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tarnold's avatar
tarnold
Explorer
Sep 22, 2016

Amps are amps...right?

Be nice ok? Size of wire determines current capacity (amps) as I believe. Recently had to replace a 30 amp fuse(U shaped). Had small size about same as a dime but needed next larger, about size of a nickel. Went to Autozone and got what I needed. They had same style that was about the size of a 50 cent piece, if you can remember what they look like. All 30 amp, so question, why the much different physical sizes, if all are still 30 amp rated?
  • mike-s's avatar
    mike-s
    Explorer
    Sep 24, 2016
    Interrupt ratings are in amps, not volts. The 11A/1000V fuse in Fluke handhelds has an interrupt rating of 20,000 amps.

    They're more expensive than the 3AGs (or 5x20s) found in cheap meters. Beware of the cheap ones if measuring AC circuits, and don't believe their CAT ratings. A 1000V CAT II meter needs to have an 84 A interrupt rating at its rated voltage. The test source is 1000 V/12 ohms. A standard 3AG has a 200 A (5x20s have 80 A) rating at 250 VAC. So, when you see a cheap meter with a CAT II rating, a 750 VAC setting, and it uses 3AG fuse, they're lying to you. If it's a 5x20 fuse, it's not even safe at 250V, forget about 750V.

    Even a low end Fluke is CAT III/1000V, which requires a minimum 4000 A interrupt rating. But the fuses are $10, not $1.

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