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1.2.1 Circuit Breakers

Overview

All START battery positive circuits protected by Mechanical Products Series 17 circuit breakers with direct battery connections.

Location: Driver rear wheel well, within 7" of START battery positive terminal (ABYC/NEC code compliant)

Circuit Breaker Specifications

Circuit Model Rating Reset Type Power Path Max Load Sizing
PMU24 Mechanical Products
(174-S2-250-2)		 250A Manual START battery+
└→ 250A CB
   └→ PMU24		 140-170A typical (radiator fan, iBooster + TCU no longer on PMU) Protects 2/0 AWG (265A @ 60°C); now generously oversized after relocation
Fwd Dist Bus (master) Mechanical Products
(174-S2-150-2, ⚠️ TBD #135)		 150A Manual START battery+
└→ 150A CB
   └→ START+ Forward Distribution Bus		 ~108A peak (~68A continuous) Protects 2 AWG forward feed; selective with downstream load CBs
BCDC Input Mechanical Products
(174-S2-080-2)		 80A Manual START battery+
└→ 80A CB
   └→ BCDC Alpha 50		 50-55A BCDC input 145-160% of max load

Total battery-side Circuit Breakers: 3 (PMU 250A, Forward Dist Bus master 150A, BCDC 80A)

PMU Circuit Breaker Sizing

250A CB sized to protect 2/0 AWG wire (265A @ 60°C). With the radiator fan, iBooster, and TCU relocated to the Forward Distribution Bus, typical PMU load drops to ~85-115A (well under half the CB rating). The CB is left at 250A (no benefit to downsizing).

Forward Distribution Bus Breakers (engine bay)

The three loads relocated off the PMU each get a breaker at the START+ Forward Distribution Bus (engine bay), fed from the 150A master above. SKUs follow the Mechanical Products Series 17 S2 pattern — confirm via ⚠️ TBD #135.

Circuit Model Rating Power Path Max Load Sizing
Radiator Fan Mechanical Products (174-S2-060-2) 60A Fwd Bus → 60A CB → fan 53A continuous 113% of max load (protects 4 AWG)
iBooster main Mechanical Products (174-S2-050-2) 50A Fwd Bus → 50A CB → iBooster 40A brief peak / 0.25A idle 125% of brief peak (protects 8 AWG)
Turbolamik TCU Mechanical Products (174-S2-025-2) 25A Fwd Bus → 25A CB → TCU 15A continuous 167% of max load (protects 12 AWG)

Total Forward Bus Circuit Breakers: 3 (fan 60A, iBooster 50A, TCU 25A)

All Circuit Breakers:

  • Manufacturer: Mechanical Products Series 17
  • Type: Surface mount
  • Reset: Manual (Type III - push to reset)
  • Terminals: 3/8"-16 studs (S2 configuration)
  • Dimensions: 3.39" L × 1.9" W (per unit)
  • Ratings: 48V DC (20-150A), 30V DC (175-200A), 14V DC (225-300A)
  • Standards: Marine-rated (SAE J1171, ABYC E-11, UL1500, IP67, MIL-STD-202)
  • Mounting: Driver rear wheel well within 7" of battery positive terminal

Space Requirements: 3 battery-side CBs (PMU 250A, Forward Bus master 150A, BCDC 80A) side-by-side with wiring clearance: ~10" × 4" (~40 sq in). The 3 forward-bus load CBs (fan/iBooster/TCU) mount in the engine bay on a bracket beside the Forward Distribution Bus, not in the wheel well.