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Electrical Formulas Wall Chart

Ohm's law, power formulas and single- and three-phase current, kVA and horsepower formulas for quick reference.

Ref EL-0063

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A quick list of the basic formulas used to relate voltage, current, resistance and power. Symbols used: E or V is voltage in volts, I is current in amperes, R is resistance in ohms, P is power in watts, PF is power factor, and Eff is efficiency as a decimal. 1.732 is the square root of 3.

How to use it

Pick the formula that solves for the value you need using the values you know. Keep units consistent: volts, amperes, ohms and watts. Divide watts by 1,000 for kW. These formulas give calculated values only; conductor and overcurrent device sizing follow the NEC, not these results alone.

Ohm's law and power

Find	Formulas
Voltage (E)	E = I × R; E = P ÷ I; E = √(P × R)
Current (I)	I = E ÷ R; I = P ÷ E; I = √(P ÷ R)
Resistance (R)	R = E ÷ I; R = E² ÷ P; R = P ÷ I²
Power (P), DC or resistive	P = E × I; P = I² × R; P = E² ÷ R

AC circuits

Find	Single-phase	Three-phase
Amperes from kW	I = (kW × 1,000) ÷ (E × PF)	I = (kW × 1,000) ÷ (1.732 × E × PF)
Amperes from kVA	I = (kVA × 1,000) ÷ E	I = (kVA × 1,000) ÷ (1.732 × E)
Amperes from horsepower	I = (HP × 746) ÷ (E × Eff × PF)	I = (HP × 746) ÷ (1.732 × E × Eff × PF)
kVA	kVA = (I × E) ÷ 1,000	kVA = (I × E × 1.732) ÷ 1,000
kW	kW = (I × E × PF) ÷ 1,000	kW = (I × E × 1.732 × PF) ÷ 1,000
Horsepower output	HP = (I × E × Eff × PF) ÷ 746	HP = (I × E × 1.732 × Eff × PF) ÷ 746

Key points

1 horsepower = 746 watts.
Three-phase formulas use line-to-line voltage, such as 208, 240 or 480 V.
Power factor is 1.0 for purely resistive loads such as heaters. Motors and other inductive loads have a lower power factor.
Resistors in series add: R total = R1 + R2 + R3. In parallel: 1 ÷ R total = 1 ÷ R1 + 1 ÷ R2 + 1 ÷ R3.
Voltage drop calculations also need conductor resistance from NEC Chapter 9, Table 8 or Table 9.

Common mistakes

Using the horsepower formula to size motor circuits. The NEC requires the motor full-load current tables in Article 430 for most motor circuit sizing.
Using line-to-neutral voltage in a three-phase formula that expects line-to-line.
Forgetting power factor and efficiency, which makes calculated current too low.

Governing standards: None for the formulas. NEC Article 430 for motor circuit values and NEC Chapter 9 for conductor properties.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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