Thread: Fuses vs Breakers for 240 V Applications

I'd like to hear some of the forum member's opinions on this. When I see fuses in an A/C service disconnect, I've been mentioning to my clients that they should consider an upgrade to a 240 V breaker, and get rid of the fuses. This is because you can have only one fuse blow, and that the equipment is still going to have 120 volts applied to it, but it won't be able to run on just half of its intended circuit. A breaker, on the other hand, will disconnect both sides of the 240 V circuit when it trips.

I recently mentioned this to an HVAC contractor, and he said I shouldn't be telling my clients this because having the fuse blow on just on side of an A/C power circuit wouldn't hurt the equipment. What say all the experts here on the forum?

Thanks a lot!

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As long as it is single phase I agree with the HVAC contractor.

You've unwittingly strolled right into a minefield ....

The first part of the issue is that circuit breakers and fuses each have their own advantages, and which one to use is generally a design decision, not a code matter.

With HVAC equipment, the issue gets murkier. It is very possible that the nameplate of the equipment specifies "maximum FUSE size." If that is the case, then there have to be fuses somewhere in the power to the unit. Only if the nameplate says "maximum OCPD" or "maximum fuse or breaker" does one have the option of using the breaker.

Another issue that comes up is disconnection of the unit. Fuses alone are not enough; there also has to be some manner of disconnecting means at the unit. This contrasts with breakers, which generally qualify as a disconnecting means.

If it's older equipment, the data plate may state: "Max. fuse size __" and if it does only fuses may be used, newer equipment allows both* & it's not a bad thing to use fuses in a A/C disco, IMO.

*Newer HVAC EQ. will say Maximum fuse size or HACR circuit breaker.

Some HVAC equipment requires fuses. While not as convenient to reset as a breaker, fuses are fine.

Originally Posted by James Duffin

As long as it is single phase I agree with the HVAC contractor.

3-pole fusible safety switches are commonly used on 3 phase HVAC equipment.

I should have specified that I was referring to the service disconnect located on the exterior wall at the location of the exterior A/C compressor unit, and that we're talking strictly about single phase applications only. Actually, since the A/C circuits are protected by either fuses or breakers at the interior service panel anyway, having additional fuses or breakers at the exterior service disconnect is probably needlessly redundant anyway.

Thanks for the insight on the use of fuses with A/C circuits. That certainly didn't take long!

Originally Posted by John Steinke

You've unwittingly strolled right into a minefield ....

Thanks John! It's a talent I have!

Mike

Originally Posted by Rollie Meyers

3-pole fusible safety switches are commonly used on 3 phase HVAC equipment.

"I recently mentioned this to an HVAC contractor, and he said I shouldn't be telling my clients this because having the fuse blow on just on side of an A/C power circuit wouldn't hurt the equipment. What say all the experts here on the forum?"


I was responding to the OP question. Blowing one fuse on a single phase cirucit will not hurt the equipment but it will on a three phase circuit. And thanks for the heads up on three phase fused disconnects.

Originally Posted by James Duffin

I was responding to the OP question. Blowing one fuse on a single phase circuit will not hurt the equipment but it will on a three phase circuit. And thanks for the heads up on three phase fused disconnects.

That's where my original assumption came from I guess. My background is more with three phase, and I know that loosing a leg there is not a good thing. Thanks!

It is a plus to have a 3-pole breaker on a three phase motor to help prevent single-phasing when one fuse blows. You just have to size the breaker per Table 430.52 to help prevent the breaker from kicking out on startup.