Roof Light Power. Without the High-Load Panel.
To wire a 25A roof light bar to your INEOS Grenadier without a High-Load Panel: use EXT1 as relay trigger only (not a power feed), run a dedicated 30A-fused lead from the battery, and use 8 AWG wire for the 8-ft roof run. This guide covers the complete relay architecture, wire gauge math, and sealed cable routing for Grenadier body geometry.
300 Watts Through a 10-Amp Switch
Quick Answer: How to Wire Grenadier Roof Lights (EXT1 → 25A Bar)
- EXT1 (10A) → relay coil pin 86 — trigger signal only, never the power feed
- Battery positive → 30A inline fuse → relay pin 30 — dedicated high-current feed
- Relay pin 87 → 8 AWG wire → roof light bar (8-ft run keeps voltage drop under 2.1%)
- Dedicated ground → chassis bolt near roof entry, not relay body
The Grenadier’s EXT1 auxiliary circuit delivers 10A — 120 watts. A serious roof light bar draws 25A — 300 watts. The factory circuit is a control signal, not a power source. Community threads keep converging on the same question: how do you bridge that gap safely?
EXT1 is one of three circuits that come standard on every Grenadier (alongside INT1 and INT2). It lives under the hood as an unterminated pigtail — bare wire leads ready for whatever connector you choose — and is controlled by a dedicated switch on the overhead console. The optional EXT5 provides 25A from the same under-hood location, but many owners only have the standard three circuits.
The pattern on owner forums is consistent: a ~25A LED light bar, a 10A EXT1 output, uncertainty about routing to the roof, and concern about long-term reliability in dust, rain, and vibration. Most installs that develop problems six months later trace back to a decision made in the first hour — running high current through EXT1 directly, undersizing wire for the route, or treating cable routing as cosmetic instead of structural.
Owner Install Report — The INEOS Forum
"I used the ext 1 switch (10 amps) to trigger a 50amp Fastronix relay under the hood. I have the Ineos lightbar that pulls 25amps so used a 50amp relay. The ext 1 wires are under the hood on the left hand side." — How to Add Roof Power thread, The INEOS Forum
This guide gives a complete electrical architecture that solves the current mismatch cleanly, using EXT1 as relay trigger logic and a dedicated fused feed for the actual power.
Grenadier Auxiliary Circuit Map
Standard (all vehicles): EXT1 Bonnet 10A · INT1 L Footwell 10A · INT2 R Footwell 10A
Optional: EXT2 RF Roof 25A · EXT3 Roof 25A · EXT4 500A (Factory Winch) · EXT5 Bonnet 25A
If your Grenadier has EXT5 (25A), it can drive up to 300W directly without a relay. This guide is for owners who have only the standard EXT1 and need to run loads above 120W.
Critical Distinction
A roof light install is not “how do I make it turn on.” It is successful only if it satisfies all five integrity requirements simultaneously: current, voltage, thermal, mechanical, and diagnostic. Miss any one and the install degrades over time.
Current Integrity
High-load current does not pass through low-current switch circuits. The factory aux output serves as control logic only.
Voltage Integrity
The light sees stable voltage under load — no major brightness drop caused by voltage sag across undersized conductors.
Thermal Integrity
Wire gauge and fuse selection remain safe at continuous current. No hot spots at terminations, no fuse holder heat soak.
Mechanical Integrity
Routing survives vibration, articulation, water ingress, and future service access. Every transition zone is sealed and strain-relieved.
Diagnostic Integrity
Future troubleshooting can isolate faults quickly. Control, power, and ground paths are independently accessible.
Low-Current Trigger, High-Current Feed
The correct architecture separates control from power. This is the single most important design decision in any Grenadier roof light install.
Common Mistake
Running full load current through EXT1. The 10A circuit carries 25A it was never designed for. The fuse blows — or worse, the harness degrades silently over weeks.
