What is the maximum allowed voltage drop?

The NEC (National Electrical Code) recommends a maximum voltage drop of 3% for branch circuits and 5% for the total including feeders. IEC 60364 recommends 3% for lighting circuits and 5% for other circuits. Exceeding these limits can cause equipment to malfunction, overheat, or have reduced lifespan.

Why does wire length affect voltage drop?

Voltage drop is caused by the resistance of the wire. Longer wire means more resistance (R = ρ × L / A), and more resistance means a larger voltage drop (V = I × R) for any given current. This is why long cable runs — such as to outbuildings or garden lighting — often require larger wire gauges than the current alone would suggest.

What is the difference between single-phase and three-phase voltage drop?

For single-phase 2-wire circuits, current travels out and back on two conductors, so you multiply the one-way resistance by 2: Vdrop = 2 × I × R. For balanced three-phase 3-wire circuits, the relationship uses the square root of 3 (≈1.732): Vdrop = √3 × I × R. This means three-phase is more efficient for carrying power over distance.

How do I reduce voltage drop?

To reduce voltage drop you can: (1) Use a larger wire gauge (lower AWG or higher mm²); (2) Shorten the cable run by moving the distribution point closer to the load; (3) Increase the supply voltage where possible; (4) Reduce the load current; (5) Use three-phase power instead of single-phase for long runs.

Voltage Drop Calculator – Wire Length & Current

System Type

Supply Voltage (V)

Load Current (A)

One-Way Cable Length (m)

Max Acceptable Drop (%)

Conductor Material

Wire Gauge System

Wire Gauge (AWG)

—

Voltage Drop

0%3% limit10%+

Resistance per metre (2-way or √3) —

Total cable resistance —

Voltage drop —

Percentage voltage drop —

Remaining voltage at load —

NEC/IEC compliance status —

💡 Wire Upgrade Recommendation

Formulas Used

Conductor resistance:
R_conductor = ρ × L / A [Ω]
ρ_Cu = 1.724 × 10²» Ω·m | ρ_Al = 2.82 × 10²» Ω·m

Single-phase (2-wire):
V_drop = 2 × I × R_conductor

Three-phase (3-wire, balanced):
V_drop = √3 × I × R_conductor

Percentage:
% drop = (V_drop / V_supply) × 100

What is Voltage Drop and Why Does It Matter?

Voltage drop is the reduction in voltage along a cable caused by the cable's electrical resistance. Every conductor has resistance — even copper — and when current flows through that resistance, energy is lost as heat and the voltage at the far end of the cable is lower than at the source. The formula is simply:

V_drop = I × R_total

Excessive voltage drop causes equipment to run below its rated voltage, which can lead to: motor overheating, reduced lighting output, nuisance tripping of electronic devices, and wasted energy. Even a seemingly small 5% drop on a 230 V supply means the load only receives 218.5 V.

NEC and IEC Voltage Drop Limits

Regulatory standards define maximum permissible voltage drops to protect equipment and ensure safe operation:

NEC (NFPA 70): Recommends ≤3% drop on branch circuits and ≤5% total (feeder + branch). These are recommendations in the NEC, not mandatory requirements — though local codes may make them mandatory.
IEC 60364-5-52: Recommends ≤3% for lighting circuits and ≤5% for other uses (motors, heating, etc.).
AS/NZS 3000 (Wiring Rules): Maximum 5% from supply point to any point of use.
BS 7671 (UK): 3% for lighting, 5% for other circuits under normal conditions.

The default limit in this calculator is 3% — the stricter NEC/IEC standard for branch circuits and lighting.

Wire Resistance Reference Table

Resistance values shown are DC resistance at 20°C. AC resistance is slightly higher due to skin effect, especially at larger gauges.

AWG	mm² (approx.)	Cu Resistance (mΩ/m)	Al Resistance (mΩ/m)
14	2.08	8.29	13.55
12	3.31	5.21	8.52
10	5.26	3.28	5.36
8	8.37	2.06	3.37
6	13.3	1.30	2.12
4	21.2	0.814	1.33
2	33.6	0.513	0.839
1	42.4	0.407	0.665
0 (1/0)	53.5	0.322	0.527
00 (2/0)	67.4	0.256	0.418
000 (3/0)	85.0	0.203	0.332
0000 (4/0)	107.2	0.161	0.263
—	1.5	11.49	18.79
—	2.5	6.90	11.28
—	4	4.31	7.05
—	6	2.87	4.70
—	10	1.72	2.82
—	16	1.08	1.76
—	25	0.690	1.13
—	35	0.493	0.806
—	50	0.345	0.564

Frequently Asked Questions

The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% for the total system (feeder + branch). IEC 60364 recommends 3% for lighting and 5% for other circuits. Exceeding these limits can cause equipment to malfunction, motors to overheat, and electronics to behave erratically. Some sensitive loads (medical equipment, precision instruments) require even tighter limits of 1–2%.

Voltage drop is proportional to wire length because resistance increases linearly with length (R = ρ × L / A). Double the wire length, double the resistance, double the voltage drop. This is why long cable runs — to outbuildings, garden sockets, pool pumps, or large factory floors — often need significantly larger wire gauges than the current rating alone would suggest. Always calculate voltage drop for any run longer than 15–20 metres.

For single-phase 2-wire circuits, current must travel to the load and return on two conductors, so the effective conductor length is doubled: V_drop = 2 × I × R_conductor. For balanced three-phase 3-wire circuits, the three phases share the return path and the factor becomes √3 (≈1.732): V_drop = √3 × I × R_conductor. Three-phase is more efficient — for the same voltage drop you can use smaller wire or run longer distances.

The most practical solutions are: (1) Increase wire size — go up one or two AWG sizes or choose a higher mm² cross-section; (2) Shorten the run — move the distribution board or sub-panel closer to the load; (3) Use three-phase where possible for long high-current runs; (4) Reduce current by splitting the load across multiple circuits; (5) Boost the supply voltage slightly at the source where local regulations and equipment ratings allow.