How to calculate electrical loads

[De Trommelslager]

For those that are unfamiliar with calculating the loads of your lights for circuit sizing purposes, this tutorial will hopefully put you at ease with the process. It is really simple math at work, so don’t let the technical jargon intimidate you.

To break this down into segments, this has been broken into four parts. The first part is the terms and descriptions of each. In the second segment, we will look at the various formulas that you will need to calculate your loads. In the third segment, we look at an example set up and look at some of the things we need to know in order to safely install a lighting system. And in the fourth segment, we look at the answers from our example and wrap it all up. So, let’s get going.


The first thing you have to know is Ohm’s Law, specifically the formulas and what the different terms mean. For this tutorial, we won’t get in depth with all of the formulas and theories, but rather we’ll skim the surface to provide what you need to calculate your lighting loads for planning purposes.

Following is a simplification of what things mean, and each is illustrated to help make understand what is happening in an electrical circuit. Think of electricity as water and an electrical circuit as a system of pipes. There are obvious differences between the two, but for this exercise we are going to illustrate the basics with the water flow concept.

Voltage
Think of voltage as the pressure in a circuit similar to the pressure in a garden hose. This is the potential energy between two points. The more voltage, the higher the pressure (or potential) is. In the formulas, E = Electro-motive force, which is measured in Voltage or Volts and represents this pressure (or potential).

Amperes
The amount of electrical flow in a circuit can be thought of in how much water is (or can) flow through a pipe. In the formulas, I = Current in Amperes and represents the amount of current flow in a circuit.

Resistance
The ability of the circuit to carry electricity is known as resistance and can be compared to the size of a pipe. More water will flow through a garden hose than through a straw. If you had to fill a swimming pool, a garden hose would be a better choice than a straw because it has less resistance. The higher the resistance in a circuit (or material), the less current it is able to carry. In the formulas, R=Resistance in Ohms and represents the ability of the circuit to carry current.

Wattage
The total amount of energy being delivered is a function of the three items above. If you have a fire hose with 100psi, there will be a substantial amount of water being delivered. We’ll say this is the most efficient for the circuit. But take the same hose with 1psi and the water delivery will be much less, and efficiency will dramatically decrease. On the other hand, let’s take the same fire hose and apply 1000psi to it; there will be a little more water coming out in comparison to the sweet spot of 100psi, but not ten times as much because the circuit is not efficient at the higher pressure. In the formulas, P=Power in Watts represents the total amount of work being done.

Recapping the terms:
E=Electro-motive Force in Volts
I=Current flow in Amperes
R=Resistance in Ohms
P=Power in Watts


There are some constants, and some values that will move. So, let’s briefly look at those and finish this segment up.

The load on your system will be a constant figure. If you have 1000W worth of lights connected, then that will remain the same (unless lamps go out, etc.). Voltage does change, and along with that so will the current. Resistance can change as well, such as with temperature changes, but for our purposes it really isn't a factor.

Voltage and Amperes are inversely proportional to each other. In other words, when the Voltage increases, the current (Amperes) will decrease. As Voltage decreases, current increases. This is not a huge problem for us, but if you have things loaded right to the breaking point, then you might find yourself with unhappy electronics should you have a Voltage drop.

It is also important to note that the current flow will be higher for a system with 110V nominal Voltage compared to a system with a nominal Voltage of 125V. Not much, but there is a difference. And keep in mind as demand increase on the power grid, voltages do move around.

Now, taking into account the voltage fluctuations may be considered "splitting hairs". The focus of this isn't really to nail things down to a hundredth of an Ampere, but rather to explain how electricity behaves.


In the next segment, we’ll look at the formulas needed for performing load calculations.

[De Trommelslager]

Now that you know the terms and have an understanding of what each represents, we’ll talk about the formulas.

Again, this is basic math, but the technical jargon tends to throw people a curve ball and creates confusion.

E=Electro-motive Force in Volts
I=Current flow in Amperes
R=Resistance in Ohms
P=Power in Watts

Here are the formulas. All you have to do is plug the known values in to find the answer to your problem.




