Calculating Electrical Loads

Overview

Productions should know what their power requirements are before arriving to set. This is done by adding up the electrical needs of all the equipment they intend to use at that location. There are two methods of doing this – quickly estimating via “Paper Amps” or precisely calculating via the “West Virginia Formula” (Watts/Volts=Amps).

Paper Amps works for most general-purpose needs.

The West Virigina Formula is necessary when calculating heavy loads or when your needs are coming up close to the rated capacity of the available circuits (when using every amp counts).

Paper Amps Method

A common practice is to use a method called “Paper Amps.” Paper Amps operates by the simple premise that every 1000watts of electrical demand requires 10Amps of electrical supply. It is not nearly as accurate as the West Virginia formula but since it errors heavily on the side of overestimating the required ampacity (1000w=10A instead of 8.3A), it is a quick and effective way to estimate your power needs based on you fixture list.

The voltage of an electrical service would have to be 100v for the draw of a 1000w light to actually be 10A. Electrical service in the vast majority of US residential and commerical locations typically ranges from 110v to 130v (for single phase, hot to neutral service). 100v is considered to be a below standard voltage supply and is uncommon on most US power grids.

West Virginia Formula

Use the West Virginia Formula (W=VA) is a simplified version of Ohm’s Law that relates voltage, current, and power to determine how much electrical demand equipment will require in order to safely and properly operate. The electrical draw of most lighting equipment is measured in Watts. The capacity of most electrical cables, outlets, and breakers (which are the components that make up a circuit) are measured in Amps. The type of electrical service available for use in the mainland US is AC (Alternating Current) expressed in Volts, which typically ranges from 110-130VAC.

The formula can be arranged to determine different information, you just need to know two values in order to determine the third:

Watts = Volts * Amps

X = 120V x 8.33A

X = 1000w

Watts / Volts = Amps

1000w / 120V = X

X = 8.33A

Watts / Amps = Volts

1000w / 8.33A = X

X = 120VAC

Abstract Example

At 120VAC, the amount of electrical current that will be drawn when operating a single 1000w lighting fixture will be 8.33Amps (1000 / 120 = 8.33A). If the circuit being used is rated for 15A, then subtracting 8.33 from 15A means that there is 6.7A of electrical supply remaining on that circuit.

Modifying the formula can create a new calculation to help determine what size lighting fixture can be placed on this circuit without overloading it (W=VA) 120VAC x 6.67A = 800.4watts is available.

That means a 15amp circuit could easily service the addition of a 650w lighting fixture. As 15A x 120V = 1800watts, and 650w + 1000w = 1650w, then 1800w – 1650w = 150w of surplus capacity.

Practical Example

Tech Scout

During the tech scout of each location, place the leads of a multi-meter into an electrical outlet to determine the voltage of that location’s electrical service. Once identified, use the determined voltage in your calculations. Typically, mains service in the continental US is 110-130VAC for residential and commercial outlets. For this example, we will use 120VAC.

Check the rated capacity of the circuits you intend to use. This is typically found stamped on the breaker switch for each circuit, which is located in the breaker box for that location.

Keep in mind when calculating loads for available circuits; make sure the total rated capacity of a circuit is actually available. Many residential houses and apartments will have wall sconces, overhead lighting, and fans attached to these circuits. Electrical circuits are often shared over multiple areas; do not assume that outlets in a house or office space are on different circuits simply because they are located in different rooms.

Use a small low wattage light, a partner, and your cell phones (for communicating) to make a guide of what outlets are on which circuit. Have one person plug the light into each outlet as the other person flips the circuit breakers to determine which one it is connected to. Always get permission from the homeowner and/or the business merchant before doing so when using a third-party location.

Fixture List

Collect the electrical draw of each fixture or globe you intend to use. This information can typically be found stamped directly onto the equipment or via a specification document available on the manufacturer’s website. The electrical draw of most lighting equipment is rated in wattage (w), but sometimes it is rated in amps (A).

A common lighting package is made up of the following fixtures and bulbs (globes):

• (4) Philips LED Practical Bulb – 7w
• (1) Aputure Nova p300C Panel – 300w
• (1) Aputure Nova p600C Panel – 600w
• (1) Amaran 150C Monolight – 150w
• (1) Amaran 300C Monolight – 300w
• (1) Aputure LS 600D Monolight – 600w

Calculate the Load

That’s four 7w globes and one 150w, two 300w, and two 600w LED fixtures for a total demand of 1978w at 120VAC (1978w / 120 = 16.48A). That means putting all 9 fixtures on to a 15A circuit will overload the circuit by 1.48A or 178watts:

7w x 4 = 28w + 150w + 300w + 300w + 600w + 600w = 1978w Total

16.48A – 15A = 1.48A

120VAC x 1.48A = 178w

Drawing 178w over the rated load of 15A (or 1800w) would eventually overheat and trip the circuit breaker, cutting off power during production. If the breaker were malfunctioning, it could lead to overheating of the electrical cables before the break trips which could eventually burn out the cables in the walls (aka start a fire).

Service Requirements

To use all 9 fixtures and globes on-set, locate an additional circuit nearby to move some of the electrical load to. Try repatching the Nova p600c to a new circuit:

Circuit 1: 1978w – 600w = 1378w; which is now well below the 1800w capacity of a single 15A circuit.

Circuit 2: 600w; which is also well below the 1800w capacity of a single 15A circuit.