07 March 2013

A Technical Guide to (RGB) Wire Selection

With the advent of RGB lighting, there is a need to now carry power over wires that are different than those the community has used in the past for AC based power - namely SPT1 and SPT2 cords.  There are a variety of factors to consider when determining the type of wire you want to use for wiring RGB lighting - whether it is for smart/pixels or basic/dumb lighting:
  • What is the amount of current required for the lights?
  • What are the conditions the wire will be used in or with? (water, UV exposure, temperature range, how much flex will it be exposed to)
  • Cost - not only for the wire itself but also the connectors used with it.
  • Availability
  • Easy of use (soldering, crimping, etc)
Current Carrying Capacity
First, lets start off with the most important function of any wire - it's ability to carry power.  The primary method of expressing power carrying ability of a wire is in amps.  But...you need to look at the voltage also.  For example, a wire rated to carry 1 amp of power (it doesn't matter is the power is DC or AC):
  • 1 amp at 5 volts is 5 watts (Current  or amps * Voltage = Watts)
  • 1 amp at 12 volts is 12 watts
  • 1 amp at 120 volts 120 watts
So, as you can see, the actual power (watts) a wire can carry varies based on it's voltage (this is part of the reason some people choose 12v RGB lights over 5v pixel lights).  This is the same reason that a high tension power wire for interstate power transfer is in the millions of volts - if the same, 1" diameter cable was at 120 volts as opposed to millions, it would have to be massively larger (in diameter) to carry the same amount of power.  So - volts matter.

So, how to do you know how much current (or amps) a wire can handle?  Well, it's complicated and at the end of this article we will show you the "real world" method to determine what wire you need to use.  There are a number of factors that go into the calculation - including material type (tin, aluminum, copper), design (stranded vs solid wire), the diameter of the wire (gauge or in the US, AWG), the temperature the wire is exposed to, how many wires are bundled together and the insulating material.  You can start with charts, such as this one that give you a rough idea of how much a SINGLE wire can carry - remember that there are always two wires required for AC and DC wiring systems.  When you look at a chart you want to find the AWG (American Wire Gauge) or gauge.  How do you know what gauge the cable is?  Well, it's complicated also for the following reasons:
  • Some vendors lie about the gauge of cable - this is very common for wiring sourced from China.  This is most common with wiring used in RGB lights. 
  • The charts most often assume you are using solid copper - the best possible (short of gold and silver) conductor of power but often due to cost reasons, you may have tin wire plated in copper or aluminum wire plated in copper or some other variation, which renders the tables invalid.
So, we would always recommend looking closely at the cable and then measuring the cable diameter with a micrometer if you have them.  If you are using CAT5 cable or other cable purchased in the US on a roll, it's usually safe to trust the AWG listing.  So, now you have your wire gauge, say, 18 gauge or AWG.  So, we look up on the table for 18 AWG and find that it can handle 16 amps per wire for "chassis wiring" or 2.3 amps for "power transmission".  Those are some pretty big differences - why is that?  Again, the difference is due to the use of the cable, insulation and other factors. 
Conditions The Wire is Used In
There is no one perfect wire because the conditions that each project it is used in vary.  For example, one person may be permanently installing lighting onto their house and does not have intentions to remove it.  In that case, issues of UV exposure (which breaks down the insulation on the cable) and temperature exposure become important factors.  In this case you might also consider using a solid wire as opposed to a stranded wire as there will not be much future movement (and thus breakage) of the wire.  For installations in very cold regions, the insulation material is an important consideration as common insulation's are made from vinyl which doesn't function well in low temperatures.

So, when selecting cable, consider how the cable will be used and select a cable that meets those specific environmental issues.

Cable Cost
Of course a big factor in cable selection is cost.  The major cost in any cable is the wire, which is most often copper.  You don't want to select a cable that has conductors that are too thin and thus unable to carry sufficient current but you don't want to have overkill as this results in higher costs, heavier cable and often less flexibility.  Also keep in mind that cable cost is also a function of the quality of the insulation, so if you cheap out on a cable that doesn't have UV resistance and the cable has to be replaced after two seasons (along with all the associated soldering and connections), you may not have saved that much in the long term.

