How to Use the Wire Size Calculator
USA mode (AWG, NEC)
- Enter the load in amps, or switch to watts and enter the voltage.
- Choose the voltage (120, 208, 240, 277 or 480 V) and single-phase or three-phase.
- Choose copper or aluminum conductors.
- Enter the one-way length from the panel to the load, in feet.
- Tick Continuous load if it runs for 3 hours or more (EV chargers, water heaters, heat).
- Set the voltage drop limit. The default is 3%.
UK mode (mm², BS 7671)
- Enter the load in amps or kW at 230 V single-phase or 400 V three-phase.
- Choose the protective device (MCB or RCBO rating). The calculator suggests one if you leave it on Auto.
- Choose how the cable is installed, from clipped direct to buried in insulation.
- Enter the cable length in metres and choose lighting (3%) or other (5%) voltage drop.
The result shows the minimum size for current capacity, the recommended size once voltage drop is checked, the actual voltage drop, and the breaker or MCB rating.
The Three Checks Behind Every Wire Size
Electricians size conductors by passing three tests, and the biggest wire required by any of them wins:
- Current capacity (ampacity). The wire must carry the load current without overheating its insulation.
- Overcurrent protection. The breaker or fuse must protect the wire, so the wire's rating has to be at least the device rating.
- Voltage drop. On long runs, resistance wastes voltage. Motors struggle, lights dim, and EV chargers slow down. Long runs often need a bigger wire than ampacity alone suggests.
Most online wire gauge calculators only do the first check. This one does all three.
USA: How Wire Size Is Calculated Under the NEC
- Find the design current. For continuous loads, the National Electrical Code (NEC) requires the conductor and breaker to be sized at 125% of the load. A 32 A EV charger needs a circuit rated for at least 40 A.
- Read the ampacity from NEC Table 310.16. The table lists ampacity by size and insulation temperature (60°C, 75°C, 90°C). Most residential breakers and devices are rated for 60°C or 75°C terminations, and under NEC 110.14(C) you must use the 60°C column for circuits of 100 A or less (or 14 to 1 AWG) unless every termination is marked 75°C. The calculator lets you pick the column and defaults to 60°C up to 100 A.
- Apply the small-conductor rule. NEC 240.4(D) caps the breaker on small copper wire regardless of the table: 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A.
- Check voltage drop. The NEC recommends no more than 3% on a branch circuit and 5% total for feeder plus branch (informational notes, not hard rules). The calculator uses:
K is about 12.9 for copper and 21.2 for aluminum, I is the load current, L is the one-way length in feet, and CM is the wire's area in circular mils.
Worked example: EV charger on a long run
A 32 A Level 2 EV charger at 240 V sits 150 feet from the panel, in copper.
- Continuous load: 32 × 1.25 = 40 A design current, so a 40 A breaker.
- 8 AWG copper, 60°C column = 40 A. Passes ampacity.
- Voltage drop with 8 AWG: 2 × 12.9 × 32 × 150 ÷ 16,510 = 7.5 V (3.1%). Fails 3%.
- Step up to 6 AWG: 4.7 V (2.0%). Passes.
Answer: 6 AWG copper on a 40 A breaker. Without the voltage drop check, you'd have installed 8 AWG and lost charging speed every day.
Common US wire sizes (copper, short runs)
| Breaker | Copper wire | Aluminum wire | Typical use |
|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | Lighting, general outlets |
| 20 A | 12 AWG | 10 AWG | Kitchen, bathroom, laundry outlets |
| 30 A | 10 AWG | 8 AWG | Dryer, water heater, small AC |
| 40 A | 8 AWG | 6 AWG | Range, cooktop, 32 A EV charger |
| 50 A | 6 AWG | 4 AWG | Range, RV outlet, 40 A EV charger |
| 60 A | 6 AWG (75°C) or 4 AWG (60°C) | 4 AWG (75°C) | Subpanel, 48 A EV charger |
| 100 A | 3 AWG (75°C) | 1 AWG (75°C) | Subpanel, detached garage |
These assume runs under about 50–75 feet and NM-B or THHN conductors. For dwelling service and main feeders, NEC 310.12 allows a smaller size based on 83% of the rating, so a 100 A service can use 4 AWG copper or 2 AWG aluminum. That exception doesn't apply to subpanels in general.
UK: How Cable Size Is Calculated Under BS 7671
The UK Wiring Regulations (BS 7671, the IET Wiring Regulations) use the same three checks with different names:
- Design current (Ib). Load in watts ÷ 230 V for single-phase. A 9.5 kW shower draws 9,500 ÷ 230 = 41.3 A.
- Choose the protective device (In). It must be at least the design current: Ib ≤ In. Standard MCB ratings are 6, 10, 16, 20, 25, 32, 40, 45, 50 and 63 A. For the shower, that's a 45 A device.
- Find the cable rating (Iz) for how it is installed. BS 7671 Appendix 4 lists ratings by reference method, because a cable buried in loft insulation can carry far less current than one clipped to a wall. Twin and earth ratings for common methods:
| Cable (twin & earth) | Clipped direct (C) | Above ceiling, ≤100mm insulation (100) | In stud wall touching inner skin (102) | Surrounded by insulation (103) | mV/A/m |
|---|---|---|---|---|---|
| 1.0 mm² | 16 A | 13 A | 13 A | 8 A | 44 |
| 1.5 mm² | 20 A | 16 A | 16 A | 10 A | 29 |
| 2.5 mm² | 27 A | 21 A | 21 A | 13.5 A | 18 |
| 4 mm² | 37 A | 27 A | 27 A | 17.5 A | 11 |
| 6 mm² | 47 A | 34 A | 35 A | 23.5 A | 7.3 |
| 10 mm² | 64 A | 45 A | 47 A | 32 A | 4.4 |
| 16 mm² | 85 A | 57 A | 63 A | 42.5 A | 2.8 |
Then apply correction factors for ambient temperature (Ca), grouping with other cables (Cg) and thermal insulation (Ci). The rule is In ≤ Iz × Ca × Cg × Ci.