- EXT1 fuse blows at 10A
- Voltage sag under load
- No thermal headroom
- Factory harness degradation
Correct Design
EXT1 triggers a relay coil (<300mA). Relay switches a dedicated fused battery feed to the light. EXT1 never exceeds 3% of its rated capacity.
- EXT1 carries <300mA coil load
- Dedicated power path to light
- Fuse protects wire, not switch
- Independent fault isolation
The relay acts as a power amplifier — translating EXT1’s low-current control signal into a high-current switching event. EXT1’s unterminated pigtail under the hood makes this straightforward: crimp a Deutsch DTP connector to the pigtail leads and wire directly to the relay coil. The factory circuit never sees more than the relay coil draw (typically 150–300 mA — under 3% of EXT1’s 10A rating). The high-current path is independently fused and routed on wire sized for the actual load and distance.
Design Principle
Control and power are separate systems. The factory aux is a signal source, not a power source. Every component in the high-current path — fuse, wire, relay contacts, terminals, ground return — is sized for full continuous load plus thermal headroom.
Current Budget and Protection Sizing
Design target: 300W roof light bar at 12V nominal = 25A steady-state. EXT1 provides the control signal; a dedicated battery feed provides the power.
| Parameter | Value | Notes |
|---|---|---|
| Steady-state load | 25A | Measured at full output, not rated max |
| Relay continuous rating | ≥40A (50A preferred) | 160–200% of load for thermal headroom |
| Main branch fuse | 30A | 125% of steady load; validate against wire ampacity |
| Wire ampacity (min) | >30A continuous | Must exceed fuse rating — fuse protects wire first |
| Relay coil draw | 150–300 mA | Under 3% of EXT1’s 10A rating |
| EXT1 trigger circuit | 10A / 120W max | Control only — pigtail under hood, overhead console switch |
Fuse selection protects wire first, device second. A 30A fuse is the practical starting point for a 25A branch — it sits at 120% of steady load. A 25A fuse can work if startup inrush does not nuisance-blow it, but leaves no margin. Standard practice per NEC continuous-load guidance is 125% of expected current, then validated against the wire gauge ampacity rating so the fuse always opens before the wire reaches its thermal limit.
If your specific light bar has published inrush data showing a brief spike above 30A at startup (common with capacitor-input LED drivers), size the fuse with a slow-blow characteristic or step up to 35A — but never exceed the wire’s rated ampacity.
Wire Gauge Is a Voltage-Drop Problem Before It Is an Ampacity Problem
Most roof-light installs fail their performance target due to voltage drop, not because the light never turns on. The light works — it’s just dimmer than it should be, and the owner may never know why.
For 12V systems, keep branch voltage drop at or below 3% for optimal light output. Drops between 3–5% are electrically safe but reduce LED driver efficiency and visible output. Above 5% is generally unacceptable.
For a Grenadier roof light bar with the relay mounted under the hood near the battery, the one-way cable distance from relay output to the roof light is conservatively 8 feet (16 ft round trip including the ground return).
| Wire Gauge | Resistance (Ω/1000 ft) | Voltage Drop | % Drop (12V) | % Drop (13.8V) | Verdict |
|---|---|---|---|---|---|
| 14 AWG | 2.525 | 1.01V | 8.4% | 7.3% | Reject |
| 12 AWG | 1.588 | 0.64V | 5.3% | 4.6% | Marginal |
| 10 AWG | 0.999 | 0.40V | 3.3% | 2.9% | Acceptable |
| 8 AWG | 0.628 | 0.25V | 2.1% | 1.8% | Excellent |
Recommendation for 8 ft Run
10 AWG is the minimum recommended gauge for 25A at 8 ft one-way — it keeps voltage drop at 3.3%, just inside the acceptable threshold. 8 AWG is the best choice at only 2.1% drop, delivering maximum light output with comfortable margin. 12 AWG is marginal at 5.3% and should be avoided for a 300W load at this distance.
