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Converters & physics calculator

Cable size calculator: voltage drop, power loss and the right cross-section

Finds the smallest cable that meets both your voltage-drop limit and its current-carrying capacity. Works for DC, single-phase or three-phase, copper or aluminium, in mm² and AWG, with power loss and temperature effects.

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Results

6 mm² copper (≈ 8 AWG) carries 32 A over 25 m with a 2.56% drop (5.9 V) and 189 W of loss.
Recommended cable
6 mm²
≈ 8 AWG · copper
Voltage drop
2.56%
5.9 V of 230 V · limit 3%
Power loss in cable
189W
2.56% of 7.36 kW
Current
32A
cos φ 1
Min. size for voltage drop
6 mm²
exact 4.96 mm²
Min. size for current
4 mm²
Clipped direct to wall or tray, 30 °C
Current capacity of chosen size
46A
ambient factor 1
Max length at this size
29m
for a 3% drop
Energy lost per year
189kWh
at 1,000 full-load hours
Voltage drop by cross-section (25 m, 32 A)

Cable size comparison

SizeAWG ≈Drop %Drop VLoss WCapacity AMax length mOK?
1 mm²16 AWG15.07%34.651,109165✗ current
1.5 mm²14 AWG10.07%23.16741207✗ current
2.5 mm²12 AWG6.17%14.194542712✗ current
4 mm²10 AWG3.84%8.832823620✗ drop
6 mm² ◀8 AWG2.56%5.91894629✓
10 mm²6 AWG1.52%3.51126349✓
16 mm²4 AWG0.96%2.2708578✓
25 mm²3 AWG0.61%1.3945112124✓
35 mm²1 AWG0.44%132138172✓
50 mm²1/0 AWG0.32%0.7424168233✓
70 mm²3/0 AWG0.22%0.5116213336✓
95 mm²4/0 AWG0.16%0.3712258467✓
  • Voltage drop: ΔV = 2 × L × I × (R·cos φ + X·sin φ), with IEC 60228 conductor resistance corrected to 70 °C and 0.08 mΩ/m reactance.
  • Current capacity is indicative, from IEC 60364-5-52 tables (PVC/XLPE, method factors, ambient and grouping corrections). Your national wiring rules (BS 7671, NEC 310, VDE 0298-4…) and the protective device rating decide the final size.
  • Motor starting currents (5–7× nominal) cause brief extra drops, so check starting conditions on long motor feeders.

How this cable size calculator works

A cable has to pass two tests. It must carry the current without overheating (its current-carrying capacity, or ampacity), and it must deliver the current without losing too much voltage along the way. Short runs are usually limited by ampacity; long runs, low voltages and solar circuits are almost always limited by voltage drop. This calculator checks both and recommends the smallest standard cross-section that passes.

For voltage drop it uses the standard IEC 60228 conductor resistances (stranded copper or aluminium) corrected to the conductor’s operating temperature — copper’s resistance rises about 0.39% per °C, so a cable at 70 °C has 20% more resistance than at 20 °C. For AC circuits it adds a typical cable reactance and the power factor. For ampacity it starts from the IEC 60364-5-52 reference table (PVC insulation, copper, clipped direct) and applies factors for the installation method, XLPE insulation, ambient temperature, grouping with other circuits and aluminium conductors.

It also reports the power lost as heat in the cable, the annual energy loss, the maximum length for each size and the AWG equivalent.

The formulas

I = P ÷ (V × cos φ) (DC, single-phase) I = P ÷ (√3 × V_LL × cos φ) (three-phase) R_T = R_20 × [1 + α × (T − 20 °C)] α_Cu = 0.00393, α_Al = 0.00403 ΔV = k × L × I × (R_T × cos φ + X × sin φ) k = 2 (DC, 1-ph), √3 (3-ph) ΔV% = ΔV ÷ V × 100 P_loss = n × I² × R_T × L (n = 2 or 3) I_z = I_table × k_method × k_insulation × k_ambient × k_group × (0.78 if Al)

Worked example: 7.4 kW EV charger, 25 m away

The default case is a single-phase 230 V EV charger drawing 32 A, 25 m from the distribution board, on copper PVC cable clipped to a wall, with a 3% voltage-drop limit.

  • Ampacity alone would allow 4 mm² (36 A in method C).
  • But 4 mm² drops 8.8 V (3.84%), above the limit.
  • 6 mm² drops 5.9 V (2.56%), carries up to 46 A and wastes about 189 W at full power. It is the recommendation (≈ 8 AWG). Its maximum length at 3% is about 29 m.
  • The exact cross-section for 3% would be 4.96 mm²; 10 mm² would halve the loss to 112 W.

How to read the results

  • Recommended cable is the smallest standard size meeting both limits. The two “minimum size” tiles show which limit governs.
  • Power loss matters for anything that runs for many hours: for solar circuits and heat pumps, upsizing one step often pays back through lower losses.
  • Max length at this size answers the common question “how far can I run this cable?”
  • The comparison table shows drop, loss and capacity for neighbouring sizes so you can trade copper cost against losses.