To find Amperes when the Voltage and Wattage are known:
W / E = I Watts divided by Voltage equals Amperes.

To find Wattage when Voltage and Amperes are known:
I * E = P Amperes multiplied by Voltage equals Wattage.

To find Ohms when Voltage and Amperes are known:
E / I = R Voltage divided by Amperes equals Ohms.

To find Voltage when Amperes and Resistance are known:
I * R = E Amperes multiplied by Ohms equals Voltage.

With the above formula diagrams, you can solve basic electrical equations. Simply plug in the known values and calculate. There are a vast number of different formulas for solving electrical problems, and the above only skims the surface. However, with the above formulas you can calculate your loads and design a safe display system.

Please note that these formulas do not take into account any environmental variables, such as temperatures, wire sizes, or other factors. Those are important considerations, but are outside the scope of this tutorial.


In the next segment, we’ll put these formulas to work in a scenario of planning a display.

[De Trommelslager]

Okay, now that you know the Ohm’s Law formulas and the basics of what the terms mean, let’s put it all into practice and plan out a lighting display.

You will need to know some information from your lighting loads, mainly the Wattage of each string of lights. For this exercise we will use some generic values, but you should use the values of your equipment for calculating loads. If you are ever in doubt, either measure the needed value with an appropriate meter or add some safety factor to a generic value.

An incandescent light string with 100 lamps uses 40.8 Watts. To find the Amperes of this circuit, we will divide the voltage (120V) by the Wattage (40.8W) and that comes to .34 Amperes. Review the formulas above to solve this problem.

A LED light string with 100 lamps draws 9.6 Watts. To find the Amperes, the same formula and calculation as above will equate to .08 Amperes.


Now, let’s plan a display. This plan calls for 9,602 lights on one controller. We have one controller with two dedicated 15A circuits feeding each side of the unit. Our plan calls for the following display items and light string counts for each channel.

Channel 1: Quantity of two – 100 LED light nets in bushes

Channel 2: Quantity of two – 100 LED light nets in bushes

Channel 3: Quantity of four – 100 Incan light string around columns

Channel 4: Quantity of four – 100 Incan light string around columns

Channel 5: Quantity of three – 100 Incan light string on roof

Channel 6: Quantity of three – 100 Incan light string on roof

Channel 7: Quantity of five – 100 Incan light string on roof

Channel 8: Quantity of seven – 100 Incan light string on roof

Channel 9: Quantity of one – 200 LED light string on yard

Channel 10: Quantity of two – 100 LED light string on yard

Channel 11: Quantity of thirty – 100 LED light string on house

Channel 12: Quantity of twenty – 100 LED light string on house

Channel 13: Quantity of one – 150W Flood light

Channel 14: Quantity of one – 100W Flood light

Channel 15: Quantity of three – 200 LED light string on tree

Channel 16: Quantity of six – 100 LED light string on tree


Now that we know what we want to use in the display, we next need to know what each channel will draw in Amperes since that is how the LOR controller is rated. Each channel is rated at 8 Amperes, and each side (1-8; 9-16) is rated at 15 Amperes. It is important that we don’t overload the channels and also important that we don’t overload our power circuits.

Regarding power circuits, the National Electrical Code states that a circuit can only be loaded to 80% of its rating for continuous loads. And the NEC defines a continuous load as three hours in duration or longer. If your lights come on at 7PM and go off at 11PM, then that is considered a continuous load, and the 80% rule applies.

A best practice is to treat your system as a continuous load and never load your power circuits above 80% capacity. On a 15 Ampere power circuit, that equates to 12 Amperes for our calculated load. On a 20 Amperes circuit, the equates to 16 Amperes.

On the channels, LOR rates each for 8 Amperes. However, loading those to 100% of the rating isn’t a best practice. For our purposes, let’s load the channels to a maximum of 75% of the rating, or 6 Amperes. You can adjust your load budgets on your controller how you wish, but don’t exceed the ratings.

With these budgets in mind, let’s calculate each channel and see where we end up. Will our display work as planned, or are we going to have to make adjustments? Are all of the channels within budget, and are our two power circuits within budget? If not, then we might find ourselves with tripping breakers, or worse yet a controller with blown out channels!

