amps the vendor can save $ on the materials in the controls and blower by requiring a fuse.
    – Ed Beal
    Commented Oct 6, 2017 at 2:49
  • is it a slow blow fuse? if so, that's a different application than a CB...
    – dandavis
    Commented Oct 6, 2017 at 5:48
  • 2
    I am from the old, old school, and that was how it was done years ago when there wasn't a dedicated circuit for the furnace. The installer or the company may be from my era. (old habits die slowly)
    – d.george
    Commented Oct 6, 2017 at 9:48
  • And yes, what size fuse got put in? Commented Oct 6, 2017 at 11:38

5 Answers 5


You have an electrical connection to the Furnace - even if it is gas. Blower motors and control circuits require electricity; should you have a short - you will need the overload protection to prevent a fire that only a pyromaniac could enjoy.

The fuse is most likely of a lower amperage than your Circuit Breaker - it guarantees the safety of the furnace - regardless of your Circuit Breaker.

Think on this Why do Microwave Ovens have Fuses in them or other appliances such as TV's have fuses in them when we have these wonderful circuit breakers installed?

In Summary: The consuming unit may not trip the breaker, and if not a fire could easily erupt first before that breaker ever went into overload.

  • 1
    Appliance engineers sometimes put proprietary fuses inside because they don't trust the general public to plug things into a properly protected branch circuit, even though it's clearly stated in the instructions.
    – Upnorth
    Commented Oct 6, 2017 at 18:31

Newer furnaces might not need an SSU if they have internal motor overload protection.

The fuse is sized to protect the blower motor from overload. It should be about a 3 amp fuse.

The 20 amp breaker is there to protect the conductors from ground fault and short circuits. The breaker alone would allow the motor to overheat and burn up if a bearing jammed.

  • How do you determine 3amp for the fuse? Strangely enough my furnace has a 30amp fuse. Which seems to defeat the purpose entirely. How would I go about choosing the correct size fuse?
    – cryptic0
    Commented Oct 2, 2022 at 0:03
  • The fuse should be sized according to the manufacturer's specifications for your furnace. If it is very old and you don't have the specifications I would start with a 1.5 or 2 amp fuse and see if it holds while the furnace operates normally. If that blows move up to a 3 amp fuse and then possibly a 5 if you have a very large furnace. If those don't hold, then you probably have a problem with the motor or the mechanical part of it that is causing a larger than normal current draw.
    – ArchonOSX
    Commented Oct 3, 2022 at 8:41

You also need a fuse to protect the hot surface ignitor in case it attempts to ignite a non-existent gas flow. I've blown mine twice in the fourteen years we've owned this house, and in both cases it was when the furnace was turned on while the gas was turned off.


Not entirely sure where the fuse is you are talking about. Most new furnaces have a 3-5 amp fuse on the low voltage side to protect the circuit board and transformer from a possible short or overload. If someone were to put too many accessories on the existing transformer the fuse should protect it. Also if a thermostat wire were to ground on the cabinet it would save the electronics.


I'm not American (nor British), anyway I still like to have fuses on expensive appliances 'just in case main breaker fails'. Or, if on a bigger circuit, to limit the ampacity available to the specific appliance (eg: all my thermostats valves have a 1A fuse, and are connected to 10A light circuit). So in case of failure (that once occurred me) I have just to replace a few cent fuse and not a thousand Euro thermostat with its thermo-electric actuator,

Also fuses usually can break bugger currents than 'standard' switch-like breakers. So if a 10kA failure occurs (AKA short circuit) it's more probable a fuse will fuse than a standard breaker will trip because standard breaker cannot extinguish the arc well as a old-style fuse.

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