Determining DC Cable Thickness

This page is designed to help you determine the thickness and type of cable for your overlander, camper, or tiny house.

This tool was created for 12V and 24V DC systems.

Selecting the correct cable type

Simply put, for 12V or 24V systems where the structure moves, you should find cabling that meets the following standard:

ISO 6722 Class B

These cables are rated for automotive voltages, temperatures, vibrations, insulation, abrasion etc. Because this standard must be adhered to across the automotive industry, they are also easy to source and inexpensive.

Unless you want to know more, you can jump to the calculator to work out which thickness you require.

You can find a link to the ISO Standard PDF in the references section at the bottom of the page.

Stranded vs Solid core

Solid core is perfectly fine for static applications such as traditional houses. For applications subject to dynamic forces (vibrations, movement etc) stranded core cables are more appropriate.

 

Solid core cables are less flexible and more likely to work harden which results in thinning and cracking. This can cause loss of continuity (broken circuit) or points of high resistance leading to thermal events.

Cables that meet the ISO 6722 standard have been tested for abrasion, water resistance, bending, and mechanical stresses and should be considered standard for all low voltage systems in applications that move.

For overlanding, expedition vehicles, van conversions and mobile tiny houses, stranded core should be used.

What to do with the ends?

The ends of stranded cables should ideally be crimped. This protects the end of the cable and provides a good electrical and mechanical connection.

Often the ends will simply be twisted and inserted into a screw down connector.

The ends should NEVER be 'tinned'. Tinning is where the end of the wire is dipped or coated in solder. This might seem like a good idea but the solder is not as solid as it seems and will change shape over time. This can cause the cable to form a bad connection or come loose which can be a serious safety issue. No competent manufacturers tin the ends of stranded cables and nor should you.

Current Rating

Cable manufacturers should provide a current rating for each thickness of cable they supply.

The current rating is in Amps and is intended to help you choose the appropriate thickness cable for your application. 

Essentially the current limit is a thermal limit, related to how much heat the cable can dissipate. All wires have a resistance (although it is designed to be low) which causes wires to heat up under load.

Exceeding the current limit for a cable might result in a "thermal event" and is a serious safety concern. Most of the tests involve the cable being suspended in free air (or water) so the current rating might be lower if the cable is to be placed inside a conduit or bundle of other wires. 

The calculator at the bottom of the page uses manufacturer figures but you should always consult the reference material from the manufacturer you buy from.

Voltage Drop

Because of the wire resistance, transmitting electricity even a few meters results in a voltage drop along the cable. What that means is that devices far away from the battery receive less than the battery voltage.

Some devices might have voltage sensing circuits that prevent operation if the voltage is too low.

The voltage loss along the cable also causes power loss which is wasting energy unnecessarily. The trade off is between the additional cost and weight of thicker cabling, and the loss of power and heat generation.

 

Targeting to keep voltage drop under 3% (there and back) is good practice, although the calculator will allow you to select 1-5%.

Length of cable run

The length of the cable run is simple to calculate but must include the true length of the cable. Sometimes a cable's route can be quite circuitous and complex and it can't hurt to overestimate.

The calculator has a toggle button which automatically doubles the length to include a return run. If you have a cable that goes to a device then all the way back, leave this toggled.

If you are using a chassis return, where the conductive chassis is connected to the negative terminal of your supply, and the chassis is capable of transmitting that current, then you can toggle off this feature.

 

DC Cable Size Calculator

Voltage (V)

Current (Amps)

Cable run (m)

Include return?

Only uncheck if you are using a chassis return or are calculating the one-way loss.

Voltage drop (%)

arrow&v

Bundle/Conduit 

Toggle if cable will not be in free air (e.g. inside conduit or thick bundle)

The above calculator is provided as a guide. Please ensure you cross reference and refer to manufacturers specifications. Please refer to someone competent and qualified if you are unsure. Sources and references are provided at the bottom of the page.

Need more detailed help?

 
Constants, formulas, and assumptions:

Below are the formulas, equations, and constants used in the calculator for cross reference. The calculator assumes, as a safety factor, that the cables are operating at their maximum specified temperature according to ISO 6722 class B (100°C). The values for resistivity and thermal coefficient of resistance have been selected as drawn copper (approx 97%) as conductive as standard annealed copper to better represent the quality of cable commonly used.

Technical document references
Other online DC Cable sizing calculators:

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