Wiring Diagrams & Planner
Relay circuits drawn the way race looms are actually built — pick the job, set the load, and get fuse ratings plus the right wire gauge for every leg. Or open the sandbox and build your own circuit on a grid: lamps glow, relays click, fuses pop, live.
Industrial PC builder
Multi-sheet schematics, connectors, live simulation and a printable harness schedule — built for a computer screen (it needs more than 801px). The sandbox below stays for phones.
Battery → fused feed → relay → pump, cabin switch controls the coil. The classic.
Fuse @ source
15 A
1.5 × load, rounded up
Power wire
14 AWG
2.08 mm² · ≤0.3 V drop
Ground wire
18 AWG
same current, 1 m
Coil / trigger wires
18 AWG
~0.25 A, any small wire
| Wire leg | Current | Length | Pick | Min. copper |
|---|---|---|---|---|
| Battery → fuse | 10 A | 2.5 m | 14 AWG | 1.46 mm² → 2.08 mm² |
| Fuse → relay 30 | 10 A | 2.5 m | 14 AWG | 1.46 mm² → 2.08 mm² |
| Relay 87 → pump | 10 A | 2.5 m | 14 AWG | 1.46 mm² → 2.08 mm² |
| Pump → ground | 10 A | 1 m | 18 AWG | 0.58 mm² → 0.82 mm² |
Parts list
- 15 A blade fuse + holder at the battery feed
- 30/40 A automotive relay (5-pin with built-in suppression is worth it)
- Blade fuse holder at the battery feed — sized below
- 5 A inline fuse on the coil feed (protects the switch loom)
- Sealed cabin switch (20 A rated switch gives you mechanical margin)
- Add an inertia cut-off or oil-pressure switch in series — cheap engine insurance
Pumps hate resistance: ground the pump to a clean chassis point near the tank, not the boot floor through paint. Prime the system and check the fuse holder doesn’t get warm in the first minutes — warm fuse holders are a fire waiting to happen.
Sizing uses 0.3 V total drop and conservative continuous ampacities — for long runs, high ambient heat or alcohol fuels’ bigger pumps, step one size up. Fuses protect the WIRE, not the device: after wiring, deliberately short each leg (with the load disconnected) and confirm the right fuse pops before you trust it on the car.
Tap a port, then tap another port to wire them (or drag port → port — a dashed line follows your finger). Tap a wire to set its colour, gauge and note — the schedule below reads like a real loom list. Tap a switch to flip it · hold a push button to fire it · tap a part to edit fuses, lamps and motors. Everything simulates live on 100 % of your phone — nothing is uploaded, ever.
Good to know
- Every positive feed gets its own fuse AT its source — a fuse at the load protects nothing if the wire upstream shorts.
- Wire gauge here is copper sized for ≤0.3 V drop plus a conservative continuous ampacity — step up one size for long runs, hot engine bays and E85/methanol pumps.
- Fuses protect the wire, not the device: after wiring, test-short each leg with the load disconnected and confirm the right fuse pops.
- Relays only carry the load when their coil fires — never switch a pump or fan directly from a cabin switch unless you like buying switches.
- The sandbox simulates the real physics client-side (relay pull-in at 7 V, diode drops, fuse ratings) — so you can blow a fuse on your phone instead of in your loom.