Pick the conductor size, its insulation rating, where it runs and how many current-carrying conductors share the bundle. The calculator gives the conductor's allowable current and the largest fuse or breaker allowed on it. Enter the one fitted to check it.
The table and the corrections
33 CFR 183.425 sets the allowable amperage of each conductor in Table 5, by AWG size and insulation temperature rating. Two notes correct it:
- Engine spaces, by insulation rating: 0.58 at 60 °C, 0.75 at 75 °C, 0.78 at 80 °C, 0.82 at 90 °C, 0.85 at 105 °C, 0.89 at 125 °C and 1.00 at 200 °C.
- Bundled current-carrying conductors: 0.70 for 3, 0.60 for 4 to 6, 0.50 for 7 to 24 and 0.40 for 25 or more.
The calculator applies both where both apply.
The fuse or breaker
183.455 requires a manually reset, trip-free breaker or a fuse on each ungrounded current-carrying conductor. Its rating may not exceed 150% of the conductor's allowable amperage on circuits under 50 volts. At 50 volts or more it may not exceed the allowable amperage, with the next standard size allowed up to 150% where none matches. A fuse rated above that lets the conductor overheat before it trips.
The fuse goes at the source of power. Where that's impractical, within 7 inches of it along the conductor, or within 40 inches if the conductor is sheathed or enclosed all the way. Cranking motor conductors are exempt.
Who the rule binds
Subpart I of 33 CFR 183 applies to boats with gasoline engines, other than outboards, for propulsion, power or generation. Surveyors use the same table on other boats as a benchmark, and ABYC E-11 sets its own ampacity and protection rules: see ABYC E-11.
Citing it in a report
"Windlass feed, 6 AWG 105 °C in the engine space, protected by a 250 A fuse. Allowable current 102 A, so the maximum fuse is 153 A. Recommend the fuse is replaced with one within the conductor's rating." Give the conductor, the fuse fitted and the maximum it should be.