NEC Low Voltage Ampacity versus Wattage question

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  #1  
Old 04-10-14, 10:40 PM
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NEC Low Voltage Ampacity versus Wattage question

Hey, I need some clarification help on a Low Voltage issue.
This is the background of what I am doing.
I am making a residential stair lighting system for my home. I have a class II power supply which feeds a PCB within an enclosure. Coming out of the enclosure I am feeding each step with a separate cable for the lights on each step; which are LED strips. Each cable will have 4 wires.

Specifications for Maximum rating of system:
(Using RGB “colored” LED strips; it will be using 4 wires per step)
One wire “power lead” going to the each step will be supplying +12V DC (Max 900mA or (1.08A with 20% added)
The other three wires will be the Neutral side from the step to the enclosure, each wire using Max. 300mA or (360mA with 20% added).

NOTE: The LED strips are made of LED’s which have three internal LED’s making the RGB colors. Each return wire "Neutral", are independent using Max 300mA.

The max distance one way would be less than 30 feet.
The max number of steps or cables are 21 steps or 21 cables. I am using conduit from the enclosure to the bottom step, then hanging the cables open air (in an enclosed stairway, not Plenum) with one hanger at every step.

My question after all that is this:
In the NEC2011 book, it does not specify any Ampacity for anything lower than 18 AWG that I could find. Also in other charts on the internet, I have found for smaller AWG; but they do not specify any voltage, only Ampacity.
Is the Ampacity the deciding rule for what AWG to use or is the actual Wattage should be considered. Since the NEC book charts (600 Volts & lower) is based on 115V (at least it's my understand that it is); wouldn’t the AWG for Low Voltage be based on wattage and not just Ampacity?
Second, could one clarify what would be the proper AWG for a four pair cable for the above scenario?

While I'm here since I am using 30VDC or less and it's enclosed space, do I really need a raceway under the steps?

Thanks for any help one can give me.
 

Last edited by Lyle3; 04-11-14 at 01:20 AM.
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  #2  
Old 04-11-14, 05:25 AM
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As a point of information, voltage drop issues are related to the number of amperes, not volts. At lower voltages, for a given number of watts, more amperes are involved so you need to be more conscious of wire size and voltage drop.

Don't forget to use the round trip distance when computing wire resistance which in turn is used in your voltage drop calculations.

To be thorough, you compute wire size two ways:
1. Taking into account voltage drop only.
2. Taking into account codes (e.g. table in the NEC), any other recommendation such as on a web site, and derating if needed (may need heavier wire) for multiple conductors running juxtaposed, and ignoring voltage drop.
You use the larger of the two sizes you come up with.

The number of volts lost flowing through a piece of wire is equal to the number of amperes flowing at that moment multiplied by the resistance of that piece of wire.

High voltages are used for long distance transmission and distribution because more watts can be delivered using fewer amperes and therefore thinner, lighter, and less expensive wires.

Specific wires are rated for a maximum of so many volts (250 and 600 are common for wire sold at Home Depot) and this depends on the kind of insulation, not their size.
 

Last edited by AllanJ; 04-11-14 at 05:48 AM.
  #3  
Old 04-11-14, 05:46 AM
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You are working with low voltage wiring so almost anything goes. You can make free air splices in the walls, you can run any size wire you want, except you will need to consider voltage drop, as Allan posted.

While I'm here since I am using 30VDC or less and it's enclosed space, do I really need a raceway under the steps?
No.

If it was me, I would use some 18/4 thermostat wire.
 
  #4  
Old 04-11-14, 11:25 AM
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I want to thank both of you for your time and explanation. I will look at the voltage drop for the distance / amps and the adjustment for that many wires being bundled together.

Thanks again.
Lyle
 
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Old 04-11-14, 11:58 AM
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Here is a voltage drop calculator that is pretty helpful: Voltage Drop Calculator
 
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Old 04-11-14, 04:08 PM
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Thanks Tolyn, I checked it out. It came up with a little bit different answers than what I came up with but it was most likely more comprehensive in what it used to calculate the answer.
Thanks I'll use it.
 
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