What Size Cable Do I Need? Current and Voltage Drop Guide

Choosing an electrical cable is not simply a matter of matching one current value to one cable size. The correct cross-sectional area depends on the load, supply voltage, cable length, conductor material, installation method, ambient temperature, protective device and the voltage drop allowed by local rules and the connected equipment.

What size electrical cable do I need?

You need a cable that can carry the design current under its real installation conditions, keep voltage drop within an acceptable limit and work correctly with the circuit breaker or fuse. A cable that appears adequate in open air may not have the same usable capacity when enclosed in insulation, grouped with other loaded cables or installed in a hot location.

• The expected load current in amperes
• The supply system: DC, single-phase AC or three-phase AC
• The one-way cable length and the complete current path
• Copper or aluminium conductors and the cable insulation type
• The installation method, temperature and grouping conditions
• The rating and characteristics of the protective device
• The permitted voltage drop and any equipment-specific requirements

Cable size is a system decision

Cable cross-section is commonly expressed in square millimetres, while some countries use American Wire Gauge, or AWG. These systems describe conductor size differently and should not be treated as exact one-to-one equivalents.

The selected cable must suit the load and the way it will be installed. Conduit, thermal insulation, underground routes, cable bundles and high ambient temperature can all change the usable current-carrying capacity.

That is why a calculator result should be treated as a shortlist for verification rather than permission to install a particular cable.

Why cable size cannot be chosen by amps alone

Current is the starting point, but it is only one part of cable sizing. The same current can require a different conductor size when the route is longer, the cable is surrounded by insulation, several circuits are grouped together, aluminium is used instead of copper or a motor has a high starting current.

Step 1: determine the design current

When the equipment nameplate provides current in amperes, that value is usually a better starting point than calculating from advertised power alone. If only power is known, current can be estimated from voltage, but power factor, efficiency and starting behaviour may also matter.

Approximate current for a resistive single-phase load:
I = P_in / V

Single-phase estimate from electrical input power:
I = P_in / (V × PF)

Single-phase estimate from output power:
I = P_out / (V × PF × η)

Balanced three-phase estimate from electrical input power:
I = P_in / (√3 × V_LL × PF)

Balanced three-phase estimate from output power:
I = P_out / (√3 × V_LL × PF × η)

Where:
I = line current in amperes
P_in = electrical input power in watts
P_out = useful output or mechanical power in watts
V = single-phase supply voltage in volts
V_LL = line-to-line voltage in volts
PF = power factor
η = efficiency as a decimal

For heaters and other mainly resistive loads, power factor and efficiency may be close to 1. Motors, pumps, air conditioners, compressors and some electronic loads need more careful treatment because their running current, power factor and starting current may differ significantly from a simple watts-to-amps estimate.

Step 2: coordinate the cable with the circuit breaker or fuse

A protective device is not chosen independently of the cable. In a simplified design check, the circuit design current should not exceed the protective-device rating, and the protective-device rating should not exceed the cable's corrected current-carrying capacity.

Simplified coordination check:
Ib ≤ In ≤ Iz

Ib = design current of the circuit
In = rated current or setting of the protective device
Iz = cable current-carrying capacity after applicable corrections

The value of Iz must reflect the real installation, not only a catalogue value under reference conditions. Temperature, thermal insulation, grouping, conduit fill, underground installation and the number of loaded conductors can require correction factors that reduce the usable capacity.

• Cable enclosed in thermal insulation
• Several loaded circuits installed together
• High ambient or ground temperature
• A long section inside conduit or trunking
• Underground installation with unfavourable soil conditions
• Multiple loaded cores within the same cable
• Equipment with long operating periods or high starting current

Step 3: account for cable length and voltage drop

Every conductor has resistance. As current travels through the cable, part of the supply voltage is lost along the route. The effect becomes more important when current is high, the conductor is small, the cable is long or the system voltage is low. Excessive voltage drop can cause poor equipment performance, difficult motor starting, dim lighting, additional heating and wasted energy.

Simplified resistive voltage-drop estimate for a two-conductor path:
ΔV = (2 × L × I × ρ) / S

Voltage drop percentage:
ΔV% = (ΔV / V) × 100

L = one-way cable length in metres
I = load current in amperes
ρ = conductor resistivity in Ω·mm²/m
S = conductor cross-section in mm²
V = nominal supply voltage

This simplified formula is useful for understanding the effect of length and cross-section, but a final AC calculation may also need conductor operating temperature, reactance, power factor, cable construction and the correct single-phase or three-phase method. For DC battery systems, include the complete positive-and-negative current path.

Step 4: choose the conductor material and cable type

The conductor is only one part of the cable. Insulation temperature rating, number of cores, flexibility, mechanical protection, fire performance, moisture resistance, sunlight resistance and permitted installation environment also matter. Use a cable type approved for the intended location and compatible with the equipment terminals.

Step 5: check the installation conditions

• Will the cable be clipped in open air, inside conduit, behind a wall or buried?
• Does any part of the route pass through thermal insulation?
• Will it be grouped with other loaded power cables?
• What ambient temperature can occur around the cable?
• Is the cable exposed to moisture, sunlight, chemicals or mechanical damage?
• How many conductors are expected to carry current at the same time?
• Are fire-resistant, low-smoke or other special cable properties required?
• Can the selected cable bend and terminate correctly at both ends?

The most restrictive part of the route can determine the whole circuit design. For example, a cable may run mainly in free air but pass through a short insulated section where heat cannot escape. That section should not be ignored simply because most of the route has better cooling.

A safer cable-sizing workflow

1. Identify the appliance, circuit or group of loads and record the manufacturer data.
2. Calculate or confirm the design current, including power factor, efficiency, duty cycle and starting current where relevant.
3. Confirm the supply voltage, phase arrangement and earthing system.
4. Measure the actual one-way route and note every installation method along it.
5. Select a preliminary cable type and conductor material suitable for the environment.
6. Check corrected current-carrying capacity against the load and protective device.
7. Calculate voltage drop and review equipment-specific limits.
8. Verify short-circuit withstand, earth-fault protection, disconnection time, protective conductor size and terminal compatibility.
9. Have the final design and installation checked according to local electrical rules.

What not to do when selecting cable size

• Do not choose a cable from appliance power alone without converting the load correctly.
• Do not use a current table without checking its installation method and reference conditions.
• Do not ignore cable length because the circuit breaker is nearby.
• Do not assume copper and aluminium of the same cross-section are interchangeable.
• Do not increase the breaker rating to stop repeated trips without investigating the cause.
• Do not conceal undersized conductors inside walls, insulation or inaccessible junctions.
• Do not mix AWG and mm² values as though they are exact equivalents.
• Do not rely on cable colour or outside diameter to identify conductor cross-section.
• Do not use a flexible cord as a permanent substitute for fixed-installation cable unless it is specifically permitted for that application.
• Do not treat a calculator result as a replacement for testing and inspection.

Practical example: planning a new high-power appliance circuit

When an electrician or electrical designer is needed

• A new fixed circuit, distribution board or subpanel is being installed
• The load includes a cooker, water heater, boiler, heat pump, air conditioner, EV charger, workshop machine, pump or motor
• The route is long, buried, outdoors, grouped or enclosed in thermal insulation
• Aluminium conductors or transitions between aluminium and copper are involved
• The circuit is three-phase, DC battery-based, solar-related or supplied by an inverter or generator
• The existing breaker trips, a cable or plug becomes warm, lights dim or equipment starts poorly
• The available fault current, earthing arrangement or protective-device operation is unknown
• Local rules require design, testing, certification or notification by an authorised person

Cable size checklist before installation

• Load current is based on reliable equipment data
• Starting and inrush current have been considered where relevant
• Supply voltage and phase are confirmed
• The full cable route has been measured
• Conductor material and cable type match the application
• Installation method and correction factors have been applied
• The breaker or fuse coordinates with the corrected cable capacity
• Voltage drop has been checked for the complete route
• Protective earthing and fault-disconnection requirements have been verified
• Terminals are suitable for the conductor size and material
• The result complies with local rules and manufacturer instructions
• The completed circuit will be inspected and tested before normal use

Frequently asked questions

Can I choose cable size only from watts?

No. Watts can be converted into an approximate current when voltage and other electrical characteristics are known, but cable length, voltage drop, installation method, conductor material, protective device and local rules still affect the final size.

Does a longer cable need to be thicker?

Often it does. A longer route increases resistance and voltage drop. Upsizing the conductor can reduce the drop, but the final choice must still satisfy current-carrying capacity, protection and installation requirements.

Is copper cable always better than aluminium?

Not in every application. Copper is compact and has lower resistance for the same cross-section, while aluminium can be lighter and economical for suitable larger circuits. Aluminium normally needs a larger cross-section and requires compatible terminals and installation methods.

Can I use the next larger cable size?

A larger conductor can reduce voltage drop and provide additional thermal margin, but it must fit the equipment terminals, protective device and installation system. Larger is not automatically correct if the cable cannot be terminated properly or the rest of the circuit design is unsuitable.

Why does an electrical cable become warm?

Some temperature rise is normal when current flows, but excessive heat can indicate overload, an undersized conductor, poor ventilation, loose or unsuitable terminals, damaged cable or a fault. Unusual heating should be investigated rather than accepted as normal.

How do AWG and mm² cable sizes compare?

AWG is a gauge system in which a smaller gauge number generally means a larger conductor. Square millimetres describe the conductor's cross-sectional area directly. Because common AWG sizes do not exactly match common metric sizes, use an approved conversion reference and then verify the electrical rating for the actual cable and installation method.

Use a cable size result as a starting point, not a guarantee

A useful cable size estimate combines load current, route length, voltage, conductor material and an allowed voltage-drop target. A complete design goes further by checking actual installation conditions, protective devices, fault current, earthing, termination and local electrical requirements. Use the HomDera calculator to compare preliminary options, then have the selected circuit verified before purchase or installation.