Low Voltage U/C Wiring Help (also posted in Lighting)

monkeystomachJune 13, 2011

I need a lot of help with this under cabinet lighting system I am trying to plan.

I have so many questions I really do not know where to start so I will try to explain and hopefully my very crude drawing will help.

I would like a low voltage linear under cabinet lighting system like these from Sea Gull (http://www.seagulllighting.com/Linear-Lighting.htm).

I have two separate runs of cabinet that are both in the kitchen, but I would not be able to hide the low voltage cable if I ran it from one set of cabinets to the other.

So my questions are as followed:

1. Can place the transformer in the crawlspace under the kitchen?

2. Can I branch off of the transformer with two branches using NM cable to run the low voltage current in the walls to the cabinets?

3. Can I directly wire the 120v 15a circuit into the transformer without using a switch? (I do not want to clutter the backsplash/walls with switches)

4. Can I use some sort of small in-line low voltage dimmer in a junction box under my cabinets and then convert the NM cable to the low voltage cable?

5. Can I use the transformer as a junction box under my house to also connect my current overhead lighting?



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1. Yes, the transformer can be located anywhere that's accessible.
2. Yes, in most locations you can use NM to run the low voltage current. Check your local laws. Yes, you can branch the low voltage wiring.
3. A switch is not necessary. However, depending on the transformer, you may need to provide a receptacle instead of direct wiring.
4. Yes, if you can find a small, in-line low voltage dimmer at a price you're willing to pay. Most low voltage dimmers that I'm familiar with are the same size as 120V dimmers (use the same plates, etc.), and are quite a bit more expensive than their 120V counterparts.
5. The transformer is not going to be an approved junction box. However, you can use the same junction box for 120V and low voltage, as long as the wiring is all rated for at least 120V.

    Bookmark   June 14, 2011 at 9:39AM
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The issue that quickly shows up with using NM for lov voltage wiring is the sometimes very high currents required by low voltage lighting.

The wire size is based on current, with voltage playing no part (voltage determine insulation thickness).

A 20 watt bulb in 120 V pulls ~0.167 amps.
On 12 V a 20 W bulb pulls ~1.667 amps, and the lower voltage puts a premium on voltage drop.

Dropping 1 V in a 120 V circuit is less than 0.83%, while in a 12 V circuit it is 8% and will result in noticeable color shifts (red tint) if a lower drop lights is nearby.

To control voltage drops wire size may need to be increased evrn further (there are online tools that will compute voltage drop given current, wire size, and conductor length).

Wire size directly affects box fill calculations and bending space.
Larger conductors are often stranded to make working with them easier.
The larger wires also require larger wire nuts, and may not even fit under screw heads on switches.

I use small electronic 'transformers' (actually small power supplies) to power low voltage under-cabinet lights and avoid using a single large physical transformer.

A 30 W power supply is less than 3 inches long x 2 inches wide and 1.5 inches thick *(most are closer to 1 inch thick).
They are available for plug or hard wire connection and can use conventional 120 V dimmers.
The 120 V wire can be as small as #14 (15 amp circuit) and is not hard to work with.
The small power supplies can be mounted under the cabinets near the back with the lights near the front.
The only complaint I have about the power supplies is they need a small strap to hold them in place (the single screw and double face tape fails pretty quickly).

    Bookmark   June 14, 2011 at 11:44AM
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Hi Brickeyee,

Do you mind telling me what brand under cabinet lighting you are describing? Or do you piece your own brands together, if that is the case please provide additional details too.



    Bookmark   June 14, 2011 at 8:55PM
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Numerous brands provide the electronic 'transformers' for incandescent lights.

None of my customers has wanted to waste money on LED lights yet.

You can purchase the separate pieces from different sources as long as you are aware of the voltages and currents involved.

A single large transformer has always been a rather PITA solution.

It requires either multiple runs of smaller wire or very large conductors.
Using #10 or #8 wires for voltage drop considerations runs up the cost something fierce.
By delivering 120 V as close as possible to the lights you can reduce a lot of issues.

    Bookmark   June 15, 2011 at 9:46AM
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