Step 4: Check voltage drop. Voltage drop = mV/A/m × Ib × length ÷ 1,000. BS 7671 Appendix 4 gives limits of 3% (6.9 V) for lighting and 5% (11.5 V) for other circuits on a public supply.
Worked example: 9.5 kW electric shower
- Ib = 41.3 A, so In = 45 A MCB.
- 6 mm² clipped direct = 47 A. Passes, but only if the cable never runs through insulation.
- If the cable crosses a loft above ≤100mm of insulation, 6 mm² drops to 34 A and fails. 10 mm² (45 A) passes.
- Voltage drop, 10 mm² over 20 m: 4.4 × 41.3 × 20 ÷ 1,000 = 3.6 V (1.6%). Passes.
Answer: 10 mm² on a 45 A RCBO is the safe choice unless the whole route is clipped direct.
Common UK cable sizes
| Circuit | Typical cable | Typical device |
|---|---|---|
| Lighting | 1.0 or 1.5 mm² | 6 A |
| Ring final (sockets) | 2.5 mm² | 32 A |
| Radial sockets, up to 50 m² | 2.5 mm² | 20 A |
| Radial sockets, up to 75 m² | 4 mm² | 32 A |
| Immersion heater | 2.5 mm² | 16 A |
| Cooker up to about 13 kW | 6 mm² | 32–40 A |
| Electric shower 8.5–9.5 kW | 10 mm² (6 mm² if clipped direct, short run) | 40–45 A |
| Electric shower 10.5 kW+ | 10 mm² (16 mm² through insulation) | 50 A |
| 7 kW EV charger | 6 mm² (10 mm² for long runs) | 32 A |
Copper vs Aluminum, and AWG vs mm²
Aluminum wire is cheaper and lighter but has higher resistance, so it needs to be about two AWG sizes larger than copper for the same current. It's common for US service entrances and large feeders, and needs connectors rated AL/CU with anti-oxidant compound. UK domestic wiring is almost always copper.
To compare AWG and metric sizes: 14 AWG ≈ 2.1 mm², 12 AWG ≈ 3.3 mm², 10 AWG ≈ 5.3 mm², 8 AWG ≈ 8.4 mm², 6 AWG ≈ 13.3 mm², 4 AWG ≈ 21.2 mm². Smaller AWG numbers mean thicker wire.
When You Need an Electrician
In the USA, most new circuits, subpanels and service changes need a permit and inspection from your local building department, and many states require a licensed electrician. In England and Wales, adding a new circuit, replacing a consumer unit, or working in a bathroom's special locations is notifiable under Part P of the Building Regulations: use a registered electrician (NICEIC, NAPIT or ELECSA) who can self-certify, or notify building control first. Scotland and Northern Ireland have their own rules.
Use this wire size calculator to plan your budget and check a quote. The person who connects the circuit should do the final calculation.
Frequently Asked Questions
What size wire do I need for 50 amps?
For copper, 6 AWG. Its 60°C rating is 55 A and its 75°C rating is 65 A, so both pass. For aluminum, use 4 AWG. On runs over about 100 feet, check voltage drop and consider 4 AWG copper.
What size wire do I need for a 100 amp subpanel?
3 AWG copper or 1 AWG aluminum at 75°C, with 75°C-rated terminations, for runs under about 100 feet. Longer runs often need 2 AWG copper or 1/0 aluminum to keep voltage drop near 3%.
What size wire for 30 amps at 100 feet?
10 AWG copper meets ampacity. At 100 feet and 24 A of actual load on 240 V, voltage drop is about 2.5%, which passes. At 120 V the same run drops about 5%, so step up to 6 AWG to stay under 3%.
Can I put 12-gauge wire on a 30 amp breaker?
No. NEC 240.4(D) limits 12 AWG copper to a 20 A breaker. A 30 A breaker needs at least 10 AWG copper.
What size cable do I need for a 32A circuit in the UK?
Usually 4 mm² twin and earth clipped direct (37 A), or 6 mm² if it runs through insulation or over a long distance. A 32 A ring final circuit uses 2.5 mm² because the current splits two ways around the ring.
What size cable do I need for a 10.5 kW shower?
The shower draws about 46 A, so it needs a 50 A device and 10 mm² cable clipped direct or in a wall. If the route crosses loft insulation, 10 mm² drops to 45 A, and 16 mm² or a re-route is needed.
How much voltage drop is acceptable?
In the USA, 3% on a branch circuit and 5% total is the NEC recommendation. In the UK, BS 7671 allows 3% for lighting and 5% for other circuits from a public supply.
Why does the calculator recommend a bigger wire than the chart?
Charts only show ampacity. On long runs, voltage drop controls the size, so the calculator steps up until the drop is within your limit.
Does wire size change for 240V vs 120V?
Ampacity doesn't depend on voltage, but voltage drop as a percentage does. The same wire and current lose half the percentage at 240 V, so 240 V circuits can run farther on the same gauge.
Get a licensed electrician to confirm your wire size
New circuits usually need a permit or Part P notification. Get local quotes before you buy cable.