EV Charger Cable Size Calculator — Installer Tool
EV Charger Cable Size Calculator (Installer Tool)
Reference cable ratings used
This tool uses the following representative base current ratings and voltage drop factors (per amp per metre) as a starting reference — these are typical figures for general guidance, not a substitute for manufacturer cable data or BS 7671 tables.
| Cable size | Typical base rating* | Volt drop factor (mV/A/m) |
|---|---|---|
| 1.5mm² | ~19.5A | 29 |
| 2.5mm² | ~27A | 18 |
| 4mm² | ~36A | 11 |
| 6mm² | ~46A | 7.3 |
| 10mm² | ~63A | 4.4 |
| 16mm² | ~85A | 2.8 |
| 25mm² | ~112A | 1.75 |
| 35mm² | ~138A | 1.25 |
*Base rating shown before installation method, temperature, and grouping corrections are applied — this tool applies those corrections automatically based on your selections above.
How this calculator works
It follows the same two-stage logic used in proper cable design:
1. Current-carrying capacity check: the design current (Ib) is calculated from the charger's power, then a protective device is sized to the next standard rating above it. The cable's required tabulated capacity is worked out by dividing the device rating by the combined correction factors for installation method, ambient temperature, and grouping — a smaller combined factor means a bigger cable is needed to compensate.
2. Voltage drop check: once a cable size passes the current check, its voltage drop over the specified run length is calculated. If it exceeds the typical 5% limit, the calculator steps up to the next cable size and rechecks — cable size is always whichever check (current capacity or voltage drop) demands the larger cable, not just the first one that passes.
Further reading:
Charging cables, sockets & accessories explained
Contactors & relays explained
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Frequently Asked Questions
Why isn't cable size just based on the charger's current draw?
Current-carrying capacity is only one of two checks. A cable can be rated to carry the current safely but still cause an unacceptable voltage drop over a long run, which affects equipment performance. The correct cable size is whichever of the two checks demands the larger cable, not just whichever passes first.
Why does the installation method change the required cable size?
How a cable is installed affects how well it can dissipate heat. A cable enclosed in conduit or surrounded by insulation can't shed heat as effectively as one clipped to an open surface, so it needs a larger cross-sectional area to carry the same current safely.
Why does ambient temperature matter for cable sizing?
Cable ratings are based on a reference ambient temperature, commonly 30°C. In hotter locations, such as a loft space in summer, the cable has less capacity to dissipate heat before reaching its maximum operating temperature, so a larger cable or reduced load is needed.
What voltage drop limit does this tool use, and why?
This tool checks against a common 5% voltage drop limit for power circuits, referenced from typical BS 7671 guidance for such installations. Excessive voltage drop can affect equipment performance and is a genuine design consideration, not just a theoretical one, particularly on long cable runs.
Can I use this tool as my final cable size decision?
No. This tool is an indicative starting reference using representative figures, not the exact values in BS 7671 or manufacturer cable data. Final cable and protective device selection must be verified by a qualified electrician carrying out a proper design calculation for the specific installation.
This calculator provides indicative estimates for general reference only. It does not constitute electrical design advice and must not be used as the sole basis for an installation. All EV charger circuit design and installation work must be carried out and verified by a qualified, registered electrician in accordance with BS 7671.
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