Wire and Cable Size Calculator
Use this calculator to figure out the minimum wire or cable size needed for your equipment.
Prefer to look stuff up in a table? No problem! Check out our wire and cable size table
Wire and Cable Ampacity
These are some of the factors that affect the cable or wire's ampacity (the capacity to carry current):
- Temperature — lower temperatures means lower resistance and higher ampacity
- Length — shorter cables means lower resistance and higher ampacity
- Cross section — bigger cables means lower resistance and higher ampacity
- Composition — pure stranded copper cables and wire should be used when building your system. Only silver has a lower resistance than copper
- Many cheap (and some not so cheap) cables and wire available from online retailers are made from aluminium and not copper! Aluminum and copper clad aluminum have greater resistance than copper and should not be used for your build! By the time you account for the larger cables needed for aluminum it usually isn't any less expensive than just using higher quality copper cables. If the composition of a cable or wire isn't listed, it is usually safe to assume it is aluminium or copper clad aluminum and should be avoided.
Voltage Drop
All cables will have some amount of voltage drop due to the resistance of the cable itself. Copper's resistance is very low but it isn't zero. For a given cable size, voltage drop will increase as current increases. Here are some guidelines for how to determine acceptable voltage drop for maximum efficiency:
- Critical equipment (DC-DC chargers, battery cables, fridge, etc) should not exceed 2% or 3%
- Non-critical items (lights, fans, etc) can go as high as 10%
- Solar panel input cables must not exceed 2%
In some situations this may be necessary to compromise on voltage drop due to mechanical considerations. For example, if 1 or 2 AWG cable simply won't fit in the space available, going with a 5% voltage drop for the start battery and ground connections of your DC-DC charger in order to use 4 AWG cable is fine. The charger will still work normally, just slightly less efficiently. Solar is an exception as that application is partularly sensitive to voltage drop - keep that at or below 2%.
Temperature
Temperatures near the engine can easily exceed 160-180°F (70-80°C). If your cable will be under the hood be sure to adjust the temperature slider to account for that. For wiring within the interior of the vehicle also keep in mind that temperatures inside parked vehicles can also get quite high.