Undersized DC wiring is the most common serious fault in owner-built RV power systems, and it is the one with the worst failure mode. Victron's own handbook puts it plainly: undersized wiring or a bad electrical contact can cause a fire.
The good news is that Victron publishes the numbers. This guide works through their method, and covers the one rule that catches almost everyone who has moved to lithium.
Start here: the specification that overrides everything
Look in your product's manual first. Every Victron manual states the recommended DC battery cable size and fuse size for that specific model. Where the manual differs from any general guidance, including this article, the manual wins.
The rest of this guide is for understanding why the manual says what it says, and for sizing the parts of the system the manual does not cover.
Sizing the cable
The rule of thumb
For cable runs up to 5 metres total, Victron gives a quick formula: divide the current in amps by three to get the required cross-section in square millimetres.
A 200 A load therefore wants roughly 66 mm², which is about 2/0 AWG.
Use this to sanity-check a design, not to finalise one.
The proper table
Victron's recommended battery cable table gives the maximum current for standard cable sizes at a voltage drop of 0.259 V. Total cable length means the positive cable plus the negative cable added together, which is the detail most people get wrong. A run of 2.5 metres out and 2.5 metres back is a 5 metre total, not 2.5.
| Cross-section | Up to 5 m total | Up to 10 m | Up to 15 m | Up to 20 m |
|---|---|---|---|---|
| 10 mm² | 30 A | 15 A | 10 A | 8 A |
| 16 mm² | 48 A | 24 A | 16 A | 12 A |
| 25 mm² | 75 A | 38 A | 25 A | 19 A |
| 35 mm² | 105 A | 53 A | 35 A | 26 A |
| 50 mm² | 150 A | 75 A | 50 A | 38 A |
| 70 mm² | 210 A | 105 A | 70 A | 53 A |
| 95 mm² | 285 A | 143 A | 95 A | 71 A |
| 120 mm² | 360 A | 180 A | 120 A | 90 A |
Contact losses are not included in these figures, so treat them as a floor rather than a target.
Converting to AWG
American cable is sold in AWG, and the conversion is not intuitive:
| AWG | Cross-section |
|---|---|
| 4 | 21.1 mm² |
| 2 | 33.6 mm² |
| 1 | 42.4 mm² |
| 1/0 | 53.5 mm² |
| 2/0 | 67.4 mm² |
| 3/0 | 85.0 mm² |
| 4/0 | 107 mm² |
Read across carefully. A cable sold as "2 gauge" is 33.6 mm², which by the table above is good for about 105 A on a 5 metre total run, nowhere near enough for a 3000 VA inverter.
Aim for under 2.5 percent voltage drop
Victron's free toolkit app calculates voltage drop from voltage, cable length, current and cross-section. Their guidance is to keep the drop below 2.5 percent.
Voltage drop is not just an efficiency question. On a 12 V system it eats directly into the headroom your inverter has before it declares a low-battery alarm, which is why an inverter that shuts down under load on a healthy battery is so often a cable problem rather than a battery problem.
If you cannot find thick enough cable, double up
Victron explicitly permits running two cables per connection instead of one very thick one, provided the combined cross-section meets the requirement. Two 35 mm² cables equal one 70 mm².
This is why the larger inverter/chargers have two positive and two negative battery terminals. They are there for exactly this purpose.
Cable mistakes Victron calls out
- Do not use cable with coarse strands. Fine-stranded flexible cable only.
- Do not use non-flexible or solid-core cable.
- Do not use AC cable for DC battery connections.
- In marine or damp installations use tinned-copper marine cable.
Insulation thickness is misleading. Check the actual conductor size from the cable marking or by stripping a short length, not by eye.
Sizing the fuse
Victron lists four selection criteria, and most guidance online covers only the first.
1. Current rating
With a single consumer on a circuit, the fuse matches either the consumer's rating or the cable's rating, whichever is lower. With multiple consumers, the fuse matches the cable.
The fuse protects the cable. That is its job.
2. Voltage rating
The fuse must be rated at or above the system's maximum voltage, and rated for DC. Most DC fuses suit 12 V and 24 V but are not necessarily suitable for 48 V and above. A fuse's AC rating is often higher than its DC rating, so read the DC figure specifically.
Some circuit breakers are not bidirectional, so on DC the wiring direction matters.
3. Interrupt rating — the rule that catches lithium installs
This is the most important paragraph in this article.
Lithium batteries can deliver enormous short-circuit currents, far higher than a comparable lead-acid bank. The interrupt rating, sometimes called breaking capacity, is the maximum fault current a fuse can safely interrupt without rupturing.
