Cable thickness matters more than people think

In the UK, cable is sized in mm² (cross-sectional area), not AWG, so make sure any sizing chart you use is in the right units. The rule of thumb is simple: the higher the current and the longer the cable run, the thicker the cable needs to be.

A good target is to keep voltage drop under 3% across the full cable run (remember to count both the positive and negative legs, so a 2m run to an appliance is really 4m of cable). Undersized cable is one of the most common causes of poor performance and, in the worst cases, overheating and fire risk. Always use flexible, multi-strand automotive cable rather than solid core, since solid cable can fatigue and crack from vibration on the road.

As a rough guide: light loads like LED lighting or USB sockets can usually run on thinner cable, but anything feeding a fridge, water pump or especially an inverter needs to be sized properly for the current it will carry, not just for the appliance's average draw.

12V vs 240V: what goes where

Most camper and motorhome electrics run natively on 12V — lighting, water pumps, fridges, USB charging and control panels. Keeping appliances on 12V where possible is more efficient, since every conversion from 12V to 240V through an inverter loses some energy as heat.

240V (via an inverter, or hook-up when available) is generally reserved for household appliances that don't have a 12V equivalent — laptop chargers, kitchen appliances, power tools, and so on. Deciding what genuinely needs 240V versus what can run natively on 12V is one of the best ways to reduce your battery's daily workload.

Inverters: sizing and wiring

Inverters can pull very high current from a 12V battery, especially under full load, so they need short, heavy-duty cable runs directly to the battery or busbar and their own dedicated fuse. As a general example, a 1200W appliance running through a 12V inverter can draw around 100A from the battery, which is why inverter cabling is usually the thickest cable in the whole system.

Keep inverter cable runs as short as practically possible to minimise voltage drop and heat build-up, and make sure the inverter is mounted somewhere ventilated, since they generate heat under load.

Fuses: protecting the cable, not the appliance

It's a common misconception that fuses protect the appliance. In reality, a fuse's job is to protect the cable from overheating if there's a fault or short circuit. That means the fuse rating should be chosen based on what the cable can safely carry, not just what the appliance draws.

For high-current circuits like the main battery cable or inverter feed, MIDI, MEGA, ANL or Class T fuses are commonly used, while smaller circuits like lighting and USB sockets typically use standard automotive blade fuses.

Always fit the main battery fuse as close to the positive terminal as possible, ideally within a few centimetres, so as much of the cable run as possible is protected.

Reminder: Sonnie's Adventures offer — 10% off for new customers 🎉

Sorting your install and need a battery? Use code FOGSTAR10 at checkout to save 10% on any Fogstar battery. Valid for new customers throughout July and August 2026, with free delivery and full authorised dealer warranty included. Don't forget to check our Sonnie's Adventures Fogstar battery offer before you buy. Sonnie's Adventures promotion, valid only at sonniesadventures.com — not redeemable at fogstar.co.uk.

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https://sonniesadventures.com/collections/fogstar

FAQs

What size cable do I need for my Fogstar battery?

It depends on the current draw and the length of the cable run. As a rule, keep voltage drop under 3% across the full run (both positive and negative legs combined), and size up for high-current circuits like inverters.

Where should the main fuse go?

As close to the battery's positive terminal as possible, ideally within a few centimetres, so the fuse protects as much of the cable as it can.

Do I need an inverter?

Only if you have appliances that don't run on 12V. Keeping as much as possible on native 12V is more efficient and reduces strain on your battery and cabling.

Can I install a Fogstar battery myself?

Many self-builders do, provided you're comfortable following correct cable sizing, fusing and safety practice. If you're unsure at any stage, it's worth getting a qualified auto electrician to check or complete the install.

Does FOGSTAR10 apply if I'm buying accessories for my install?

The code applies to Fogstar batteries specifically, for new customers, throughout July and August 2026.