Both columns are shown because the Grenadier’s charging system delivers approximately 13.8V when the engine is running. The percentage drop improves slightly at higher source voltage, but the absolute voltage lost is identical — which is why designing to the 12V baseline is the conservative and correct approach.
Resistance values are from NEC Table 8 (ASTM B3, uncoated copper at 20°C). At elevated temperatures inside the engine bay — where the relay and initial cable run live — actual resistance increases. Add 10–15% margin when routing through hot zones.
The ground return path carries the same current as the positive feed. Size the ground wire identically to the power wire. Undersized ground returns are the single most common source of “mystery dimming” in roof light installs.
Relay Topology and Grounding Strategy
Relay (Control Side)
The relay coil is triggered by the EXT1 output pigtail under the hood. Connect via a Deutsch DTP connector pair for a weatherproof, serviceable junction. Coil ground connects to a known clean chassis ground point — not the EXT1 pigtail ground, which should remain isolated. If the relay does not have internal transient suppression (check the datasheet), add a flyback diode or TVS across the coil terminals. Without suppression, the inductive kick when the relay de-energizes generates voltage spikes that can feed back into the EXT1 circuit and disturb other electronics.
Relay (Power Side)
The high-current path runs: Battery positive → Fuse (as close to source as practical) → Relay contact (normally-open) → Roof light positive. The fuse must be within 18 inches of the battery or bus bar connection — this protects the entire wire run from a short circuit, not just the load end.
Ground Return
The roof light negative returns through a matched-gauge ground path to a robust chassis or bus bar ground. This is where many installs fail silently.
Ground Path Failures
Avoid undersized ground wire, paint-contaminated ground bolts, and star-grounding multiple high-current loads on a single small bolt. Many “mystery dimming” complaints are return-path problems — the voltage drop equation applies to the ground wire identically. Sand to bare metal, use a star washer, apply dielectric compound after torquing, and verify with a millivolt-drop test under load.
Routing Strategy for Grenadier Body Geometry
Owner reports consistently identify workable entry and routing paths through kick-panel grommet access, A/B/C pillar cavities, and roof-side trim interfaces. Regardless of the specific path chosen, enforce these six rules at every transition point:
No Unsupported Spans
Every run near a vibration zone must be clamped at intervals. Free-hanging wire fatigues at flex points and eventually fractures conductors inside intact insulation.
No Bare Pass-Throughs
Every panel crossing uses a grommet or bulkhead fitting. Never route wire through a drilled hole with only electrical tape or silicone as protection.
No Edge Contact
Abrasion sleeve (split loom or woven sheath) wherever the loom approaches metal or plastic edges. A single chafe point becomes a dead short after enough vibration cycles.
Drip Strategy
Include drip loops before every cabin entry. Route cable so water flows away from entry points, not toward them. A single bead of water following a wire into the cabin becomes corrosion in weeks.
Service Loops
Leave controlled slack where trim removal may be needed later. A taut wire behind a pillar panel means the next person to service that area either cuts the wire or damages the trim.
Strain Relief
At every transition zone — especially the roof light connection point — the cable must be mechanically decoupled from the fixture. P-clamps, not zip ties, at load-bearing anchor points.
The Part That Determines the 12-Month Outcome
A perfectly sized wire can still fail early if routing and sealing are weak. Electrical reliability and mechanical restraint are one system.
The initial install can test fine. Voltage drop passes. Current draw matches spec. The real test is the first 12 months of dust infiltration, humidity cycling, thermal expansion, and vibration fatigue. Most installs that fail at the 6–12 month mark trace back to a sealing or retention shortcut made during the original build.
Adhesive-Lined Heat Shrink
Use on every splice and terminal exposed to humidity cycles. The adhesive liner creates a moisture barrier that standard heat shrink does not provide. Shrink with a heat gun, not a lighter — uneven heat causes incomplete seal.
