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Load Calculations

Branch Circuit Load Calculations

Loads Added to Existing Installations

Section 220.16

Section 220.16 addresses loads that are added on to existing installations. Fundamentally, these rules say that loads on additional branch circuits will be calculated the same as the original circuits. The calculation of added loads must also extend to the service or feeder to determine that the existing supply is adequate. That calculation is covered in Parts III and IV.

Some of these load requirements are clarified in Section 220.18, which talks about the specific requirements for a certain type of load.

Subsection 220.18(A) covers motor-operated combination loads, which typically are sized based on nameplate information. Subsection 220.18(B) addresses inductive and LED lighting loads, which are based on the input rating to the ballast, transformer or other power supply, not on the rating of the lamps.

For a common example covered by Subsection 220.18(B), consider the markings on a typical compact fluorescent lamp (CFL). The lamp is rated at 14 W, which would seem to be about 0.12 amps (14W/120V). However, the lamp is marked 120 V, 0.200 A, or 120 V x 0.2 A = 24 VA. The load must be based on this 24 VA. The same type of rule applies to all types of discharge lighting. The load is based on the input ratings of the luminaire, including the ballast as in the CFL example, not on the output ratings of the lamps. Incidentally, this CFL, which includes an integral ballast, seemingly has a power factor of 14/24 = .58 or 58% pf. This is not precisely correct, because the 14 W is an output rating and the 24 VA is the input rating. So part of this difference is due to efficiency or luminous efficacy, which is a consideration for any lighting source no matter how high the power factor.

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

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