Victron's requirement is unambiguous: in every lithium battery installation, at least one fuse in the DC line must have an interrupt rating equal to or greater than the maximum prospective short-circuit current of the battery bank. They describe this as a mandatory safety requirement.
If the interrupt rating is too low, the fuse can fail catastrophically instead of protecting the circuit.
Interrupt ratings vary enormously between fuse types:
| Fuse type | Max DC voltage | Interrupt rating |
|---|---|---|
| Class T (Eaton Bussmann) | 160 V | 200,000 A |
| Class T (various manufacturers) | 125–300 V | 20,000 A |
| NH blade fuses | 250 V | 25,000 A |
| ANL (Blue Sea) | 80 V | 6,000 A |
| AMX(L) (Eaton Bussmann) | 125 V | 3,000 A |
| MEGA fuse (Littelfuse) | 70 V | 2,500 A |
| MRBF terminal fuse (Blue Sea) | 58 V | 2,000 A |
| MEGA fuse (Littelfuse) | 58 V | 1,000 A |
Read that table against your battery's datasheet. A MEGA fuse is an excellent, widely used DC fuse, and on many lead-acid systems it is entirely appropriate. On a large lithium bank its interrupt rating may be well below the prospective fault current, and that is precisely the situation Victron's rule is written to prevent.
Class T is Victron's recommended solution for lithium because of its very high interrupt rating and fast response. Their Lynx range includes a Class-T Power In busbar built for this.
We do not currently stock standalone Class T fuses. If your system needs one, check your battery manufacturer's specified prospective short-circuit current and source a Class T fuse rated at or above it. Do not substitute a MEGA fuse to save time.
4. Speed
Slow-blow fuses suit DC circuits with high inrush, which includes inverters with large input capacitors and anything with a motor. Fast-blow fuses suit most AC applications, though AC loads with electric motors such as refrigerators and air conditioners need a slower fuse again.
For an inverter's DC feed, you want a slow fuse. MEGA fuses are slow; ANL fuses are fast.
Fuse types available
| Type | Range | Voltage | Speed |
|---|---|---|---|
| MIDI | Smaller branch circuits | 32 V DC | Slow |
| MEGA | 40–500 A | 32 V DC | Slow |
| ANL | 35–750 A | 32 V DC | Fast |
| NH | Up to 1000 A | 440–550 V DC | Various |
Each needs its matching holder: MEGA, ANL, MIDI. Larger inverter feeds commonly use the 400 A and 500 A MEGA fuses.
Distribution and busbars
Once you have more than a couple of DC loads, run everything to a distribution point rather than stacking rings on the battery terminal.
Victron's busbar sizing rule is simple: apply the recommended cable cross-section to the busbar's cross-section. A 10 mm by 5 mm bar is 50 mm², so it suits about 150 A over runs up to 5 metres.
The cross-section of the connection between the battery and the distribution point must equal the sum of the required cross-sections of everything fed from it. A busbar fed by a cable smaller than its loads is a hidden bottleneck.
Options: plain busbar for straightforward distribution, six-way MEGA fuse holder with 250 A busbar where each branch needs its own fuse, or the Lynx Distributor for a modular system.
Busbars are not insulated. Shield them. A dropped spanner across an unshielded pair of busbars on a lithium bank is a very serious event.
Two rules for multi-inverter systems
Victron is specific about systems with more than one inverter or inverter/charger:
- Fuse each unit individually, with the same fuse type for each, so every DC path has the same resistance.
- Do not use one large fuse for the whole system. A fault inside a single inverter rarely draws enough current to trip an oversized fuse, so the current keeps flowing at a dangerous level and damages wiring instead of blowing the fuse.
Victron also prefers, though does not mandate, keeping a continuous negative connection and fusing or switching only the positive of each unit.
Isolation
A main fuse can serve as your isolation point, since pulling it breaks the circuit for maintenance. If you do fit an isolator switch, fit it in the positive cable only, and choose one rated for the DC current the system can actually produce. A poor-quality switch adds resistance and voltage drop.
Check that any switch you buy is rated to break DC current under load. Not all are.
A short pre-flight checklist
- Cable cross-section from the manual, cross-checked against the table using total length.
- Voltage drop under 2.5 percent.
- Fine-stranded flexible cable, tinned copper if damp.
- Fuse rated for DC at your system voltage.
- Interrupt rating at or above your battery's prospective short-circuit current.
- Slow fuse on inverter feeds.
- Terminals torqued to the manual's figure, then re-checked after a few weeks.
- Busbars shielded, and fed by a cable sized for the sum of the loads.
If you send us your inverter model, battery bank and cable run lengths, we will work through the sizing with you before you order. Everything ships from Upland, California.