Typical values (copper, 70 °C, 3% drop, single-phase 230 V)

Load Current 10 m 25 m 50 m
2.3 kW socket circuit 10 A 1.5 mm² 2.5 mm² 4 mm²
3.7 kW (16 A) 16 A 1.5 mm² 4 mm² 6 mm²
7.4 kW EV charger 32 A 4 mm² 6 mm² 16 mm²
11 kW (3-phase 400 V, 16 A) 16 A 1.5 mm² 1.5 mm² 4 mm²

Three-phase circuits need much smaller cables for the same power: the current per conductor is lower and the √3 factor replaces the factor of 2.

Tips

  • Use the one-way route length, including vertical runs and detours. The formula already accounts for the return conductor.
  • Raise the voltage before adding copper. Doubling the voltage halves the current and quarters the percentage drop for the same cable.
  • Size solar DC cables for 1–2% drop and check them at the string’s operating current; see the string sizing calculator.
  • Battery cables carry the largest currents in a system and must also match the fuse rating. Size them with the battery’s continuous and peak current from the battery bank calculator.
  • Derate hot locations. Cables in roof spaces or along PV arrays can sit at 50–70 °C ambient, which cuts PVC cable capacity sharply; use XLPE or solar cable.

Limitations

Current-carrying capacities here are indicative values derived from IEC 60364-5-52 reference tables with simplified correction factors; national standards (BS 7671, NEC Article 310, VDE 0298-4 and others), the protective device and short-circuit withstand requirements determine the legal minimum. Reactance is a typical value for multicore cables and is higher for widely spaced single cores. Harmonic currents in neutral conductors, motor starting and fault-loop impedance are not checked. Have installations designed and verified by a qualified electrician.

Method & assumptions

Current: DC and single-phase I = P ÷ (V × cos φ); three-phase I = P ÷ (√3 × V_LL × cos φ). Resistance per metre from IEC 60228 values at 20 °C, corrected R_T = R_20 × [1 + α(T − 20)] (α = 0.00393 Cu, 0.00403 Al).

Voltage drop ΔV = k × L × I × (R cos φ + X sin φ), k = 2 for DC/single-phase (out and back), √3 for three-phase. Power loss = n × I² × R × L (n = 2 or 3 conductors). The recommended size is the smallest standard cross-section that meets both the voltage-drop limit and the derated current capacity (IEC 60364-5-52 method C table × installation-method, insulation, ambient-temperature and grouping factors; aluminium × 0.78).

Frequently asked questions

How do I calculate voltage drop in a cable?
For DC and single-phase AC: ΔV = 2 × length × current × resistance per metre (the factor 2 accounts for the outgoing and return conductor). For three-phase: ΔV = √3 × length × current × (R·cos φ + X·sin φ). Example: 32 A over 25 m of 6 mm² copper at 70 °C (3.68 mΩ/m) gives 2 × 25 × 32 × 0.00368 = 5.9 V, or 2.6% of 230 V.
What is the maximum allowed voltage drop?
IEC 60364-5-52 recommends no more than 3% for lighting and 5% for other uses from the origin of a public low-voltage installation; the US NEC recommends 3% for a branch circuit and 5% in total. For solar PV, designers usually target 1–2% on the DC side and 1% between inverter and grid connection point, because every percent is lost energy.
What size cable do I need for a 32 A EV charger?
Current capacity alone allows 4–6 mm² copper for 32 A in most installation methods, but voltage drop usually decides: for runs up to about 20 m, 4 mm² stays under 3%; from about 20 to 29 m you need 6 mm², and up to about 49 m 10 mm². Your local code and the circuit breaker rating set the minimum.
What size cable for a 12 V system?
Low voltages need thick cables because the same power means high current. A 1 kW load at 12 V draws 83 A; keeping drop under 3% over 4 m (one way) needs about 50 mm² copper. The same load at 48 V draws 21 A and needs only 4 mm². That is why larger off-grid systems use 24 V or 48 V.
How do I convert mm² to AWG?
Common equivalents (next larger AWG): 1.5 mm² ≈ 14 AWG, 2.5 mm² ≈ 12 AWG, 4 mm² ≈ 10 AWG, 6 mm² ≈ 8 AWG, 10 mm² ≈ 6 AWG, 16 mm² ≈ 4 AWG, 25 mm² ≈ 3 AWG, 35 mm² ≈ 1 AWG, 50 mm² ≈ 1/0, 70 mm² ≈ 3/0, 95 mm² ≈ 4/0. AWG sizes are not identical to metric sizes, so always round up.
Is aluminium cable as good as copper?
Aluminium has about 61% of copper's conductivity, so it needs roughly 1.6 times the cross-section (typically one or two standard sizes larger) for the same voltage drop and carries about 78% of the current at the same size. It is much lighter and cheaper, which is why it dominates large feeders and utility cables, but it needs suitable terminals and is generally not used below 16 mm².

Sources & references

  1. IEC 60364-5-52: Low-voltage electrical installations: Selection and erection of wiring systems (current-carrying capacity, voltage drop)
  2. IEC 60228: Conductors of insulated cables (DC resistance values)
  3. NFPA 70 National Electrical Code: Article 310 (conductor ampacity) and informational notes on voltage drop
  4. IEC 62548-1: PV arrays design requirements (DC cable sizing)

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