Ohm's law links the three measurable quantities of a DC or resistive AC circuit — voltage, current, and resistance — through a single relationship: the current that flows through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. Add the power identity P = V × I, and four quantities collapse into two independent ones: pick any two and the other two follow.
V = I × R (Ohm's law)
P = V × I (DC power)
P = I² × R (substitute V)
P = V² ÷ R (substitute I)
The calculator above shows all four fields at once. Type into any two and the remaining two appear automatically — no toggles, no mode-picking. That covers every scenario you'll meet on a domestic or light-commercial NZ installation: known load and supply voltage, known cable run with measured resistance, manufacturer plate showing watts and volts, or fault current measured against a known loop impedance.
Almost every calculation a NZ electrician does on the design or test side leans on Ohm's law somewhere. A few common examples:
You almost never need all twelve permutations. In day-to-day AS/NZS 3000 work the same three forms come up over and over:
The calculator above assumes a purely resistive load — heating elements, filament lamps, resistive appliances. For real-world AC circuits with motors, transformers, fluorescent or LED drivers, fridges, washing machines, and most modern electronics, the load is reactive: current and voltage are out of phase, and you need impedance Z (with a power factor) rather than plain resistance R.
For those circuits:
For most cable-sizing decisions on a domestic 230 V supply you can treat the load as resistive without losing meaningful accuracy. For commercial and industrial design with significant motor load or harmonic content, use a power-factor-aware calculation. AS/NZS 3000 Clause 1.7.5 and AS/NZS 3008.1.1 Section 5 cover harmonic current contributions if you need to go that far.
Resistance R applies to DC and to resistive AC loads — voltage and current are perfectly in phase. Impedance Z applies to all AC circuits: it's a complex quantity that combines resistance with reactance from inductors (motors, transformers, ballasts) and capacitors. For purely resistive loads, Z = R and the two are interchangeable.
The mV/A/m figures in AS/NZS 3008.1.1 Table 40 are calibrated for the cable conductor at its rated operating temperature (75 °C for V-75 PVC, 90 °C for X-90 thermosetting), not the cold 20 °C resistance you'd measure with a continuity tester. Conductor resistance rises about 0.4% per degree, so a 75 °C conductor reads roughly 22% higher than at 20 °C. The mV/A/m tables already include that correction.
No — the calculator uses whatever voltage you enter. NZ low-voltage public supplies are nominally 230 V with a +10% / −6% tolerance per the Electricity (Safety) Regulations 2010, so for worst-case voltage-drop design at minimum supply use 216 V. AS/NZS 3000 Paragraph B4.5 calculates maximum Z_s using bare 230 V (Z_s = U_o ÷ I_a) — no Cmin derating, unlike BS 7671 Table 41.3 Note 1 in the UK.
Use the magnetic trip current of the breaker as I, the supply voltage as V, and solve for R. A Type B 32 A MCB needs 5 × 32 = 160 A to trip in < 0.1 s; at 230 V that means the loop impedance from the fault back to the source must be ≤ 230 ÷ 160 = 1.44 Ω. AS/NZS 3000 Paragraph B4.5 uses this bare Ohm's-law calculation directly — no Cmin derating like BS 7671 in the UK. Type C needs 10 × I_N, Type D needs 20 × I_N.
Ohm's Law calculator: V = IR. Solve for voltage, current, resistance or power given any two values. Free, instant results.