Every Wattonomy design is sized and checked against the recognized electrical standard for where you are — and it shows its working. This page is the full method: the standards we use, the exact rules behind each number, and how we validate every release.
We size each conductor and overcurrent device to the published standard for your region, size the battery and solar from your real loads and climate, then automatically check every output against the standard's own tables and the manufacturer's datasheet before you ever see it.
There’s no single global electrical code, so we use the one that governs where you are.
Different countries have different electrical rulebooks. We simply use the right one for where you are — so your design matches the rules a local inspector or surveyor would actually check against.
A conductor has to satisfy two independent requirements, and we size it to whichever demands the bigger wire:
The conductor must safely carry the current of the overcurrent device protecting it, continuously, at temperature. We start from the standard's published conductor tables ABYC E-11 Table 6A Table 4A (single conductor) and apply the real-world deratings the standard requires:
Even a conductor that can carry the current is wrong if it wastes voltage over a long run. We hold critical circuits to a 3% voltage drop over the conductor's actual measured run length: ABYC (3% volt-drop)
The final gauge is the larger of the ampacity result and the 3%-drop result — so a short high-current run and a long low-current run can land on very different wire, exactly as they should.
Two different things can make a wire “too small”: it can overheat carrying the current, or it can quietly lose too much voltage over a long run. We check both and use the size that satisfies both — so your gear gets the power it expects and nothing runs hot.
The job of overcurrent protection is to protect the cable — not the device — so the rule is simple and strict: the fuse rating must never exceed the conductor's ampacity, sized to 125% of the continuous load ABYC/NEC 125%.
A fuse must be able to safely break the largest fault current the battery bank can actually deliver. A lithium bank can deliver enormous short-circuit current, so the fuse's interrupting rating (AIC) has to match it:
A fuse’s real job is to protect the wire, not the gadget — so we never let the fuse be bigger than the wire can handle. And because a lithium battery can dump a huge jolt in a fault, the fuse also has to be able to actually stop that jolt (that’s the “AIC” part most builds miss).
Sized from your actual daily energy use, your chosen days of autonomy, and the usable depth of discharge for the chemistry, then rounded up to whole battery modules:
Cold is handled separately and honestly: where your climate is cold enough to reduce usable capacity or block lithium charging, the tool warns you (LiFePO₄ can’t charge below about +5 °C, and capacity falls in the cold) rather than quietly inflating the bank.
Sized to replace your daily consumption under realistic conditions, not lab conditions — derated for panel tolerance, temperature, soiling and charge-controller efficiency against your location's usable sun-hours.
This is a safety check, not just a capacity one. Panel open-circuit voltage rises as it gets colder, so we check the array's string voltage against the controller's maximum PV-input voltage at the coldest expected temperature for your climate — the failure mode that quietly destroys controllers. We also flag that LiFePO₄ cannot charge below about +5 °C and route around it where the climate implies a problem.
We work out how big a battery you need from what you actually run and how many cloudy days you want to ride out, then enough solar to refill it on a realistic (not perfect) day — and we make sure your panels won’t over-volt the controller on the coldest morning.
Citing a standard is only worth something if the output actually matches it. So before any change ships:
One principle sits above all of this: we never contradict a manufacturer's published recommendation, and where a value can't be verified we fall back to the conservative standard figure rather than guess.
Before anything ships, the tool re-checks itself against the official tables and the manufacturers’ own datasheets — many thousands of times — so a rare edge case is caught as reliably as a common build. And it shows you the working, so you never have to just take our word.
Claims are cheap, so here is validation you can inspect. Each row is a safety rule we guarantee, checked against an independent source — the laws of physics, the standard’s own rule, or the manufacturer’s datasheet — not against ourselves. A checker re-runs every row on each release and across thousands of scenarios; if any check failed, this table would not publish.
| What we guarantee | Checked against (independent) | Example | Result | Pass |
|---|---|---|---|---|
| Cable stays within a 3% voltage drop | Ohm’s law from copper resistivity (independent) | 12 V, 600 W, 3 m run → 4 AWG | engine 2.1% vs Ohm’s-law 2.0% — both within 3% | ✓ |
| Cable stays within a 3% voltage drop | Ohm’s law from copper resistivity (independent) | 24 V, 800 W, 6 m run → 16 mm² | engine 2.3% vs Ohm’s-law 1.8% — both within 3% | ✓ |
| Fuse is ≥125% of the load and never above the cable’s ampacity | ABYC E-11 / NEC 125% rule + protect-the-cable | 50 A continuous load | fuse 80 A ≥ 125% (63 A) and ≤ cable 136 A | ✓ |
| Solar string can’t over-volt the controller in the cold | Voc temp-coefficient formula + datasheets (independent) | 3× Jinko Tiger Neo 430 W at -10 °C | cold Voc 125.9 V ≤ controller max 150 V | ✓ |
| Battery holds enough usable energy for your days of autonomy | Energy balance (independent): bank Ah × V × usable DoD ≥ use × days | 1600 Wh/day × 2 days = 3200 Wh needed | bank delivers 3840 Wh usable (400 Ah, 80% DoD) | ✓ |
| Component specs match the manufacturer datasheet | Victron published coordinated fuse + cable | MultiPlus 12/1600 | engine 200 A / 50 mm2 = published spec | ✓ |
| Component specs match the manufacturer datasheet | Victron published coordinated fuse + cable | MultiPlus 12/2000 | engine 300 A / 70 mm2 = published spec | ✓ |
| Fuse can interrupt the worst-case short circuit | Safety invariant: interrupting capacity ≥ prospective fault current | Class T main fuse on a lithium bank | interrupts 20,000 A vs ~10,650 A fault — covered (larger banks add per-battery fuses) | ✓ |
All 8 checks above passed, and the same safety invariants were re-verified across 384 real-load scenarios (2,264 individual checks) when this page was built (2026-06-08). If a single check had failed, this table would not have published.
Here is a real Wattonomy result, traced step by step: a 12 V camper with a Victron MultiPlus 12/2000 inverter and a 3 m battery cable.
Result: 2/0 AWG cable + 300 A Class T fuse — every figure traceable to the manufacturer datasheet and the standard. On a run without a manufacturer-coordinated spec (a solar or branch run), the cable is instead sized to the larger of its ampacity and its 3%-drop result, exactly as in section 2.
The handful of terms worth knowing — nothing left undefined.
Wattonomy gives you a validated, standards-referenced design and a sourced parts list: a genuine, well-engineered starting point you can build from with confidence. It is not a stamped engineering drawing, and software can't see your actual boat, van or cabin.
So before you build or energise anything, verify every component, rating and connection against the products you actually buy and the rules that apply where you are — and have the design reviewed by a suitably qualified (and where required, licensed) professional. That final human check is part of doing it right, not a sign anything's wrong.