Wire Gauge (AWG) Calculator

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(V DC)
Fuse too small

Load Current Maximum current flow expected from the load.

Fuse size As recommended by the manufacturer.


Wire Length Round-trip (positive + negative).

(pos+neg, feet)

Voltage Drop The loss of energy as current moves through a wire results in voltage drop. A larger wire offers less resistance and therefore minimize voltage drop. We recommend 3%.


Fuse/Breaker If you don't know, we will use:


Derating Factors Factors that impact the ampacity or the voltage drop of wires

Recommended Wire


AMPACITY: ?A Maximum current that a wire can carry continuously without exceeding its temperature rating.
DERATED AMPACITY: ?A Corrected ampacity when taking derating factor(s) into account.
Fuse too small
Ancor Marine Duplex Wire 14AWG

*You can select the wire length on Amazon

Made with in our van.

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How this wire calculator works

Based on Load Current:

  1. Using load_current and length, we first find the wire gauge that meets the selected voltage_drop.
  2. We find a wire gauge for which the ampacity is higher than the fuse size (fuse size per owner’s manual, otherwise we compute it as follows: fuse_size = load_current x 1.4).
  3. We compare 1. and 2. above and keep the wire with the largest gauge.
  4. Voilà!

Based on fuse size:

  1. We assume that: load_current = fuse_size/1.25 (it is generally accepted that a fuse must be at minimum 125% the size of the load current)
  2. Using load_current and length, we compute the wire gauge that meets the chosen voltage drop.
  3. We find a wire gauge for which the ampacity is higher than the fuse size.
  4. We compare 2. and 3. above and keep the wire with the largest gauge.

Derating Factors:

Wires can carry a certain amount of current continuously and no more; more current means the wire will overheat (and melt) as it cannot dissipate the heat that is generated by too much current flow. That characteristic is called AMPACITY (maximum current that a wire can carry continuously without exceeding its temperature rating). Certain factors reduce the ampacity rating of wires and the voltage drop as well:

Ambient temperature of 50°C (122F) or more

A wire located in ambient temperature of 50°C (122F) or more loses its ability to dissipate heat, and, therefore, its ampacity is reduced by 15%. (note: this is per ABYC standards "In Engine Room"). This is most likely the case for a wire running in the wall/ceiling of a van because temperature in there is much higher than in the living space (that's especially true for darker color vans).

Load runs continuously for 20 minutes or more

A wire that carries a current flow for a long duration (~20 minutes) builds up more heat. As the temperature of a wire increases, so does the resistance to current flow = more voltage drop. To mitigate this factor, the load current is increased by 25% for the voltage drop calculation (but not for the fuse/breaker size calculation). This is not an ABYC requirement at the moment; however, it is generally accepted by marine product manufacturers (such as Blue Sea).

Wire in conduit, insulation, or bundled with 2 (or more) wires

A wire located in a conduit, sheath, running through insulation, or bundled with 2 (or more) wires loses its ability to dissipate heat, and therefore, its ampacity is reduced by 30%. This is not an ABYC requirement at the moment; however, it is generally accepted by marine product manufacturers (such as Blue Sea).

Making things easy

Did we mention you actually don't need to use this calculator? :P Our wiring diagram features customizable components (solar, alternator, shore, inverter, 12V loads), a built-in wire gauge (AWG) calculator, and it will output the wire lengths & terminals you need to purchase. It doesn't get easier than this! Here it is in action:

Maxxfan Wire Sizing Example


Method 1: Calculate Wire Gauge from Load Current (preferred)

Better accuracy

Load Current

Always use the maximum current that the load is expected to draw. The Maxxfan can draw up to 2.8A at the highest speed in steady state (according to our Simarine Pico Monitor). However, any load with a motor draws more current during startup for a very short period of time, so we'll go ahead and add about 50% buffer to account for startup. If you have no clue what current to use as an input, it's OK to use the FUSE SIZE that the manufacturer recommends. As a result, you'll get slightly oversized wires (which is quite good for safety, performance, and durability).

