Calculate Lighting Loads

Calculating Lighting Loads

Branch circuits that supply lighting loads are calculated either at Continuous Duty or Non-continuous Duty (less than 3 hours of continuous operation).

I (Amperage - also known as Current)

VA (Volt Amp - also known as Watt)

V (Volt)

DF % {Duty Factor for Continuous = 125 %, for Non-continuous = 100 %}

Formula to Use:

I = (VA * DF %) / V

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Branch Circuits that supply Non-continuous Duty loads (i.e. lamps, etc.) are calculated at 100 % of the total VA of the circuit.

Example:

How many receptacles (outlets) are allowed on a 15 Amp, 120 Volt, 1 Phase circuit; used at Non-continuous duty?

Answer.

Use : I = (VA * 100%) / V

(1st Step)

I = (180 VA * 100 %) / 120 V = 1.5 Amp per outlet.

(2nd Step)

15 Amp (Breaker) / 1.5 Amp = 10 Receptacles

So the total amount of receptacles you can put on a 15 Amp circuit is 10.

Branch Circuits that supply Continuous Duty loads (i.e. lamps, etc.) are calculated at 100 % of the total VA of the circuit;

In other words, the OCPD (circuit breaker) will be rated at 125 % of the total load.

Example:

How many receptacles (outlets) are allowed on a 15 Amp, 120 Volt, 1 Phase circuit; used at Continuous duty?

Answer.

Use : I = (VA * 125%) / V

(1st Step)

I = (180 VA * 125 %) / 120 V = 1.875 Amp per outlet

(2nd Step)

15 Amp (Breaker) / 1.875 Amp = 8 Receptacles

So the total amount of continuous duty receptacles you can put on a 15 Amp circuit is 8.

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