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I have two blown ac disconnect fuses. Are they supposed to be different? One is an Edison ECNR30 and the other is Fusetron FRN-R-35. My electric box amp that runs out to the ac is 30 amps.

What should I buy to replace them? Can I just get two ECNR30s? Or two Fusetron FRN-R-35s?

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    what does the manual say about the required fuses?
    – ratchet freak
    Commented May 13 at 12:53
  • @ratchetfreak Manual (or more likely, label on the unit) will specify min/max circuit size and protection, but that is provided by the breaker for the circuit, not by the disconnect. The disconnect is required for line-of-sight service cutoff.
    – manassehkatz-Moving 2 Codidact
    Commented May 13 at 14:16
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    What make and model is your disconnect? The FRN-R and ECNR are class RK5 fuses, which have a change of case size between 30A and 35A (in other words -- you can't fit a 30A fuse into a 60A disconnect without an adapter as a result)
    – ThreePhaseEel
    Commented May 14 at 2:26
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    Photograph of the nameplate on your A/C unit?
    – Harper - Reinstate Monica
    Commented May 14 at 19:35

2 Answers 2

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If it is truly "just" a disconnect, then the two fuses should match but it probably doesn't matter what they are as long as:

  • At least as large as the circuit breaker protecting the entire circuit (30A)
  • No larger than the capacity of the disconnect box itself. Some are 30A. But from what I have seen, many more are 60A. Why? Because a 60A disconnect can work for a 20A, 30A, 40A, 50A or 60A circuit. It is normally only a disconnect - i.e., a method of manually disconnecting power within line of sight when working on the equipment. The overcurrent protection is a secondary function.
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AIUI (but my specific unit is happy with breakers, so I have unfused disconnects.) Typically the labelling on the outdoor unit (and in the manual) will specify either:

  • Max Overcurrent Protection Device (which is fuse or breaker)

or

  • Max Fuse (which means you may have a breaker feeding the circuit, but the maker calls for a Fuse in the line.)

In the latter case you need the fused disconnect, with fuses of that size (or smaller, possibly, where "Min Circuit Ampacity" on the same label is a lot less than Max Fuse.) This has to due with the difference in how fast fuses and breaker respond to overloads, and how the unit was designed and tested. Some are fine with breaker curves, some need a faster disconnect in a fault condition.

The usual way you get 30 and 35A fuses in the same disconnect on a 30A breaker protected circuit is someone replaced one with the wrong fuse. It's not inconceivable for 35A to be the right size (since compressor starting can cause surges above the breaker rating, which a breaker will pass for a short period of time) but the way you know the right size and type of fuse is to consult the nameplate and/or manual for the device and check, not to guess.

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  • You can also end up with a 35A fuse in a disconnect because there's already a 30A breaker and a 35A fuse is what the tech had on the truck.
    – KMJ
    Commented May 13 at 16:19
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    @KMJ -- the problem is that a 30A RK5 and a 35A RK5 are different physical (case) sizes, so I have absolutely no clue how the installer got them to fit into the same disconnect without a fuse reducer on the 30A side
    – ThreePhaseEel
    Commented May 14 at 2:27
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    Oy, I missed that. Good point.
    – KMJ
    Commented May 14 at 2:54

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