I encourage you to work out at least a few of these loads for practice. This will help you get comfortable with the process. It isn’t a hard task to do, and once you know how, only takes a few minutes time.


In the next segment, we'll give look at a few of the calculations and see the loads for each channel. Then we can determine where this display plan needs adjustment.

[De Trommelslager]

Okay, so here are a few of the calculations. If you worked some or all of the calculations, then compare your answers below.


Channel 1: .16A Quantity of two – 100 LED light nets in bushes
9.6W / 120V = .08A ; .08A * 2 = .16A

Channel 2: .16A Quantity of two – 100 LED light nets in bushes
9.6W / 120V = .08A ; .08A * 2 = .16A

Channel 3: 1.36A Quantity of four – 100 Incan light string around columns
40.8W / 120V = .34A ; .34A * 4 = 1.36A

Channel 4: 1.36A Quantity of four – 100 Incan light string around columns

Channel 5: 1.02A Quantity of three – 100 Incan light string on roof

Channel 6: 1.02A Quantity of three – 100 Incan light string on roof

Channel 7: 1.7A Quantity of five – 100 Incan light string on roof

Channel 8: 2.38A Quantity of seven – 100 Incan light string on roof

Channel 9: .16A Quantity of one – 200 LED light string on yard
9.6W * 2 = 19.2W ; 19.2W / 120V = .16A

Channel 10: .16A Quantity of two – 100 LED light string on yard
9.6W / 120V = .08A ; .08A * 2 = .16A

Channel 11: 2.4A Quantity of thirty – 100 LED light string on house

Channel 12: 1.6A Quantity of twenty – 100 LED light string on house

Channel 13: 1.25A Quantity of one – 150W Flood light
150W / 120V = 1.25A

Channel 14: .84A Quantity of one – 100W Flood light

Channel 15: .48A Quantity of three – 200 LED light string on tree

Channel 16: .48A Quantity of six – 100 LED light string on tree


Channels 1-8: 8.14A Total
Channels 9-16: 7.37A Total


In this example, our controller’s channels are well under our allowed 6 Ampere budget, and each side of the controller is under the allowed 12 Ampere budget for the 9600 lights. According to the calculations, this display plan will not overload any channels or circuits.

There is a mixture of different light types in this example and probably doesn't look anything like your display. However, the calculations are the same. All you need to know is your voltage and the load rating (either Watts or Amperes) of the load.

Not so bad, right? It does take a little time, but after crunching the numbers, you aren't blindly plugging in lights, tripping breakers and smoking controllers.


There is an Excel 2007 spread sheet calculator that is very well done and will take a lot of the work out of the process. I will ask permission from the author to include a link here, and will update the post when/if granted. Until then, search the forums for "load calculator" and you should be able to find it.


One last point... No amount of calculating will allow you to know exactly what your load is. Having an Amperage meter and a Volt-Ohm meter are essential tools for our hobby. Even after you do your calculations, it is wise to verify your results.

Learn how to safely use your meter(s), and always remember that working on live circuits is dangerous. Many people are killed each year by electricution!


Best wishes with your lighting display!



This tutorial may contain errors and omissions. This information in no way should substitute for obtaining electrical services from a licensed electrical contractor. Working with live electrical circuits is hazardous and could result in serious injury or death. Use this information at your own risk!

[Dave Batzdorf]

Nicely done.

[Denis Chaput]

http://www.quartzhillchristmas.com/12.html

Can also download this excel spreadsheet which will help simplify it for the newbies

[Guest guest]

The quartzhill calculator is a must-have..right up there with a killowatt meter, etc...one helps you plan, one helps you confirm.

And the author of the calculator never seems to mind the link being shared.

Also, as some newbies may notice, someone has been trying to sell something similar on ebay...but the quartzhill calc is free, so don't waste your time flipping cash to some unknown seller on ebay.

[David Rise]

This needs to be put up there with Wills dictionary. Very helpful stuff.

[CKSedg]

Very good. Thanks for having the patience to do this. Those who don't understand electricity can use this to help them very well.