How this instrument works
Push current along wiring and some of your supply never arrives. Every conductor has resistance, and Ohm's law — published by Georg Simon Ohm in 1827, after he abandoned drifting voltaic piles for steadier thermocouple readings — fixes what that costs: volts equal to current times resistance get consumed inside copper itself. Those volts are not destroyed. They reappear as heat, spread along your cable run.
Magnitudes surprise people. Copper resistivity sits near 1.72 × 10⁻⁸ Ω·m, so 2.5 mm² house wiring carries about 6.9 mΩ per metre; one 30-metre circuit, counted out and back, totals roughly 0.41 Ω. Draw 16 A and 6.6 V disappear — near 2.9% of 230 V nominal, just inside what BS 7671 permits for socket circuits. NEC informational notes point at similar territory: 3% across branch circuits, 5% including their feeders. Such limits exist because motors stall, LED drivers flicker, and contactors chatter when starved.
Two assumptions sit underneath this arithmetic. First, resistance alone — fine for DC and modest AC conductors, but long 60 Hz feeders in steel conduit, or anything above roughly 250 kcmil, need impedance and power factor rather than plain R. Second, resistance quoted at 20 °C. Copper climbs about 0.393% per degree, so conductors sitting at their 75 °C insulation rating carry near 22% more resistance than any table admits, with loss to match.
- Enter Current — what your load genuinely draws, not what its breaker permits.
- Enter Total conductor resistance across one complete loop: supply leg plus return leg, since current travels both.
- Read Voltage drop, which your load never receives, and Power lost as heat, which stays inside your cable.
- Divide Voltage drop by your nominal supply for a percentage, then compare against your 3% or 5% wiring limit.
- Switch units where scale demands — milliohms for busbar links, kilovolts for transmission arithmetic.
Worked example — a 12-metre extension lead
A 12-metre extension lead wound from 18 AWG copper carries roughly 21 mΩ per metre in each conductor — counted out and back, close to 0.5 Ω. Plug in a 20 A load. Voltage drop = 20 × 0.5 = 10 V. Power lost as heat = 20² × 0.5 = 200 W.
On a 120 V supply your appliance receives 110 V, and 200 W — a small fan heater's worth — never leaves that lead. Coiled on a drum it has nowhere to shed, which is why drums print derated coiled ratings, and why an undersized cord does not merely underperform: it scorches.
Halve your load to 10 A and voltage drop halves to 5 V, but heat falls to 50 W, a quarter of before. Conductor loss follows current squared, which is why one generous cable beats two marginal ones.
Questions
Should resistance cover one leg or a full round trip?
A full round trip. Current leaves through one conductor and returns through another, so both contribute. Counting a single leg is far and away this instrument's most common misuse — it makes every result look twice as good as reality. Across 30 metres of circuit, enter resistance covering 60 metres of conductor.
Why does halving current cut heat by a factor of four?
Because dissipation follows I²R, not I·R. Voltage drop is linear in current; heat is quadratic. Take 20 A down to 10 A through 0.5 Ω and drop falls from 10 V to 5 V, while heat falls from 200 W to just 50 W. That same square law explains long-distance transmission: raise voltage tenfold, carry a tenth as much current for equal power, and shed a hundredth of your losses.
How much drop is acceptable?
Roughly 3% of nominal supply across branch circuits, 5% overall. In North America those figures appear as informational notes in NEC 210.19 and 215.2 — guidance rather than enforceable requirements. BS 7671 in Britain is firmer: 3% for lighting, 5% for other uses. Low-voltage work wants tighter still. A 12 V solar or marine run usually targets 3% or better, because 0.36 V matters far more at 12 V than 7 V does at 230 V.
Does this hold for AC as well as DC?
For short runs and ordinary conductors, yes — resistance dominates and reactance stays negligible. It breaks down on long 60 Hz feeders, large conductors above roughly 250 kcmil, and anything pulled through steel conduit, where inductive reactance and power factor matter and you need impedance Z instead of R. Skin effect also nudges effective resistance upward as frequency climbs, though at 50–60 Hz that is a percent or two on ordinary sizes.
Should I use resistance measured hot or cold?
Hot, if you want a worst case — and you usually do. Copper gains about 0.393% resistance per degree Celsius. Published tables list values at 20 °C, but conductors working at their 75 °C insulation limit sit roughly 22% higher. Voltage drop and heating both scale with that, so a run looking compliant cold can breach 3% once loaded and warm.
If 10 V drops away, does current really stay at 20 A?
Not exactly. This instrument takes current as given, which is reasonable over short intervals and useful as a worst case. In reality a resistive load sees less voltage, draws less current, and settles somewhere below your entered figure. Constant-power loads such as switched-mode supplies and motor drives behave in reverse: starved of voltage, they pull more current, deepening drop further — a feedback loop that has cooked plenty of undersized wiring.