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 difficult to overload circuits. The voltage of the 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 homes typically ranges from 110v to 130v. 100v is considered to be a below standard voltage supply and is uncommon on most US power grids.

Keep in mind when calculating loads for available circuits; make sure the rated capacity of a circuit is available. Many homes 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 from different rooms in a house or office space are on a different circuit. When tech scouting a location, 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.

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

Place the leads of a multi-meter into an electrical outlet to determine the voltage of that building’s electrical service. Typically, mains service in the continental US is 120VAC for residential and commercial outlets. 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

A common lighting package is made up of the following fixtures:

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

That’s four 7w, three 300w, and two 600w LED fixtures for a total demand of 2128w at 120VAC (2128w / 120 = 17.73A). That means putting all 9 fixtures on to a 15A circuit will overload the circuit by 328watts:

17.73A – 15A = 2.73A

120VAC x 2.73A = 328w

Drawing 328w over the rated load 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).

To use all 9 fixtures on-set, locate an additional circuit nearby to move some of the electrical load to. Try repatching both the Nova p300c and the Nova p600c to a new circuit:

Circuit 1: 2128w – 600w – 300w = 1228w; which is now well below the 1800w capacity of a single 15A circuit.

Circuit 2: 600w + 300w = 900w; which is also well below the 1800w capacity of a single 15A circuit.