Wire sizing

Four checks

Choose wire that passes: ampacity after temperature/bundling derating; allowed voltage drop at load current; terminal/lug compatibility; and mechanical protection. Use the largest size demanded by any check.

Round trip

For a two-wire DC circuit, length is positive out plus negative back. A load 10 ft from the fuse block has about 20 ft of conductor. Chassis return does not make resistance disappear.

Voltage drop formula

For copper, use a recognized calculator/table or ΔV = I x R_total. Percent drop = ΔV / system voltage x 100. Measure the finished circuit under real load because connectors and fuses add resistance.

AWG and cable type

Smaller AWG number means larger conductor; 2/0 is larger than 1/0. Fine-stranded, flexible cable with appropriate insulation is suited to mobile vibration. Marine-grade tinned copper helps in damp/corrosive environments; use listed wire required by the governing rules.

Bigger is not always better

Oversize cable can exceed terminal capacity, bend radius and lug tooling; bad adaptation creates more resistance. 2/0 and 4/0 appear when 12 V inverter currents reach hundreds of amps or runs are long - never from inverter watts alone.

Example

Current

Round trip

Target

Design note

Fridge branch

6 A

30 ft

3%

Calculate; often 12-10 AWG depending on rules/temperature

50 A DC-DC

50 A

40 ft

~3%

Large feeder; size from an approved calculator/table

3 kW inverter, 12 V

~278 A

6 ft

very low

Often 4/0 territory, but exact manual controls

CHAPTER CHECK

• I can explain the purpose of every component covered here. • I can identify the exact manufacturer limit that controls my design. • I have added the decision, rating or measurement to my schematic/worksheet.