Hoping for an OK expecting a NO

[Southern Oregon Bob]

I prewired all my walls and I'm set up to have a 3 gang switch at the head of the bed. Fan, 2 reading lights, and the cabin dome. I ran #12 wire in and out for fan and #14 in and out for each light. I went and got the 3 gang switch I planned on using ( 110v 3 rocker switches in a single box. When I got it home it only has 1 hot going in and 3 switched going out. If I used the #12 for the hot in and #12 to fan and #14s to the lights and put them all on a 20amp fuse will that be safe? Will the fuse not brake soon enough if I'm just using a light or is the single #12 not large enough to run them all at once? If I fished in all #12 would that help or do I just need to forget what I thought would be the perfect switch? Sorry that got so long, I guess what I'm asking is, is there is a good way to put two lights and the vent fan on 1 circuit? Bob

[Kevin A]

I prewired all my walls and I'm set up to have a 3 gang switch at the head of the bed. Fan, 2 reading lights, and the cabin dome. I ran #12 wire in and out for fan and #14 in and out for each light. I went and got the 3 gang switch I planned on using ( 110v 3 rocker switches in a single box. When I got it home it only has 1 hot going in and 3 switched going out. If I used the #12 for the hot in and #12 to fan and #14s to the lights and put them all on a 20amp fuse will that be safe? Will the fuse not brake soon enough if I'm just using a light or is the single #12 not large enough to run them all at once? If I fished in all #12 would that help or do I just need to forget what I thought would be the perfect switch? Sorry that got so long, I guess what I'm asking is, is there is a good way to put two lights and the vent fan on 1 circuit? Bob

Most household extension cords are 14 or 16 gauge wire, I'm sure people have loaded those with more than a fan and two lights. If you're running 12 gauge wire, I sure can't see how you'd have a problem. I've got a 25' 12 gauge extension cord that I run my table saw on.

I just reread your post, Are your lights & fan 12 volt or 110? I assumed they were 110 because you mentioned using the 110 volt switch. If you are running 12 volt, I would advise against using the 110 volt switch, it's not designed to handle the amperage of the 12 volt system.

[Nitetimes]

110 or 12v? If you are using 110v the single 12ga in is more than enough for what you are running but use a 15 amp breaker (also more than enough) because you have 14ga going out.
If you are using 12v, those switches are a little heavy for what you need.

[Southern Oregon Bob]

It is a 110v switch that I was planning to use for my 12v systems. The price is right and they come with 2 or 3 rocker switches in a single box. Are y'all saying that the 110 switches won't work with a 12v system? That is another question I had. If a terminal bar is rated for 110v at 20amps what is the formula for converting that rating to 12v at (X)amps? Electrical seems so easy until you get shocked or start a fire. Bob

[angib]

If a terminal bar is rated for 110v at 20amps what is the formula for converting that rating to 12v at (X)amps?

Amps is what overloads conductors, largely irrespective of voltage, so it's a 20amp bar at 12v. This can cause a practical problem at 12v as the low voltage means that currents are relatively high.

But 110v and 12v switches are designed differently - 12v switches are designed to have a low resistance to minimise voltage loss across the switch contacts - losing 1v across a switch is no great deal at 110v, but it's a killer at 12v. The 110v switch will work at 12v, particularly at first, but will get worse over time, as it won't have a 'wiping' action to keep the contacts clean and conductive.

Andrew

[Southern Oregon Bob]

Ok I give up, the 110v switches are out. Thanks Bob

[TRAIL-OF-TEARS]

Bob,
hey angib is right the amps are the boogers. but there is a formula you can use to find amps. Amperes= watts/volts example if you have a 1200 watt device at 110v that would be 10A. if you have a 1200 watt device at 12v it would be 100A. I would say, that 20A terminal bar at 110v would be good for 240 watts at 12v. still 20A. I hope this helps.

[GregJ]

Bob,

Don't give up hope. What Andrew said is correct but there are some things that may help. Many specialty switches are rated for AC and DC, so you may want to check. If you have the switches already look on the body of the switch it’s self. The ratings are generally marked there.

The other thing to look at is your load. If you fuse (or breaker) the line at 15A you’re limited to about 150 watts and that’s at the high end. If you had two 10 watt light bulbs a 10 watt fan (30 watts), you might be fine unless the switches are pretty bad.

The math goes like this (simplified)—

volts x amps = watts so 120v x 1 amp = 120 watts or 12 volts x 10 amps = 120 watts

Resistance is measured in Ohms. A contact or wire has resistance that causes a drop in voltage: amps x ohms = volts.

If the contact has a resistance of 1 ohm in the example above, the switch will have a 1 volt drop at 120 volts (1a x 1 ohm) and a 10 volt drop at 12 volts (10a x 1 ohm). The actual load (light or fan) as viewed from the power source, will get 119 watts with 120volts and only 20 watts of the 120 watts coming from the battery with the 12 volt system (the other 100 watts will heat the switch (10v x 10a)).

Note, a 1 ohm switch contact would be pretty bad so something realistic, and by cutting the load, you might be OK.

You see the concern with switches and wire size in a 12 volt system and why it’s often good to use heavier wire than strictly called for. You can also see why an inverter may make sense.

[Southern Oregon Bob]

Thanks for the great explanations, that really helps. I looked on the switch and found the ratings and it does not look good. Any suggestions on a good switch source. Every place I look seem to have the same 2 switches in 12v and I was hoping to find something a little nicer. Thanks again, I always enjoy learning something Bob

[GregJ]

Bob,

RV, auto and boat supply houses are your best bets. Some items at HD are DC rated, but have you to check.

FWIW: If I had the switch already, I might be tempted to try it, and replace it if there is a problem... your mileage may very and of course you would do it at your own risk as would I. Keep an eye on the switch for heat buildup...

Back to the math-- normal-- 48w/12v = 3a, 12v/3a = 4 ohms.

Increase the switch resistance to 1 ohm so 4 ohms + 1 ohm = 5 ohms, 12v/5ohm = 2.4a, 12v x 2.4a = 28.8w-- 5.7 w dissipated as heat in the switch and 22.8 in the dim light bulb (note, in real life the bulb will draw a little more since the filament is not as hot as it should be- 6w and 25w?). So how hot would the switch get with 6 watts? Note: see why the low wattage bulb is safer?

Remember it's easier/cheaper to conserve power than to make or store it...