Where every number comes from

1. Ampacity: how much current a wire may carry

From the U.S. Coast Guard's boat electrical rules, 33 CFR 183.425, Table 5 "Allowable Amperage of Conductors", by gauge (18 AWG to 4/0) and insulation temperature rating (60 to 200 °C). In engine spaces the table's note 1 factor applies (0.58 at 60 °C up to 1.00 at 200 °C). When three or more current-carrying conductors run together in one sheath, harness, bundle or conduit, the table's note 2 factor applies as well: 0.70 for 3, 0.60 for 4 to 6, 0.50 for 7 to 24 and 0.40 for 25 or more. Tell the wire check how many run together and it multiplies both factors in; the fuse ceiling below is worked from that corrected amperage.

The same section sets minimum sizes: 16 AWG for a conductor run on its own, 18 AWG only inside a multiconductor sheath (183.425(d)–(e)).

Who those rules are written for

33 CFR 183 Subpart I applies to boats with gasoline engines other than outboards (183.401), and it excludes some circuits such as electronic navigation equipment and circuits under one amp (183.425(g)). Circuit Locker uses its tables as a sound, public yardstick for any small boat's DC wiring; it is not telling you which rules bind your boat.

2. Fuse or breaker ceiling

33 CFR 183.455: every ungrounded current-carrying conductor is protected by a fuse or a manually reset, trip-free breaker at the source of power. Only when that is physically impractical may it sit within 7 inches of the source, measured along the conductor; and only when that too is impractical may it sit within 40 inches, if the conductor is inside a sheath or enclosure (such as a junction box, control box or enclosed panel) for its whole length from the source to the fuse (183.455(b)). Under 50 volts the rating may be at most 150% of the wire's Table 5 amperage (183.455(c)(1)). Circuit Locker shows that ceiling and the common ratings between the load's current and the ceiling. The ceiling protects the wire; a device maker's own fuse size, when lower, protects the device.

3. Voltage drop

The circular-mil formula used in marine wiring guides (for example Ancor's voltage-drop tables): CM = 10.75 × I × L ÷ E, where 10.75 is copper's resistance in ohm-circular-mils per foot, I the current in amps, L the length of the whole circuit in feet (the positive wire out and the negative back, so twice the one-way run), and E the volts you can afford to lose. The smallest AWG size whose area (NEC Chapter 9, Table 8 values) is at least CM meets the limit.

Limits: 3% for circuits where a drop matters (panel feeders, bilge blowers, electronics, navigation lights) and 10% for others such as cabin lighting, as marine wiring guides summarise ABYC practice. ABYC standards are voluntary and are not reproduced here.

4. The answer

The wire is the thickest of three sizes: the one the drop limit needs, the one the ampacity table needs, and the federal minimum. The page shows which one decided it.

5. Energy budget assumptions (yours to change)

• Usable share of a bank: 50% for lead-acid and AGM, 80% for LiFePO4, as defaults. These are common planning figures, not facts about your battery: use its data sheet.
• Solar: watt-hours a day ÷ (sun hours × 75%). The 75% stands for wiring, controller and panel losses; your sun hours depend on where and when you sail.
• Charging hours: amp-hours a day ÷ charging amps. Lead-acid batteries take current more slowly as they fill, so real charging takes longer.

6. What Circuit Locker does not do

• No AC shore power, inverter output or generator wiring.
• No inspection or survey: it plans, it does not check what is installed.
• No product recommendations, prices or sales. It is not a dealer or a service shop.

Found a mistake in a number? The math is open on this page; a correction to WeldonPC.com is welcome.