I'm hooking up a 300 watt toroidal magnetic transformer located in my garage; it's rated for inside and outside use, I'm using its plug into a GFI circuit, and I'm using 12/2 (12vac) power cable rated for in-ground burial use. I'm planning on drilling an approx 0.5 inch hole and using 1/2 pvc conduit from transformer through the wall and down the exterior side into the ground. The max load that will be placed on this transformer is expected to be about 40 watts, using LEDs, with a max cable run of about 160 feet.

Is there some other requirement that I should be aware of?

  • thanks hildred. I do have some spare 10 gauge, but I like your idea of adding a fuse on the output side; I think I'll try a 15A fuse to start with. Commented Jun 27, 2015 at 7:48

1 Answer 1


40 watts at 12 vac is about 4 amp which leaves plenty of headroom on the 12ga cable (which is normally rated for 20 amps (with heat loss and safety factors included). But what if your wire gets cut? Shorted? the worst case max current that a 300 watt transformer can put out (if it really is a 300 watt not a 310 watt or 315 watt) is 25 amps which is flirting with the danger limit of the cable. Soi I would either put a 15 or 20 amp fuse on the secondary side of the transformer, use 10ga wire, or put a 2amp fuse on the primary side of the transformer. 2 amp fuses could be a trick to find, 10 ga is expensive, but a 15 or 20 amp automotive fuse will work just fine considering it is only 12 v. And since you are pulling less than 5 amps a smaller fuse would be fine.

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