What Size Cable and Circuit Protection do I Need?

One of the most common questions we get daily is what size circuit breaker and cable is needed for an installation. It is a common misnomer that the circuit breaker is there to protect devices, when in actual fact, circuit protection is there to protect the cable, not the devices.  It protects the cable from over current or short circuit events, so the cable doesn’t catch on fire due to excess current draw. 

 

Circuit protection must be sized such that the circuit breaker or fuse has a current rating approx 25% lower than the maximum current the cable can handle.  Having a circuit breaker current rating higher than the maximum current rating of the cable means that the cable could overheat and catch on fire before the circuit protection does anything to protect it.    

 

Unfortunately, it is not only the unaware DIYer that gets this wrong - we hear stories all the time of installers and recreational vehicle manufacturers incorrectly sizing cable and circuit protection, and there's at least one major brand that sells pre-made wiring kits with incorrectly sized circuit protection (sometimes off by a big margin).  In this case, they have paired 8AWG cable with an 80A fuse - 8AWG (or 8B&S) is rated to carry up to 74 amps, meaning that by the time the current reaches 80 amps, the cable will have melted (or worse, caught on fire!).  An extreme example is a well know inverter seller in Australia, supplying a 3000w inverter with 4B&S cable rated for 135A and a 450A mega fuse.  Cable catches on fire well before the inverter even gets to half power. 

 

 

Where does Circuit Protection Need to be Placed? 

Circuit protection needs to be placed on all loads connected to the positive terminal of the battery, ideally within 30cm. This means that in the event of a current overload, it is only a short distance/time that the cable has to withstand the overload before the circuit is tripped (and therefore stopping the circuit from producing power).  

 

If a positive busbar is being used, you will need to connect a circuit breaker/fuse to every cable feeding off from this busbar.  A primary circuit breaker is also strongly recommended between the battery and the busbar itself.  

 

What Size Circuit Protection Do I Need? 

To size your circuit breaker/fuse, you need to determine the maximum amount of amps the load will draw.  

To work out how many amps you will be drawing, use the equation: 

Watts/Volts = Amps 

Whereby the Volts is 11/22/44V for 12/24/48V systems respectively, as the battery can still be drawn down at these voltages.   

Once you have the max number of amps being drawn, multiply this number by 1.25 (to give a 25% buffer), then round to the nearest standard size circuit breaker/fuse as your required circuit protection.  

 

What size cable do I need? 

To size your cable, take the circuit breaker/fuse size calculated above, and multiple this again by 1.25 (for the 25% buffer), and then round to the nearest standard size.  Using these two calculations above will ensure the current draw is always below the circuit protection avoiding nuisance tripping and the cable is always protected by the circuit protection. 

 

What About 12V Sockets? 

For sizing cable and circuit protection sizes for individual 12V sockets, be sure to take the total amount of watts capable of being drawn from the socket. For example, for our PD100W Socket, this has a 100W and a 36W USB point, totalling 136 watts.   

So the calculation would be: 

136W/11V = 12.36 Amps 

(136 being the total watts, 11 being the voltage number used for a 12V system) 

 

So you would use a 15A fuse.  

Then multiplying 15 by 1.25 for the cable size, would give you 18.75A. Using the table below, you can see that for twin core (which is generally used for these installations) you will need a 4mm twin core for this current rating.  

 

Keep in mind that for all “USB A” sockets, the Amps given (1 to 3A per port) is at 5V, not 12v. In order to size the circuit protection correctly, you will need to first work out the watts by multiplying the amps (2.4A) by 5 (as Amps x Volts = Watts).  So in the case of our dual 2.4A USB sockets, you would multiple 2.4 x 2, giving you 4.8A, then multiply this by the 5 volts (4.8x5 = 24W), and then use this wattage calculation to work out the circuit protection size: 

24W/11V = 2.18A 

So you would use a 5A fuse, and 2-3mm auto cable.  

 

Remember also, that whilst a plug can be rated to a higher current (eg. the Engel Posifit sockets are rated to 15A), the only device being plugged into them may not draw this much current (most fridges would not draw more than 10A), so you can use this current to size the circuit protection.  

 

 

What about Solar Panels? 

Solar panels themselves are power sources, therefore they need to be fuse protected.  

To size an MC4 solar fuse (installed on the positive cable of the panel), take the short circuit current of the solar panel (in the solar panel specifications table) then round up to the nearest fuse size, and use this size to multiply by 1.25 to find the cable size. Remember, you will need to account for voltage drop over the distance of the cable also (this is a whole other lesson, stay tuned!). 

 

What if I Join Multiple Panels Together?

If you are adding solar panels together in parallel, use an MC4 solar fuse on each panel separately, then a combined double pole solar circuit breaker/isolator should be used to protect the whole string, using the short circuit current of each panel multiplied by the number of panels (rounded up to the nearest size). 

If you are adding solar panels together in series, you should still fuse each panel separately, then a combined double pole solar circuit breaker/isolator should be used to protect the whole string, using the short circuit current of one panel (then rounded up to the nearest size). 

 

Should I use a Circuit Breaker, Fuse Block, Midi-Fuses or MC4 Fuse?  

Things to consider when deciding on the type of circuit protection are the space you have to work with, the waterproof rating required, the ease of replacement for fuses, and the need for isolation capability.  

For sockets (which are often installed in multiples), a 6 or 12 way fuse block is ideal.  Remember you should always size the cable and circuit protection for the overall current running back to the battery also.     

With fixed solar panels, AllSpark solar panels are all supplied and pre-fitted with MC4 solar fuses, which are perfect for creating waterproof connections that won't "rattle" free on corrugations. Solar circuit breakers are then ideal for protecting the main cable once all panels are linked together.  

Inverters are best fitted with manually re-settable circuit breakers, which allows you to isolate them if needed.  

For engine bays, midi-modular fuse blocks are the most popular option, being that they are waterproof and robust and have a smaller physical size than circuit breakers, but fuse blocks can be used if they are located up towards the bonnet, where they are unlikely to be submerged.  Circuit breakers are still preferred if you have sufficient space for these to be installed.  No spare fuses to carry and gives you an isolation point for maintenance or when not in use for extended periods. 

Keep in mind that resettable circuit breakers, whilst a higher upfront cost, allow for easy re-setting of the circuit once the cause of the trip is established. Using midi-fuses or blade/micro fuses means you have to ensure you have spares with you in case of emergency, should one blow when you are in a remote location and unable to easily source a replacement.  

 

The Bottom Line 

Circuit protection is essential to protect your system, but most importantly, to protect yourself. When travelling in outback Australia, you can be hundreds of kilometres (or more!) from help, so having your vehicle catch on fire because of an incorrectly sized circuit breaker or cheap pre-made harness can have devastating consequences.  

As always, feel free to reach out if you need assistance, or would like a technician to double check your calculations.  

 

***The information provided here is general in nature, and if you aren't confident with installing 12V (or 24V or 48V), then you should always seek the help of a professional, qualified and knowledgeable auto-electrician. ***