I have a countertop 15 amp GFCI receptacle in my kitchen and I would like to replace the fuse for it.

It's currently on a time-delay 30 amp fuse (it's a cartridge/capsule style fuse), which is too much, since the wire going to the receptacle is 12 gauge aluminum (which as far as I know is rated for max 15 amps).

My question is, should I use a time-delay fuse or a fast acting fuse for this receptacle?

I've read that time-delay fuses are for appliances which may have large electric motors, but I'm not sure what's considered large in this context. The outlet will connect things like a toaster, kettle, blender, slow cooker, microwave, etc. This receptacle is the only one on this circuit.

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    Remember either to use CO-ALR rated devices or pigtails with Alumiconns, because copper lugs don't play nice with aluminum wire. (which in my view is copper's fault). Jan 23 '20 at 23:02
  • I live in Ontario, Canada, and can't seem to find AlumiConn anywhere. I use the purple IDEAL Twister Al/Cu wire nuts, which I've been told is fine by local electricians. Though I know there is much disagreement about it, I've not had any issues thus far. Jan 23 '20 at 23:51
  • They may be fine in Canada. In the US they burn up a lot. Jan 24 '20 at 0:31

Yes, use 15A time delay, the short inrush of motors will not damage the wire. In the US even circuit breakers operate on a time delay, they call it "inverse-time", which really means the same thing.

Not sure what type of fuse is used in your country, in the US a FRN15 fuse would have a similar 500%/10 second delay that a typical thermal magnetic breaker would trip.

  • Looking at Lowe's, looks like it's gonna be the "Cooper Bussmann CSA Type P 15-Amp One-Time Cartridge Fuse": lowes.ca/product/fuses/…. Jan 24 '20 at 0:04
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    That would be good in Canada. Jan 24 '20 at 0:21
  • Just realized that Type P is actually non-time-delay, so I guess I should get the Type D time-delay one. Jan 24 '20 at 0:22
  • Just read through inspectapro.net/blog/… and it seems that I should be good with the regular Type P. Jan 24 '20 at 0:31
  • The article you refer to is speaks about "threaded metal base" fuses.Not sure it applies. I'm giving up, specs for CSA approved fuses like the Lowes page is conflicting about delay or not. I don't know why Cooper makes CSA only fuses while the UL fuses are CSA also. Lots of documents available for UL fuses. Jan 24 '20 at 0:52

You are correct about 15A for #12 aluminum. While some of the current-capacity rules in 310.15 can be complicated, NEC 240.4 wipes them out and forces #12 aluminum to 15A.

Remember when terminating aluminum wire, you must use CO-ALR rated receptacles. Copper lugs/screws/terminations are intolerant of aluminum wire. In the rush to aluminum in the 1960s, Underwriter's Laboratories hastily approved the AL-CU rating, which required only minor changes to existing copper-only designs. This caused a spate of house fires, resulting in the "Revised" CO-ALR spec. You see CO-ALR receps are quite different. Breakers get a separate rating system; their Cu-Al mark is OK.

While fuses are not prohibited, the normal practice is to use circuit breakers. Circuit breakers have a generous and slow trip curve.

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  • I'm not familiar with NEC or how much of it applies to me in Ontario, but thank you for this information. I have an old 100A fuse-box so I'm not sure how I'd install a breaker. Replacing my fuse box with a new 200A breaker-box is probably something I'll have done in the next 1-2 years though. Jan 23 '20 at 23:54
  • CEC 14-104 (2) Except as provided for by Subrule (1)(c), the rating of overcurrent protection shall not exceed...(d) 15 A for No. 12 AWG aluminum conductors. Also COALR requirements in the OESC (Ontario Code) & Cdn Code, except I understand CSA approves a Marlette connector for splicing to aluminum. Jan 24 '20 at 1:28

Use a standard 15 Amp fuse. You said the wire is #12 AL. That's good for 15 amps. Read and pay attention to Harper's comment.

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    #12 Al is not good for 20A. See NEC 240.4(D)(4), which says so explicitly, and overrides anything in the normal current table (310.15(B)(16)). It does this for all #14, #12 and #10 conductors. Jan 23 '20 at 23:23
  • @JACK [citation needed] that aluminum #12 was ever allowed 20A. The lower conductance of aluminum was a well known fact, even at the time. Jan 23 '20 at 23:46
  • @Harper-ReinstateMonica As usual, you're right... just read fine print... wire good for 20AMP but * states 15AMP fuse/breaker..... never used the stuff so unfamiliar with it.
    – JACK
    Jan 23 '20 at 23:54
  • The receptacle was on a MWBC (multi-wire branch circuit), with two hot (black) wires going in, and one neutral (white) wire coming out. I capped the one black and just connected one black and one neutral to the GFCI (and the ground, of course). The receptacle is the only one on the circuit. Jan 24 '20 at 0:01
  • Oops, didn't catch that it was aluminum - very odd (to me, but I'm not a pro) to have aluminum (which was a latecomer, relatively) together with old fuses. Jan 24 '20 at 0:37

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