There is always a "right" cable for every need out there in the market, though often those "special" cables are so expensive that the "right" cable can't be used.  So after determining what gauge and insulation that is required, see what vendors carry that cable.  Often moving to a more "standard" cable, such as in the case of CAT5, results in many more vendors, greater competition and as a result, lower costs and higher availability.  So, don't rule out a cable that is close to the specs you need.

Ease of Use
This is one of the least considered factors in cable selection.  Easy of use includes a variety of issues, such as:
  • Can the cable be purchased with pre-attached and tested connectors?  This is common with CAT5 and SPT cables which are standardized in wiring, color and design. 
  • How easy is it to attach connectors?  Some wire can be very hard to work with due to the small gauge, multiple layers of insulation, strain reliefs or other factors.  Having to attach connectors to each cable, correctly (to prevent possible mis-wiring related damage) can often take a considerable period of time depending on the number of cables you need.
  • What types of connectors can be used and do they meet your needs?  CAT5, for example, is ubiquitous and has many types of splitters / combiners, connectors (female / male, etc), waterproof and non-waterproof and more.  If your design calls for direct attached cable, such as screw terminals, is the wire strong enough to handle multiple screw downs on it?
  • Connector costs is one of the biggest factors.  Usually in most systems you will have a connector based system for hooking up elements as they are, in the Holiday Lighting world, temporary in nature.  So, while a spool of cable might be cheap up front, if it requires expensive connectors, the overall cost of that connection method goes up and the connectors could be more expensive than the cable itself.  Be sure to think end-to-end on what you need from your connector - does it need to be waterproof or just water resistant?  Does it matter if the connection is water resistant at all (such as the case with SPT cord.)  Don't spend money on waterproofing connections that don't benefit from it.
The Real World Example
So, I've discussed a variety of the issues you should consider on how to select a wire or more specifically, a wiring "system" - how does this work in the real world?

Let's say that you want to hookup four flood lights to a single controller.  We will be using 100ft of cable between the power supply and each flood, resulting in a total of 500ft of cable between the start (power supply/controller) and the last flood.  What cable do you need and what factors would you consider?  They are:
  • What is the power consumption of the flood?  Well, maybe the vendor says this is a "10 Watt" flood.  What does that mean?  Does the flood REALLY use a total of 10 Watts?  Well, don't trust your vendor - get our your multi-meter / VOM and actually measure the current draw of the single flood.  We have an article on our Knowledge Base that describes this process and here is a video that you can follow to learn how to do it:

    After you have the actual power consumption of a single flood, you can then determine the overall power draw.  So, lets say that the flood actually did consume 10 watts of power or .833 amps at 12 volts or a total of 2.5 amps at 12v DC for all four floods.
  • Knowing the current consumption of the flood lights is just part of the equation - now you need to determine the power consumption of the cable itself.  Wait...what?  Yes, the wire itself uses the same power that is used to power your lights.  Think about a water hose - if you hook a few 100ft sections of water hose together, you'll end up with a pretty low water "pressure" (aka voltage) at the end of the hoses, compared to the pressure coming directly out of the faucet.  The same applies to your power supply - if you have a power supply that provides exactly 2.5 amps of power at 12v DC, you will need even more power just to replace the losses from the cable itself. 
  • So, what do you do?  You have two options, you can choose to waste the power by using a thinner wire (wire diameter/thickness goes down as the gauge goes up) and just purchase a larger power supply to "over come" the losses in the cable (up to a point) OR you could purchase a lower gauge wire (thicker) that more efficiently carries the power and results in few losses.  This is why when you look at the wire gauge table, you will notice that there is a rating that references "ohms per ...".  Ohms is a measurement of the resistance that the cable puts up against the power flowing through it.  Of course as you can imagine, a smaller diameter cable will have a higher ohms per foot rating than a thicker, larger diameter cable. 
  • So, what do you do with this ohms per whatever rating?  Let's take an 18 AWG wire - it has about 6.3 ohms of resistance per 1000 feet or 3.15 ohms per 500 feet - the amount of cable we will be using in this example.  So, we will put these values through an ohms law calculator and we come up with a number of .48 watts.  That means for each wire (we need two) we will loose .48 watts or about 1 watt total for the entire length of the cable pair.  As such, we would then need to add the 1 watt of power consumption from the cable to the total power consumption of the floods (40 watts) for a total of 41 watts. 
  • So, say you have a larger power supply, maybe 100 watts and you wanted to reduce your wiring costs and use 24 gauge wire - could that work?  Let's see.  24 AWG looses about 25 ohms per 500 feet for a total loss (per pair of wires) of 5.7 watts, plus the 40 watts for the floods, for a total of 45.7 watts - so you are good to go - right?  Nope.  24 AWG can carry about .6 amps per wire, for a total of 1.2 amps of power carrying capacity per pair or about 14.4 watts total.  So, 24 AWG is out because it just isn't large enough. 
  • But wait!  What if you were using CAT5 which ccommonly uses, eight, 24 gauge wires.  If you use two wires for the DMX signal, that leaves you with six, 24 AWG wires - is this enough?  The total current carry capacity of the six wires is 3.6 amps or 43 watts.  That's a little tight for four, 10 watt floods, so what losses does the cable have over 500 feet of cable?  Again, it has a loss of 5.7 watts per pair or about 17 watts total.  Add the 17 watts to the 40 watts and you have 57 watts which is "iffy" on a cable designed for about 43 watts at 500 feet. 
  • So, what if you use it anyway?  Will something bad happen?  It depends.  The more overloaded the wire is, the higher its resistance will become, so where there is some head room, there isn't a free ride here.  What will occur is that you'll end up wasting power (about 15-20 watts) that you would otherwise have not wasted...but maybe the CAT5 cable was much cheaper than the purchase of the 18 gauge cable and that difference was much greater than the cost of the lost power (and the larger power supply to provide that power).
  • Here is a simple test - take a VOM or multi-meter and measure the voltage at the power output from the power supply and then take a second measurement at the end of the cable, with all items turned on with full output (white for RGB lights) and compare the voltages.  Let's say you start off with 12.5v at the power supply, you may end up with 10v at the end.  Is that bad?  Well, it depends on your controller and lights.  You *might* notice a drop in light output on the 4th flood verses the 1st flood or you may not - it all depends.  Some controllers will have no problem running on that 10v, some others may not be able to handle that drop.  This is most common on pixel strings where you can see 50%+ voltage drops.
So, after all that you can see that there is no one single "best" answer as to what wire you should use or how many items you can put on a length of wire - there are a number of factors to consider and what we always recommend doing is using the math and ohms law as a starting point, building your design and then testing voltages and power consumption of the devices to determine if the cable is overloaded or has too large of a power drop.

Feel free to post additional questions on the feedback section.



  1. Users might want to consider the "80% rule" when specing out thier wiring needs. For example, to power the 4 10W floods @ 12V we need 3.33 amps.
    4*10W = 40W
    P(in watts)=I(amps)* E(voltage)
    40=I*12 .. I=3.33amps

    Considering 3.33amps as 80% of the total capacity of the wire,we calculate that to give us a cushion we need to spec out wire that would carry 4.16 amps

    TotAmps * .80 = 3.33amps
    TotAmps = 3.33/.80
    TotAmps = 4.16

    A quick glance into the tables says we need a #15. Remember we are giving ourselves overhead here so a #16 (rated @3.7amps) would probably do the trick, and be easier to come by.

    (Disclaimer: I am NOT an electrican nor an electrical engineer. If there is something incorrect in this post or dangerous, please by all means edit or delete the post.
    Also, all calculations are based on generalizations. Note the differences in current needs when you do not factor in the losses to the wire. This is part of what the 80% does for you. It gives you a little headroom to account for the loss that most do not include.)

    As I stated in another post, this is what i really like about HolidayCoro. David is not just a vendor, but a teacher and I certainly appreciate all the time and effort put in to the blog, the videos and the how-to's.

    Bob Moody

  2. You are correct. The problem even with the 80% rule is that it too is just a ball park solution and because of a vast number of issues that affect current flow over wires (temp, voltage, insulation, proximity of wires to each other, fake copper vs real copper, stranded vs solid, etc) that it's really hard for Holiday Lighting people to determine the actual wire gauge needed. This is why I'd recommend the tables and math as a starting point and then testing the final solution to ensure there are no safety or voltage drop issues that could result in low light output or in the worst case, erratic functioning of pixels or controllers.


  3. In the "Current Carrying Capacity" the chart link that says "such as this one" only links to blogger.com, not to a wiring table.

  4. links don't appear to be working...

    1. We have updated the link - we did not find more than one invalid link.


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