: 4A

Wire Length

The wire length input is always the round-trip length. The round-trip length is the sum of the positive and the negative wires. Remember that a duplex wire packs the positive and the negative into a single wire, so the round-trip length is equal to twice the duplex length.

: 30FT

Voltage Drop

There is a loss of energy (voltage drop) as current moves through passive elements (terminal, fuse, wire, etc.); a smaller wire means more voltage drop. For example, if we size the wire for 3% voltage drop (bigger wire), the voltage will go from 12V at the fuse block down to 11.64V at the Maxxfan. If we size it for 10% voltage drop (smaller wire), the voltage at the Maxxfan will be 10.8V; this can become a problem as the battery voltage goes down (low SOC). For best performance, we recommend 3% (5% would be OK as well).

: 3%


Overcurrent protection devices (fuse/breaker) protect the wire (not the load) from being used over its ampacity; it's the weakest link! Therefore, a fuse/breaker should be smaller than the ampacity of the wire but big enough so that it doesn't blow during normal operation of the load. The easiest way to determine the fuse size is to follow the manufacturer recommendations! So check the owner's manual or specifications sheet. Can't find it? OK then. As a general rule, the fuse size can be determined as follows: LOAD CURRENT x 1.4 and then round up to the next available fuse. In the Maxxfan scenario: 4A x 1.4 = 5.6A = 7.5A fuse. However, it looks like the manufacturer recommends a 10A, fuse so we'll stick to that.

: 10A

Recommended Wire

AWG 12

Method 2: Calculate Wire Gauge from fuse size

Easier, but may result in slightly oversized wires (which is totally fine in terms of safety and performance; the downside is the cost!)

Fuse Size

In the scenario where we have no clue how much current the Maxxfan draws (load current), we'll use the fuse size instead. Using our Google super skills, we find that the manufacturer recommends a 10A fuse:

: 10A

Wire Length

Same same!

: 30FT

Voltage Drop

When basing our calculation from the fuse size, we like to use 5% voltage drop (instead of 3%). This is to "compensate" for the slightly oversized wire that this method gives.

: 5%

Recommended Wire

AWG 12

1500W Inverter Wire Sizing Example


As always, we want to use the maximum current as an input. 1500W/12V = 125A is an oversimplification. Actually, the inverter can work down to 10.7V and the efficiency ratio is around 85%, so: 1500W/10.7V/0.85 = 165A. But according to the specifications sheet, the "maximum input current" is 200A. We always follow the manufacturer recommendations, so:

: 200A


As always, we want to use the round-trip length as an input. That's the sum of the positive and negative wires:

: 4FT

Voltage Drop

Reputable inverter brands are quite aggressive with voltage drop; we've seen it in the 2% range. Why? Because they care about how their product performs in the real-world, not just on the specifications sheet. For example, most inverters stop working below 10.7V; if the inverter was wired for 10% voltage drop, it means it won't work when the battery charge (SOC) is at around 60%-70% (for AGM), which can happen very frequently in the real world...

: 2%


According to the Samlex owner's manual, a fuse of 200A is recommended:

: 200A

Per calculator:


The inverter is the most finicky and dangerous part of an electrical system. While you can use this calculator for sizing the wires, we highly recommend to follow the manufacturer recommendations from the owner's manual (fuse size, wire gauge, and wire length). That'll ensure the inverter installation is safe and performs as it should. (Expect to find different recommendations between this calculator and different inverter brands; no one seems to use the same calculation/factors...)

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About us



Hello! We’re Isabelle and Antoine 🙂 In 2017, we sold our house (and everything in it), quit our engineering careers, and moved into our self-built campervan. Every day is an opportunity for a new adventure... We’re chasing our dreams, and hopefully it inspires others to do the same!

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