Estimate conductor resistance, voltage drop, and delivered voltage for a wire run.
This tool provides estimates for informational purposes only and is not a substitute for professional advice. Individual results vary based on personal circumstances and assumptions.
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Voltage drop is one of the most important and most overlooked factors in electrical installation design. When current flows through a wire, the wire's resistance causes a portion of the supply voltage to be lost as heat β this is voltage drop. If the drop is too large, appliances and equipment at the end of a long run will receive insufficient voltage, leading to poor performance, overheating, or failure. This voltage drop calculator gives you the actual drop and percentage loss for any combination of wire gauge, length, current load, and material β helping you choose the right wire for every circuit.
All conductors (wires) have electrical resistance. Even copper, one of the best conductors available, has a small but nonzero resistivity. When current flows through a wire, Ohm's Law means a voltage is "dropped" across the wire's resistance: V_drop = I Γ R_wire.
The wire resistance depends on three factors:
The formula: V_drop = (2 Γ L Γ I Γ Ο) Γ· A where L is one-way length, I is current, Ο is resistivity, and A is wire cross-sectional area. The factor of 2 accounts for the round-trip current path (both the live/hot and neutral/return conductors).
The NEC (NFPA 70) recommends β but does not mandate β that voltage drop on branch circuits not exceed 3% and that the total voltage drop (feeder + branch circuit) not exceed 5%. These are guidelines found in NEC informational notes, not hard code requirements, but they are universally used as design standards by electricians and engineers.
The UK standard BS 7671 (IET Wiring Regulations) specifies maximum voltage drop limits for final circuits from the origin of the installation:
These are hard requirements in UK regulations, not just recommendations. Exceeding them means the installation does not comply with BS 7671.
US wire sizing uses American Wire Gauge (AWG), where a lower number means a thicker wire. AWG is a logarithmic scale β every 6 AWG steps roughly doubles the wire's cross-sectional area.
Common AWG sizes and their applications:
UK and European wiring uses cross-sectional area in mmΒ² rather than AWG. Common cable sizes:
The UK uses a ring circuit system for socket outlets, where cable runs in a loop with both ends connected to the same MCB. This means each socket sees current from two directions, effectively halving the current in each cable run and reducing voltage drop.
Outdoor structures, garden sheds, workshops, EV charger installations, and outbuilding sub-panels often involve cable runs of 30β100 meters or more. At these distances, even a 6mmΒ² cable carrying 32A can exceed the 3% drop limit. Always check before installation.
Voltage drop is far more critical in 12V or 24V systems than in 120V or 230V systems. A 1V drop on a 230V supply is only 0.4% β trivial. The same 1V drop on a 12V system is 8.3% β significant. Solar, marine, RV, and LED lighting systems require careful voltage drop calculations to function correctly.
EV chargers draw sustained high current for hours. A 7.4 kW home charger on a 230V supply draws about 32A. Over a 30-meter cable run with 6mmΒ² copper, the voltage drop is approximately: 2 Γ 30 Γ 32 Γ 0.01724 Γ· 13.3 = 2.49V = 1.08% β acceptable. On a 50-meter run, the same calculation gives 4.15V = 1.81% β still within limits. Always verify for your specific installation.
When your calculated voltage drop exceeds the allowed limit, you have these options:
The NEC recommends no more than 3% voltage drop on branch circuits and 5% total (feeder + branch combined). These are recommendations in the NEC, not mandatory requirements, but all professional electrical work follows them.
The most effective solution is to use a larger gauge wire (lower AWG number in the US, larger mmΒ² in the UK). You can also shorten the cable run, use copper instead of aluminum, or split the load across multiple circuits.
For typical household circuits of 10β15 meters (30β50 feet) with standard loads, voltage drop is rarely an issue with correctly sized wire. It becomes significant on runs over 30 meters (100 feet) or with high current loads.
Because voltage drop is a fixed voltage, not a percentage. 2V of drop on a 12V system is 16.7% β catastrophic. The same 2V on a 230V system is less than 1% β irrelevant. Always calculate the percentage, not just the absolute drop value.
Disclaimer: This calculator provides estimates for planning purposes. All electrical installations must be designed and installed by qualified electricians following local electrical codes (NEC in the USA, BS 7671 in the UK).