Power-Factor Correction and Lighting-Level Calculations
Theory from NEETS Module 2 (NAVEDTRA 14174A) 4.6.4 and Construction Electrician Basic (NAVEDTRA 14026) ch. 6 / Intermediate (NAVEDTRA 14027) ch. 6, read 2026-09-20 (read-log-neets.md, read-log-theory-manuals.md); NFPA 70-2023 430.32 IN (460.9) and Table 240.4(G) as read in read-log-2023.md — Article 460 itself awaits the next viewer pass · Reviewed 2026-09-21
Two General Theory topics are pure arithmetic: power-factor correction and lighting-level (footcandle) calculations. The electrical-theory lesson has the Ohm's-law and three-phase formulas; this lesson adds the power triangle, capacitor sizing, and the lumen method, paraphrased from the public-domain NEETS and Navy Construction Electrician courses.
1. True, reactive and apparent power (NEETS 2-4.6.4)
- True power (P, watts) is the power actually used — dissipated in resistance: P = I_R² × R = E × I × cos θ. The kilowatt-hour meter bills it.
- Reactive power (Q, var) is the power that inductance or capacitance borrows and returns to the source each cycle: Q = I_X² × X (inductive and capacitive vars subtract from each other). Motors, magnetic ballasts, transformers and welders draw inductive (lagging) vars.
- Apparent power (S, volt-amperes) is what the source "sees" through the impedance: S = E × I = I_Z² × Z. It is not P + Q; like impedance, it combines at right angles — S² = P² + Q² (the power triangle).
- Power factor = P ÷ S = cos θ = R ÷ Z (series circuit), from 0 to 1 (0–100 %). A PF of 0.6 means only 60 % of the apparent power does work; the conductors, transformer and generator must still carry all of S.
| Formula | Unit | |
|---|---|---|
| True power | P = E × I × PF (1-φ); P = √3 × E_L × I_L × PF (3-φ) | W |
| Apparent power | S = E × I; S = √3 × E_L × I_L | VA |
| Reactive power | Q = √(S² − P²) = S × sin θ = P × tan θ | var |
| Power factor | PF = P ÷ S = cos θ | — |
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