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Old 12-08-2022, 09:08 PM   #1
jsweet89's Avatar
Join Date: Oct 2020
Posts: 26
Wire gauging on branch circuits

Apologies in advance if this has already been asked and answered. Couldnít find it anywhere on this site.

Iím going to have a big branching circuit for my ceiling puck lights. 12ga running the length of the bus, using lever nuts to branch off wires across the bus to connect to the puck lights.

For those wires that branch off the 12ga, is it okay to use a smaller wire, like 14-16ga, to run to the lights? Is anything special needed where they branch off? Would I need an inline fuse at that point?

Any advice is appreciated. Thank you greatly.

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Old 12-08-2022, 11:15 PM   #2
Bus Crazy
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Location: Florida
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Coachwork: Integrated Coach Corp.
Chassis: RE-300 42ft
Engine: 466ci
Rated Cap: 90
Lever Locks

More info please...
AC or DC?

Sounds good. Trunk line, plus branches:
Two - 12g
One - 14 g
at each junction (light), right?

Check out the Electrical Rough in thread below for more suggestions.
Ceiling: Framing & Electrical Rough-in
Convert Hatch to AC & Roof Patch
🇺🇸 Frederick Douglass: "If there is no struggle, there is no progress.Ē
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Old 12-09-2022, 12:43 PM   #3
Bus Crazy
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Location: Northern California (Sacramento)
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Year: 1999
Coachwork: El Dorado Fiberglass
Chassis: Ford E450
Engine: V10 Gas
There are some great discussions on sizing wires if you do a quick search of this forum. I recommend you read up a bit-understanding electrical and the basic precautions (like fusing) is a good skill and may keep your bus from going up like a roman candle.

You'll need to size the wire running the length of the bus to support the current, as well as ensure the voltage drop to the last puck light is not noticeable.

If you have ten puck lights (and I'm assuming you're using LED-type), a quick check of Amazon indicates they are about .2 amps apiece at 12VDC. My experience is voltage drop is not much of a problem with these lights-they don't dim appreciable until you're down to about 7-8VDC, so you have some latitude.

The minimum wire gauge for a 2 amp load is 18 gauge, however you'd probably notice the dimming at the end of the run due to voltage drop. To keep the voltage drop reasonable over 25 feet (I see you have a shortie) when I look up a 2 amp load, 50' run (there and back again), a 14 gauge wire will give you about a half a volt drop-very much acceptable, and should not be noticeable on the pucks at the end of the run.

If you plan to branch horizontally off that main run to feed individual pucks, that wire can be smaller because the load is only .2 amps. I'd probably use 18 gauge there.

Please make sure you know the amp draw of your pucks. And if you're burying the wiring behind some beautiful ceiling finish, I'd definitely mock up the system before installing to ensure the visual result is what you hope it will be.
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Old 12-09-2022, 01:02 PM   #4
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I completely agree with Sir Rucker, above, and I'm also a big fan of the Wago connectors that DeMac shows. I would even say that your voltage drop, given use of 14GA wire, would be even lower than Rucker suggests...because the return path (through the chassis) should have a much lower resistance than a return of 14GA wire.

As far as your fuse question goes, I would not put inline fuses on the branches...especially if those would be buried in the ceiling. Instead, I'd fuse the whole circuit (as close to the power source as possible) based on the smallest size wire in the branch. Overcurrent protection, whether a fuse or circuit breaker, should be sized to protect wire that might chafe and short. So I'd use a 5 amp fuse at the start of your circuit, which should offer plenty of protection for the wires and still be large enough to not blow from the light string load.
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12v, lights, wiring

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