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NOTE: This is an unformatted excerpt from our online fire protection training library.

Load Calculations

Load Calculation Requirements

Voltage Requirements

Article 220 can legitimately disregard power factor by using volt-amperes (VA) for all calculations. There will be no additional calculation to account for power factor, and conductors will be big enough (and therefore safe) even if the power factor is relatively poor.

This is true because the apparent power of a given circuit in VA is never smaller than the true power in watts. This does not resolve the cost, efficiency and other design issues related to low power factor, but the conductors will always be big enough. The notes to Annex D say that the power factor of all the loads is assumed to be the same for convenience in the example calculations.

Sample Calculation:

Assume a circuit uses 960 W (true power). If pf* = 1.00, then 960 W = 960 VA. However, if the power factor is .50 or 50%, then 960 W = 960/0.5 VA = 1920 VA. With the same voltage in each circuit, the current is doubled in the circuit with 50% power factor while the true power in watts remains the same. When VA is used for all calculations, conductors will be large enough for the reduced power factor. The installation will then be safe, if not ideal.

* In fairly simple terms, Power Factor (pf) = true power in watts(W) / apparent power in VA, so W = VA x pf, or VA = W/pf. When pf = 1, W = VA for a given circuit. (Power factor can never be greater than 1.)

NOTE: This is an unformatted excerpt from our online fire protection training library.

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