"I have a giant tree to light up this year. I'm concerned how many Christmas light strings I can hook up together. What is the maximum and how much power will that require?"
"How long can I make my extension cords? I don't have any electrical outlets close by."
"What can I use to measure the amount of current my display is taking?"
Do The Above Questions Sound Familiar?
People get really excited about Christmas lights (understandably so!) and it's easy to install way more lights than your circuits can handle. So how should we determine what the limits are? With some basic math and a little understanding of electrical characteristics, you can have a safe, functioning, and amazing display.
Step 1: Find Out How Many Amperes Your Electrical Outlet Has
Most homes use 15 amp circuit breakers. This can be confirmed by looking at your electrical panel. There will likely be a "15" or "20" on the breaker switch indicating the number of amps the breaker will trip at. However, if you have other lights or electrical appliances on the same circuit, the available amps will be less (unless they are switched off). As a general rule of thumb, it's best to not exceed 70% of the maximum available amperes on the circuit. This allows a "buffer" for electrical fluctuations which may be caused by rain, when initially plugging in lights, or increase in draw with the age of the wire and bulbs, which will slowly draw more current as they get older. If you live in a dryer climate, you may consider less of a buffer.
Step 2: Learn The Power Required For Your Lights
Once you know how much power is available, it's time to calculate how many lights you can safely install. Let's assume you have a 15 amp circuit without any other devices or lights on it. Setting aside a 30% buffer means you now have 10.5 amps available. Then you must look up how much power your chosen product uses. The watts per unit is published on the product pages of the website, but it may also be found on the product itself, either engraved or on a tag. By dividing your watts by your area's voltage, you can calculate the amps per unit.
Formula: P / V = I or Watts / Volts = Amps
However, the actual power required is not just a simple calculation unfortunately. Inherent resistance in the wires and plugs will cause additional power consumption. And as the product ages, it will further increase the power consumption. Lets look at two of our most popular products as case studies: the retrofit bulb (G30 or C9), and the lighted string set (C6 or 5mm mini-light).
G30 or C9 Bulb
Our G30 or C9 bulbs draw 0.45W per bulb. Using the formula above, that is equal to 0.00375 amps per bulb. (0.45 / 120 =0.00375). If we have 10.5 amps available, that's 2,800 bulbs! (10.5 / 0.00375 = 2,800). But PLEASE DON'T hook up that many bulbs. Remember that there is resistance in the wire and 2,800 feet of 18AWG wire is a lot of resistance. In our experience, we typically see 3.5 to 5 amps for every 250 feet of G30 or C9 bulbs, depending on the age of the wire. That means you should be able to hook up 500 bulbs to the circuit described above. However, the SPT2 plugs attached to the ends of the cable can only withstand 8 amps. A 30% buffer brings that down to 5.6 amps. Therefore, you should limit your runs to 250 feet. If you have a 15 amp circuit available, then you'll be able to install two x 250-foot runs. So if you are installing lights on a very long roofline at a shopping plaza for example, you should require at least one electrical outlet every 500 feet.
C6 or 5mm Mini Light Strings
The C6 or 5mm mini-lights draw either 4.8W or 2.4W depending on the colour (red, yellow, and multi-colour are at 2.4W). Using the formula above, we calculate that each 4.8W string consumes 0.04 amps. (4.8 / 120 = 0.04). If we have 10.5 amps available at our electrical outlet (this includes the 30% buffer on a 15 amp circuit), we calculate that we can connect 262 strings to the outlet. However, our adapters have a 5 amp fuse. This is required because the wire is 20AWG and CSA sets the limit for this size of wire at 5 amps. Using a 30% buffer brings that number down to 3.5 amps. So the maximum number of 4.8W strings that you can connect end-to-end is in theory 87 strings. In our experience, we find that 75 strings is a better number for a little extra buffer because 75 strings equals 1875 feet of wire, which is a lot of wire on one run. CSA takes this buffer a step further by recommending a maximum of 45 strings in one run. Either way, you should be able to install approximately 3 x 75 strings = 225 strings to single 15 amp outlet with a 30% buffer (or 10.5 amps), but not through a single adapter. You will need to instead break those 225 strings up into at least 3 runs requiring 3 adapters (225 / 3 runs = 75 strings = approx. 3.5 amps to meet adapter requirements x 3 runs = 10.5 amps which meets electrical circuit requirements).
Step 3: Measure Extension Cords Distances
Extension cord lengths should be kept at less than 100 feet, particularly if you are using 18AWG SPT2 cable, in which case you might want to consider less than 50 foot lengths. The longer the cord, the more resistance and power it will consume. The relationship between distance and power consumption is not a linear relationship however, so you can't simply say "allow 'x' amps per 'y' feet". If you require to plug in greater than about 100 feet away, consider increasing the gauge of extension cord to 16AWG or 14AWG. If the distance is significant, then 12AWG could be a good solution. Of course, if the loads from the Christmas light display are high, you'll want to reduce these distances. Always error on the side of caution here.
In summary, the G30 / C9 bulbs which are 0.45W per bulb should be limited to 250 foot runs. The C6 or 5mm mini-lights should be limited to 75 strings connected end-to-end (note CSA requires 45 string max).
In all cases, when you think you're close to your limits, it's a great idea to measure. One of the handiest tools you can keep in your tool-kit for troubleshooting is a clamp meter, which is available at most hardware stores. This will give you accurate information that will ensure you're not overloading adapter fuses or electrical circuit breakers.