Circuit breaker sizes! Please Help!

Hello! My name is Bethany! I am coming up on time to put in my electrical and I was wondering if I could ask some advice on circuit breaker sizes? I have:

2 X 175 watt solar panels
40 amp charge controller —40 amp fuse to–>
170 amp hr lithium battery

12v fuse block with:
8 puck lights
2 usb outlets
Dometic fridge
suburban furnace
Maxxair fan
water pump

1000 watt inverter charger----1 AC power outlet

So I suppose I am asking for some advice on what size fuses I need in between the fuse box and battery? and also the inverter and the battery? and any other suggestions and/or advice in general would be greatly appreciated!

Thank you in advance!

This is where a little education and common sense can help.

You basically need three things: 1) Ohm’s law 2) a DC wire size chart 3) knowledge of your electrical loads.

  1. You’ll use Ohm’s law to turn watts into amps. This is so you’ll be using the same units (amps) for all of your calculations. Always use “input” volts – where it’s getting its energy from, in this case 12 volts. Watts/volts=amps, for example, your 1000 watt inverter divided by 12 volts equals 83 amps. With inverters you should always add 50% due to inverter inefficiencies, and to handle surges, so 125 amps. Since 125 amps is too much load to run through your 40-amp charge controller, it has to be connected directly to the battery with a fuse larger than 125 amps. I would use a 150 amp fuse, and the fuse has to be within a few inches of the battery, so there’s less chance of a fire. Protect the positive battery connection with electrical insulation to prevent a short, and the battery MUST be secured to prevent it from becoming a projectile during an accident, and to prevent the positive terminal from contacting anything metal inside the van – even in a rollover accident. There MUST be a grounding strap between the grounding post on the inverter and the body/frame of the van, and between the negative terminal of the battery and the body/frame of the van. It’s also a very good idea to plug a GFCI in between the inverter and anything that’s plugged into it. A 1,000 watt inverter can electrocute you!

  2. I typically use wire that’s rated for double the load that it is connected to. For each electrical circuit use a fuse that’s 20 to 50% higher amperage than the loads. NEVER install a fuse larger than the wire rating! The fuse must always be smaller than the wire rating, and slightly larger than the load. For example, if your DC fridge uses 6 amps and it’s within 6 ft of the fuse block, use 14 gauge wire and a 10 amp fuse.

  3. Smaller similar loads can be connected to the same fuse, and connected in parallel, positive to positive and negative to negative. Larger loads should always have their own fuse - not shared with others.

Most charge controllers today are equipped to manage the load (some however, may be damaged by inverters, so that’s another reason to connect inverters directly to the battery). This is to prevent over-discharge of the batteries, as the controller will disconnect the load if the voltage goes too low – protects both the battery and some devices. So connect your 12V fuse block to the inverter’s “load” connections.

Be sure that your charge controller is designed for the type of battery you will be using. Yours is lithium (I’m assuming LiFePO4 chemistry), so make sure the controller and battery are compatible. Also be sure that your battery has an internal or external battery management system (BMS). Most good quality lithium batteries nowadays have a BMS built-in.

Like the inverter, the connection between the battery and the controller should have a fuse, and that fuse should be located very close to the battery. In your case with the 40 amp controller, install a 45 or 50 amp fuse. This set of wires will go to the battery connectors on the controller. Remember to use the appropriate size wire.

For the fuse block, and to size the charge controller, add up all your loads: 6 amps (fridge), 10 amps (two USB ports), 5 amps (LED lights), etc…. The sum of all your loads should be smaller than the load limit of the charge controller. The fuse block is connected to the controller’s “load” connectors.

The solar panels should be connected to the solar connectors on the controller. Be cognizant of the controller’s maximum input voltage. If you have a PWM controller your panels need to be connected in parallel to maximize efficiency, keeping input voltage as close as possible to 12 volts. If you have an MPPT controller you may be able to connect the panels in series to increase the voltage (check the owner’s manual). Connecting in series adds the voltage together in the same way as two 1.5 volt batteries in series makes 3 volts. Read the manual for your controller. Ground the panel frames or negative terminal to the van body.

