How to Size a Cable for a 100 Amp Circuit
Selecting the correct cable size for a 100A circuit involves two independent checks, both of which must be satisfied:
- Current-carrying capacity: The cable must be rated to carry the design current continuously without overheating. Ratings are listed in AS/NZS 3008.1.1:2017 and vary by cable size, conductor material (copper or aluminium), and installation method. A cable run in a tightly enclosed space carries less current than the same cable in free air because it cannot dissipate heat as effectively.
- Voltage drop: The resistance of the cable causes a voltage drop along its length. AS/NZS 3000 (the Wiring Rules) recommends that the total voltage drop from the point of supply to any socket outlet or fixed appliance does not exceed 5% of the nominal supply voltage (11.5V on a 230V circuit, or 20V on a 400V three-phase circuit). Longer runs or higher currents increase voltage drop and may require a larger cable than the minimum current rating alone would suggest.
You must satisfy both requirements; the larger of the two results governs cable selection.
Voltage Drop Formula
For a single-phase AC circuit:
V_drop = (2 x L x I x R) / 1000
Where L is the one-way cable length in metres, I is the current in amps, R is the conductor resistance in milliohms per metre (mΩ/m), and the factor 2 accounts for both the active and neutral conductors.
For a three-phase circuit:
V_drop = (1.732 x L x I x R) / 1000
The 1.732 factor (square root of 3) accounts for the three-phase geometry; only one conductor length (not two) is used because the return current flows through the other two phases.
Current Ratings for Common Cable Sizes (Copper, 75 degrees C)
| Size (mm²) | Free Air (A) | In Conduit (A) | Direct Buried (A) | Resistance (mΩ/m) |
|---|
| 16 mm² | 73 A | 59 A | 72 A | 1.15 |
| 25 mm² | 98 A | 80 A | 95 A | 0.727 |
| 35 mm² | 119 A | 96 A | 115 A | 0.524 |
| 50 mm² | 142 A | 115 A | 138 A | 0.387 |
| 70 mm² | 180 A | 146 A | 174 A | 0.268 |
| 95 mm² | 215 A | 175 A | 209 A | 0.193 |
Ratings are based on AS/NZS 3008.1.1:2017 for 75 degrees C insulation in an ambient temperature of 40 degrees C in air or 25 degrees C in ground. Aluminium conductors have approximately 60% of the current-carrying capacity of copper for the same cross-section.
Installation Method Derating
When a cable cannot dissipate heat freely, its current rating is reduced by a derating factor:
- Free air or open tray: No derating (factor 1.00). The cable has maximum ventilation and radiates heat to open air.
- Direct buried: Factor approximately 0.97 to 1.00 depending on soil thermal resistivity. The ground provides reasonable cooling for buried cables.
- In conduit above ground: Factor approximately 0.80. Airflow in the conduit is restricted, raising conductor temperature.
- Enclosed in wall or ceiling cavity: Factor approximately 0.70. Very limited heat dissipation. This is the most restrictive common installation method.
When multiple cables are run together (bundled or in the same conduit), additional derating applies for the number of current-carrying conductors. This calculator assumes a single circuit with no additional bundling derating.
Worked Example
A 100A single-phase supply, 30 m cable run (one-way), copper cable in free air, 230V, 5% max voltage drop:
- Maximum allowable voltage drop: 5% x 230V = 11.5 V
- Test 25 mm² copper (rated 98A in free air, derating 1.00): 98A < 100A. FAILS current rating.
- Test 35 mm² copper (rated 119A in free air, derating 1.00, derated rating 119A): current OK (119A > 100A).
- Resistance of 35 mm² copper: 0.524 mΩ/m.
- Voltage drop: (2 x 30 x 100 x 0.524) / 1000 = 3.14 V (1.37%). Within 5% limit.
- Result: 35 mm² copper satisfies both current (119A > 100A) and voltage drop (3.14V < 11.5V) criteria.
Important Notes
This calculator provides indicative guidance only. Cable sizing must be confirmed by a licensed electrician (registered with the Electrical Workers Registration Board) before any installation. Other factors that may require a larger cable include: ambient temperature above 40 degrees C, cables grouped together in bundles, fault level withstand requirements, cable insulation type, and local network operator requirements. For sub-mains and main switchboard connections, your lines company may have additional requirements.
Related Calculators
Sources and method: AS/NZS 3008.1.1:2017 Electrical installations - Selection of cables - Cables for alternating voltages up to and including 0.6/1 kV - Typical Australian and New Zealand conditions. AS/NZS 3000:2018 Wiring rules. Electrical Workers Registration Board (EWRB) New Zealand (ewrb.govt.nz).
This calculator is for guidance only and does not replace advice from a licensed electrician. Cable sizing must comply with AS/NZS 3000, AS/NZS 3008.1.1, and any requirements of your local network operator. Only a registered electrician may carry out electrical installation work in New Zealand.
