Power Guide: Watts, VA, Power Factor & Circuit Sizing
Types of Electrical Power
- Real Power (P): Actual work done. Measured in Watts (W). P = V × I × PF
- Reactive Power (Q): Energy stored and released by inductors/capacitors. Measured in Volt-Amperes Reactive (VAR). Q = V × I × sin(θ)
- Apparent Power (S): Total power delivered by the source. Measured in Volt-Amperes (VA). S = V × I
Power triangle: S² = P² + Q²
Power Factor: PF = P / S = cos(θ). Ranges from 0 to 1.0. PF = 1.0 means all power is real (purely resistive). Motors typically have PF 0.6–0.9.
Power Formulas
| Circuit Type | Real Power (W) | Apparent Power (VA) | Current (A) |
|---|---|---|---|
| Single-phase | P = V × I × PF | S = V × I | I = S / V |
| 3-phase (line-to-line) | P = VLL × I × PF × √3 | S = VLL × I × √3 | I = S / (VLL × √3) |
| 3-phase (line-to-neutral) | P = 3 × VLN × I × PF | S = 3 × VLN × I | I = S / (3 × VLN) |
Common Power Factors by Load Type
| Load Type | Typical PF | Impact on Sizing |
|---|---|---|
| Incandescent bulb | 1.0 | W = VA |
| LED driver | 0.9–0.95 | Minimal correction needed |
| Fluorescent (no PFC) | 0.5–0.6 | Needs capacitor correction |
| Induction motor (loaded) | 0.75–0.85 | Common in HVAC |
| Induction motor (light load) | 0.3–0.5 | Very poor PF |
| VFD (variable freq drive) | 0.95–0.98 | Good with input reactors |
| Welding transformer | 0.3–0.5 | Severe PF penalty |
Why Power Factor Matters
Low power factor means higher current for the same real work. This increases:
- Wire size: Must be sized for VA (current), not watts
- Transformer loading: A 100 KVA transformer with PF 0.7 can only deliver 70 KW of real power
- Utility penalties: Many utilities charge penalties for PF below 0.85–0.90
- Voltage drop: Higher current causes more voltage drop in conductors
Three-Phase Power
Three-phase power uses three alternating currents, 120° apart. It's more efficient for high-power loads:
Advantages:
- Constant power delivery (no zero crossings)
- Thinner wires for same power (3 conductors vs 2 for single-phase)
- Self-starting motors (rotating magnetic field)
| System | Voltage | Use |
|---|---|---|
| 208Y/120V 3-phase | 208V line-to-line, 120V line-to-neutral | Commercial buildings |
| 480Y/277V 3-phase | 480V line-to-line, 277V line-to-neutral | Commercial/industrial |
| 240V 3-phase delta | 240V (high-leg: 208V to neutral) | Older commercial |
| 600Y/347V 3-phase | 600V line-to-line | Canadian industrial |
Common Mistakes
- Confusing watts and VA: A 1,000VA UPS with 0.7 PF only delivers 700W. Always check both ratings.
- Ignoring PF for wire sizing: Wire must be sized for current (amps), not watts. A 960W motor at 120V, 0.8 PF draws 10A, not 8A.
- Using single-phase formulas on 3-phase: Missing the √3 factor. 3-phase current = VA / (VLL × √3).
Standards Reference
- NEC Article 220 — Branch Circuit Calculations
- IEEE 1459 — Power Definitions
- NEMA MG-1 — Motors and Generators
- IEEE 519 — Harmonic Limits
Frequently Asked Questions
What's the difference between watts and VA?
Watts (W) is real power — the actual work done. VA (volt-amperes) is apparent power — total power delivered. For resistive loads (heaters, bulbs), W = VA. For reactive loads (motors), VA > W due to power factor. A motor with 960W and 0.8 PF has 1,200VA apparent power.
How do I calculate 3-phase power?
For 3-phase: P = V_LL × I × PF × √3. Example: 480V, 30A, PF 0.85: P = 480 × 30 × 0.85 × 1.732 = 21,195W ≈ 21.2 KW. The √3 factor accounts for the 120° phase relationship.
What is a good power factor?
A power factor above 0.90 is considered good. Most utilities require PF above 0.85 to avoid penalties. Motors are the most common cause of low PF — they typically run at 0.75–0.85. Power factor correction capacitors can improve PF to 0.95+.
Does low power factor increase my electric bill?
Yes, in two ways. First, your wiring carries more current for the same work, increasing I²R losses. Second, many commercial utilities charge a PF penalty when it drops below 0.85–0.90. Residential customers typically aren't penalized for PF.
How do I size a UPS for my equipment?
Add up the VA ratings (not watts) of all equipment, then add 20% margin. A computer drawing 300W at 0.6 PF has 500VA. For 3 computers: 1,500VA + 20% = 1,800VA UPS minimum. Check both the VA and watt ratings of the UPS — both must exceed your load.