All connections must be solid, tight, and free of looseness and corrosion. Only use STRANDED copper wire for anything in a vehicle. Solid wire will eventually vibrate and break. Again, use the correct size wire for the load, and don’t compromise on the connections/connectors. Protect all of the wire harnesses from chafing and abrasion, especially where they might move. Pull hard on all the connections to be sure they’re tight. If they come loose with a firm tug you’re not doing it right.

If your electrical panel area looks like a rat’s nest you’re not doing it right. Everything has to be tidy, tight, secured from movement, and protected from any kind of damage, heat, chafing, or corrosion. Protect from moisture. Even stranded wires can break if subjected to a lot of movement or vibration; better to secure and protect them.


Thank you so much for this! I am so grateful to have this information to reference!! I never was that good at the numbers part but it looks like I have some calculations in my future! :pray:t3:


Sorry for the delayed response, It’s been A WEEK! I was thrown a curve ball by the company I purchase the setup form and informed after I received everything that my inverter and battery were not compatible. So I have opted to send back the lithium battery and change it for 2 200AH AGM batteries. This definitely throws a wrench in my plans but I am trying to accommodate with patience.

Also, a few questions :slightly_smiling_face:I never intended to connect my 12v fuse block to my inverter. Is there a reason you suggest doing this? I don’t intend to use my inverter often, thus why I planned for a smaller one with a remote on/off and other than using it for the one outlet it will mainly be off. But I love your suggestion of a GFCI. Do you think a 20 amp GFCI outlet would be sufficient or should I look into one with a higher amp rating?

Also how do I go about calculating the load of all of my 12v appliances? How do I find out the loads of each? I know their amp draw but I am very new to configuring the rest of it.

And what about the grounding strap? I won’t have anything but wood underneath my inverter so I’m not sure how to add the strap so it touches the van body (I assume you mean the metal?).

I truly am very grateful for all your help!!

I never said that you should connect the inverter to the fuse block - quite the opposite; connect it directly to the battery. Read my post again - I stated that in two places.

Every appliance should have an information tag/plate/inprint on it, or in the instructions stating how much power it uses. It will be either in amps or watts. If it’s in watts I gave you the formula for converting watts to amps.

I don’t understand why your Lithium battery is incompatible with your inverter. Will it not supply enough amperage? Did you get a 24 volt inverter and a 12 volt battery? Confused on this.

Your GFCI should match or exceed the power of your inverter. Again, use the same formula I discussed above, but this time we’re talking about the AC side of the inverter. for example, if you have a 1000 watt inverter, 1000w/110volts=9 amps; get a 10 or 15 amp GFCI.

Get a ground strap that reaches the frame or body of the van. If it has to be 5 ft long, so be it.

Please read my post carefully until you understand it. There’s a lot of information there. If you don’t understand something, try to Google it. If it’s all Greek to you, then maybe this is beyond your capability, and you should hire a professional.

:ok_hand:t6:…Thanks mate.

Hi Bethany

I recently watched the following videos from Greg Virgoe and found they answered pretty much every question I had for electrics, incl. cable type/size and fuse size. There’s a small amount of maths, but even I could understand it so I’m sure everyone else will find it mega simple :slight_smile:

Hmmm… I’m unable to post links (new user) so try searching for ‘Greg Virgoe’ on YouTube. He has a really comprehensive playlist for his entire van conversion, and near the end of the playlist there are a couple of videos that include very simple wiring diagrams and fuse information.

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THIS is super helpful! Thank you very much! I know what I’m doing with the majority of the build but wiring and circuit breaker size are definitely confusing me. Thank you for suggesting this! It really helps! :pray:t3::pray:t3::pray:t3: