View Full Version : Wire Sizing at Transformer

05-28-2012, 09:45 AM
I am installing a small system with a 300W single tap transformer.

Due to the layout, there will be three (3) #12 leads leaving the transformer.

One with 105W, one with 80W, and one with 70W.

The transformer lugs will not adequately accommodate three (3) #12 wires.

I plan to make an in-ground splice at the base of the transformer.

My question is this. I know a #12 wire is suppose to accommodate 240W but do I need to upsize the wire from the transformer to the in-ground splice (a distance of only 5 feet), i. e., using #10.

Not a big deal to put in the 5 feet of #10, I was just curious as to the need to do so.

Thanks / Regards,


05-28-2012, 10:45 AM
I would certainly upsize the splice. People split hairs about awg amperage. Some swear by 12/2 stranded being 20 amps max 240 watts and others believe a 16 amp max 192watts (80 percent rule). From personal experience, I can tell you that I would stick with no more than 192 watts on 12 gauge wire. Reach down and touch a 12 gauge wire that has been under 240 watts of load for a few hours, the jacket will be pretty darn hot. The ten gauge is just a safer approach.

05-28-2012, 02:40 PM
Definitely #10 from the splice to the transformer. I would also suggest putting the splice in the transformer enclosure rather than burying it - easier to troubleshoot down the road. Also, good to use something like a transformer test lug (http://www.cast-lighting.com/products/accessories/installation-accessories/130/) rather than a crimp or wire nut.