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LS swap wire gauge, fuse and relay chart
What size wire, fuse and relay each circuit in an LS swap needs: a calculator for any load, a gauge chart by amps and length, the common swap circuits, and relay pin numbers.
- Wire: no more than 4% voltage drop (about 0.5 V on a 12 V circuit), counting the whole circuit: the feed wire and the ground wire back.
- Automotive wire runs a little smaller than its gauge number and runs warm under the hood, so we add margin. When it's close, go one size bigger.
- Fuse: at least a third above the running draw (a fuse run near its rating blows on its own), and never bigger than the wire can carry. The fuse protects the wire.
- Wire carries less in a bundle with other power wires: 3 together lose about 30%, 4–6 about 40%. Go one or two sizes up inside a loom.
Wire, fuse and relay calculator
Enter the load's running draw and the wire lengths.
Wire gauge by amps and circuit length
Total circuit length = feed wire plus ground wire. A rear fuel pump that grounds to the frame next to the tank is about the feed length plus a couple of feet.
Wire gauge (AWG) by running draw and total circuit length in feet, with the fuse for that draw.
| Draw | 5 ft | 10 ft | 15 ft | 20 ft | 25 ft | 30 ft | 40 ft |
|---|---|---|---|---|---|---|---|
| 5 A 7.5 A fuse | 16 | 16 | 16 | 14 | 14 | 12 | 12 |
| 10 A 15 A fuse | 16 | 14 | 12 | 12 | 10 | 10 | 8 |
| 15 A 20 A fuse | 16 | 12 | 10 | 10 | 8 | 8 | 6 |
| 20 A 30 A fuse | 12 | 12 | 10 | 8 | 8 | 6 | 6 |
| 25 A 40 A fuse | 10 | 10 | 8 | 8 | 6 | 6 | 4 |
| 30 A 40 A fuse | 10 | 10 | 8 | 6 | 6 | 4 | 4 |
| 40 A 60 A fuse | 8 | 8 | 6 | 6 | 4 | 4 | 2 |
Biggest fuse for each wire size
For 105 °C-rated wire (TXL or GXL automotive wire, or marine wire) in an engine compartment, run on its own. Use 16 AWG as the smallest single power wire.
| Wire (AWG) | Biggest fuse it can take under the hood |
|---|---|
| 16 | 21 A |
| 14 | 29 A |
| 12 | 38 A |
| 10 | 51 A |
| 8 | 68 A |
| 6 | 102 A |
| 4 | 136 A |
Common LS swap circuits
GM's own fuse sizes from the donor trucks, the pump and fan draws the makers publish, and the fuse and wire from our rules above. Fuel pump draw goes up with pressure, and an LS rail runs about 58 psi, so the draws are a floor.
| Circuit | Draw or factory fuse | Fuse | Relay | Wire | Must stay hot while cranking |
|---|---|---|---|---|---|
| Computer, constant battery feed | GM: 10 A (2007+ E38 trucks); 20 A shared with the fuel pump relay (1999–2002 0411 trucks) | 10–15 A | No relay needed | 16 AWG or bigger | – |
| Computer, key-on feed | GM: 10 A (0411 trucks), 15 A (E38 trucks) | 10–15 A | Ignition relay | 16 AWG or bigger | Yes |
| Injectors and coils (one fuse per bank: 1-3-5-7 and 2-4-6-8) | GM: 15 A per bank (0411 trucks), 20 A per bank (E38 trucks) | 15–20 A per bank | Ignition relay | 16 AWG or bigger | Yes |
| Electronic throttle | GM: 15 A (throttle control fuse) | 15 A | No relay needed | 16 AWG or bigger | – |
| O2 sensor heaters | GM: 15 A per pair (0411 trucks), 10 A per pair (E38 trucks) | 10–15 A | Ignition relay | 16 AWG or bigger | – |
| Fuel pump, 255 LPH | 10.4 A at 60 psi | 15 A (20 A is fine) | Standard 30/40 A mini relay | 12 AWG for a rear pump (20 ft circuit) | Relay coil: yes |
| Fuel pump, 340 LPH | 12 A at 43 psi | 20 A | Standard 30/40 A mini relay | 10 AWG for a rear pump (20 ft circuit); 12 AWG up to 17 ft | Relay coil: yes |
| Fuel pump, 450 LPH | 14.1 A at 40 psi (more at 58 psi) | 20–25 A | Standard 30/40 A mini relay | 10 AWG for a 20 ft circuit; go to 8 AWG past 23 ft | Relay coil: yes |
| Electric fan, single 12–16 in. | 8–22 A running (check the fan's label) | 30 A for a 22 A fan | Standard 30/40 A mini relay | 12 AWG | – |
| Electric fan, two-speed Taurus type | 33–40 A on high, 100+ A for a moment at start | 60 A | High-current relay rated 60 A or more (or one relay per load) | 8 AWG | – |
| Fans, GM factory | GM: 40 A fuse per fan relay (low and high), E38 trucks | – | – | – | – |
| A/C compressor clutch | GM: 10 A | 10 A | A/C relay (the computer grounds it) | 16 AWG | – |
| Transmission controller (T42 / T43) | GM: 15 A battery + 15 A key-on | 15 A each | No relay needed | 16 AWG or bigger | – |
| Starter relay trigger | GM: 10 A (crank fuse) | 10 A | Starter relay (a neutral safety switch can't carry the solenoid current) | 16 AWG | It is the crank circuit |
| Fuse block main feed | GM crate controller kits: 8 AWG minimum to the fuse block stud | – | – | 8 AWG minimum | – |
Check your pump or fan maker's instructions too: when they call for a bigger wire or a specific fuse, use theirs.
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Relay pin numbers: 30, 85, 86, 87, 87a
The standard 4-pin automotive relay has terminals 30, 85, 86 and 87. The 5-pin (changeover) version adds 87a.
| Terminal | What it is | Goes to |
|---|---|---|
| 30 | Common contact | Fused battery positive (the fuse from the chart above) |
| 87 | Normally open: connects to 30 when the coil is powered | The load: fuel pump, fan, A/C clutch. The load's other side goes to ground. |
| 86 | Coil positive | Key-on 12 V (fused), or the switch or computer output that turns the load on |
| 85 | Coil negative | Ground, or the computer's relay-control pin (GM computers switch the ground side) |
| 87a | Normally closed (5-pin relays only): connected to 30 when the coil is off | Usually left empty. Use it for something that should run until the relay clicks. |
- 85 vs 86 matters when the relay has a diode across its coil: 86 is positive, 85 negative. Wire it backwards and the diode shorts the coil feed.
- Heat costs current. A typical mini relay carries about 50 A at room temperature, 35 A at 85 °C and only 15 A at 125 °C. Mount relays away from the headers.
- 87a is the weaker side, about 20 A on a typical mini relay against 35 A on 87.
- Pin by pin for the fuel pump and fan relays on an 0411, P59 or E38: fuel pump and fan relay wiring.
Parts: 12 V automotive relays · Relay with socket and pigtail · Fuse and relay block · 10 AWG TXL wire · Weather Pack connector kit · Fuel pump hotwire kit
Let us set the fans and pump up in the tune. A Start-up File ($79) sets the fan temperatures, injector data, idle and conservative fuel and spark, built on a read of your computer.