Basic Electrical Laws calculator
Voltage Calculator
This voltage calculator is a focused view of Ohm's Law for situations where circuit voltage is the primary unknown. Given any two of voltage, current, resistance, or power, the underlying engine applies standard relationships such as V = I × R, V = P / I, and V = √(P × R). The tool is intended for low- and medium-voltage DC and AC resistive work (for example 120/240 V branch circuits, control circuits, and resistive heaters); reactive effects, harmonics, and detailed protection studies still require separate analysis.
Updated July 10, 2026
Example Calculations
How to Use
How to use this voltage calculator
This calculator uses the same core engine as the Ohm's Law calculator, but presents it from a voltage-first perspective. It is suited to US-style branch circuits and equipment nameplate checks where you need to confirm or back-calculate line voltage.
1. Decide which quantities you know
- For most field work you will know current (I) from a clamp meter and either resistance (R) of the load or its power (P) rating.
- The calculator accepts any two of V, I, R, and P; it then solves the remaining quantities using standard Ohm's Law and power identities.
2. Measure using RMS instruments where applicable
- For AC work on 50/60 Hz systems, use an RMS meter and treat the load as predominantly resistive.
- For non-sinusoidal or strongly reactive loads, use this tool only as a first approximation and pair it with impedance or power-factor analysis using the impedance calculator.
3. Enter data and compute voltage
- Enter your known values in consistent units (volts, amperes, ohms, watts). Leave the field you want the tool to solve blank.
- The engine will calculate voltage and the other missing quantities, then scale results into appropriate units (for example mA, kW) for readability.
Reference formulas for solving voltage
| Known quantities | Voltage formula | Typical use |
|---|---|---|
| Current (I) and resistance (R) | V = I × R | General DC or single-phase branch circuits with predominantly resistive loads. |
| Power (P) and current (I) | V = P / I | Loads where nameplate power is known and current is measured with a meter. |
| Power (P) and resistance (R) | V = √(P × R) | Heaters or other resistive elements specified by power and resistance. |
Worked voltage examples (for quick checks and tests)
| Scenario | Inputs | Expected results (with units and tolerance) |
|---|---|---|
| 20 A branch circuit with ≈6 Ω load | I = 20 A, R = 6 Ω | V ≈ 120 V (±0.5 V), P ≈ 2.40 kW (±1%) |
| 4.5 kW water heater drawing 18.5 A | P = 4.5 kW, I = 18.5 A | V ≈ 243 V (±1 V), R ≈ 13.2 Ω (±2%) |
| 1.5 kW resistive heater with 10 Ω element | P = 1.5 kW, R = 10 Ω | V ≈ 122 V (±1 V), I ≈ 12.2 A (±2%) |
4. Cross-check against equipment and wiring limits
- Compare calculated voltage against equipment nameplate ratings, especially for motors and control power transformers.
- If calculated voltage is significantly lower than nominal, evaluate conductor size and run length using the voltage drop calculator and, where necessary, conductor sizing with the wire size calculator.
5. Apply code-oriented judgement
- NEC design practices commonly reference 3% branch-circuit and 5% total system voltage-drop targets as informational notes; these are not automatic pass/fail thresholds, but they are useful comparison points.
- Always resolve final conductor sizes, protection, and settings using the current edition of NFPA 70 (NEC) and project specifications; this calculator only supplies the underlying electrical relationships.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
Can I use this voltage calculator for three-phase systems?
The underlying math follows scalar Ohm's Law relationships and can be applied phase-by-phase. For balanced three-phase work, you may treat each phase as a single-phase circuit using line-to-line or line-to-neutral values as appropriate, but detailed three-phase power and voltage-drop calculations are better handled with tools that include power factor and phase relationships.
Does this tool account for power factor or harmonics?
No. The calculator assumes a resistive relationship between voltage, current, resistance, and power. For circuits with significant reactive or non-linear loads, treat results as an idealised reference and perform a separate power-factor or impedance analysis before making design decisions.
Is this calculator sufficient for NEC compliance decisions?
The tool provides numerically correct Ohm's Law relationships but does not embed NEC rules. Use it to obtain electrical quantities, then apply code requirements such as continuous-load factors, conductor ampacity, and voltage-drop guidance using the current NEC and project specifications.
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