Are these figures published standards or rules of thumb?

Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.

Why does this calculator sometimes say there is no answer?

Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.

Do I still need an RV technician if I use this?

For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Wiring calculator

RV Fuse and Breaker Size Calculator

Quick answer

A fuse for a continuous load is sized at 125 percent of that load and rounded up to the next standard rating. A 40A continuous draw needs 50A of overcurrent protection, and the conductor must be rated to carry at least that.

Result

Fuse or breaker size 50 A

Calculated requirement 50 A

Conductor must be rated at least 50 A Published standard

The formula

fuse amps = continuous amps x 1.25, rounded up to the next standard size

A fuse protects the WIRE, not the appliance. Sizing the fuse above the conductor ampacity defeats the entire point of fitting one.

The figures shown above are computed with these exact inputs: Continuous load 40 A, Sizing factor 1.25.

What this calculator assumes

Some of these are published standards and some are conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Continuous Load Factor: 1.25	Published standard	NEC 210.20(A), overcurrent device sized at 125 percent of a continuous load
NEC Table 310.16	Published standard	NEC Table 310.16, 75 C copper

Components that match this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Intermediate

BOJACK

BOJACK 200A Inline ANL Fuse Holder with Fuse

$15.99

Takes 0 through 4 AWG, which covers the cable sizes an RV inverter circuit actually uses.

Best for: Fusing an inverter feed at the battery.

Check price on Amazon

Beginner

iRhapsody

iRhapsody 200A ANL Fuse Holder Kit, 3 Pack

$13.99

Three holders in one pack, which is usually what a full build needs once the inverter, the solar and the DC-DC feed are all fused.

Best for: Fusing several circuits in one build.

Check price on Amazon

Beginner

RECOIL

RECOIL ZAFH2-200 Inline ANL Fuse Holder

$13.99

A sealed body with matched input and output gauges, which keeps the fuse holder from becoming the weak point in the run.

Best for: A tidy inline fuse on a heavy cable.

Check price on Amazon

Frequently asked questions

How does the fuse and breaker size calculation work?: fuse amps = continuous amps x 1.25, rounded up to the next standard size. A fuse protects the WIRE, not the appliance. Sizing the fuse above the conductor ampacity defeats the entire point of fitting one.
Are these figures published standards or rules of thumb?: Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.
Why does this calculator sometimes say there is no answer?: Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.
Do I still need an RV technician if I use this?: For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Researched guidance, not professional advice. These results are planning figures from published standards and manufacturer specifications. Never exceed a conductor's rated ampacity, fuse every positive conductor close to the battery, never run a generator in an enclosed space because carbon monoxide is lethal and odorless, and have permanent AC wiring done by a qualified RV technician or licensed electrician.