ABYC E-11 wire gauge, ampacity, and fuse sizing for small-craft DC circuits. Reads a CSV circuit list, prints a sizing table, and flags circuits whose inputs are still guesses.
npx wire-wright circuits.csv
For each circuit, the conductor is the larger of:
- Voltage-drop minimum — circular mils = 10.75 × amps × round-trip feet ÷ allowable drop volts, at 3% (critical circuits) or 10% (non-critical).
- Ampacity minimum — smallest conductor whose allowable amperage, derated for engine spaces and bundling, carries the load (ABYC E-11 Table 6A).
The fuse/breaker is the smallest standard rating at or above 125% of a continuous load (100% if flagged non-continuous), and never above the derated ampacity of the chosen conductor — the conductor is upsized if no standard rating fits that window.
Ampacity values, engine-space correction factors, and bundling derates are
transcribed from ABYC E-11 Table 6A as republished with ABYC's permission
at https://boathowto.com/wiresize/wiresize_tables_abyc.pdf. The
voltage-drop formula (K = 10.75, copper DC) is the standard E-11 method;
this implementation's output is test-verified against the ABYC 3% and 10%
conductor-size lookup tables from the same document (npm test).
| column | required | meaning |
|---|---|---|
name |
yes | circuit label |
amps |
yes | continuous current, A |
length_ft |
yes | conductor length source → device → source (round trip), ft |
drop_pct |
no | 3 (default, critical) or 10 |
voltage |
no | system voltage, default 12 |
engine_space |
no | true/false, default false |
bundle |
no | conductors bundled together, default 1 |
insulation_c |
no | insulation rating °C, default 105 |
continuous |
no | default true; false uses 100% fuse target |
amps_source, length_source |
no | guess / spec / measured — anything not measured is flagged in output |
status |
no | e.g. planned / installed — non-installed flagged |
notes |
no | free text, passed through |
- DC only, copper conductors, AWG 18 through 4/0.
- Sizing is per-circuit; it does not sum loads, check panel totals, or verify fuse interrupt ratings (AIC) — lithium banks need the fuse type their battery manufacturer specifies (typically Class T).
- Output is informational. Physical installation is governed by ABYC E-11 itself and whoever surveys your boat.
MIT.