Dielectric Protection
Apply dielectric grease or compound to connector interfaces exposed to the environment. This displaces moisture from contact surfaces and prevents galvanic corrosion between dissimilar metals at terminal junctions.
Vibration Decoupling
P-clamp the cable at intervals between the light bar and the first fixed body point. The light bar vibrates at a different frequency than the body — without decoupling, the cable becomes a fatigue stress concentrator at the fixture terminal.
Post-Install Inspection
After your first off-road cycle, inspect every cable run for loom polish marks. Shiny spots on the outer sheath are the earliest indicator of a chafe trajectory — the precursor to insulation failure months later.
Commissioning Checklist
Every item must pass before final trim reassembly. Completing trim closure with unverified electrical work creates a time bomb — when the problem surfaces, you’re pulling panels to diagnose what should have been caught with a multimeter.
Static Current Test
Measure steady-state current draw at full light output with a clamp meter on the positive feed. Compare against manufacturer spec. A significant deviation indicates a wiring fault or defective driver.
Voltage at Load
Measure voltage directly at the roof light terminals with the light running at full output. This is your real-world delivered voltage — not the battery voltage, not the bus bar voltage.
Drop Verification
Compare source voltage (at fuse holder output) vs. load voltage (at light terminals). Log the percentage drop. If it exceeds your design target (≤3% preferred, ≤5% acceptable), the wire gauge or a termination is the issue.
Thermal Soak Test
Run the light for 15–20 minutes continuously. Check relay body, fuse holder, and every inline junction by touch or IR thermometer. Anything uncomfortable to hold indicates a resistance problem that will worsen over time.
Ingress Test
Water test around every cable entry point at the roof transition. Inspect the cabin side for any moisture migration. A garden hose at moderate pressure for 2 minutes per entry is the minimum bar.
Vibration Check
Short rough-road loop (15–20 minutes on mixed terrain). Recheck every clamp, every terminal, and every witness mark. Verify no new polish spots on loom sheathing. Re-torque any fasteners that show rotation.
Failure Signatures and Fastest Diagnosis Path
Diagnose by measurement, not by replacing parts in sequence. Each failure mode has a characteristic signature that points directly to the fault location.
Symptom
Light flickers over bumps
First Check
Connector retention at the light bar. Then relay control-side ground. Flex each connection point while monitoring — the flicker will localize the fault.
Symptom
Light is on but noticeably dim
First Check
Voltage at the light terminals under load. If significantly below source voltage, measure at intermediate points to localize the drop. Suspect ground return if positive-side drop is small.
Symptom
Fuse opens after minutes, not instantly
First Check
Thermal rise from high-resistance termination. A loose crimp or corroded junction draws correct current but generates enough heat to eventually trip the fuse via conducted thermal load.
Symptom
Intermittent total cutoff after rain
First Check
Ingress at the roof transition or an inadequately sealed splice. Water bridges across corroded terminals and creates a temporary short that clears as it dries — then returns next wet cycle.
Diagnostic Discipline
Every fault in a 12V system is either a voltage problem or a current problem. Carry a multimeter to the vehicle, not a bag of spare parts. Measure voltage under load at the source, at the relay, and at the light. The numbers will tell you where the fault is faster than any amount of visual inspection or parts swapping.
Practical Design Template
Copy this template, fill in your values, and keep the completed record with your build documentation. Every number should come from measurement or manufacturer data — not estimation.
Grenadier Roof Light — Electrical Design Record
Bottom Line
If your Grenadier has only the standard EXT1 circuit, you can still build a 300W roof light system that performs like a factory install — or better.
Use EXT1 as control logic only — its 10A, 120W capacity is perfect for triggering a relay coil at under 300 mA. Run a dedicated, independently fused power feed from the battery, sized by voltage-drop math: 10 AWG minimum, 8 AWG recommended for an 8 ft run at 25A. Route and seal the cable as first-class engineering work, not an afterthought. Commission with measurements, not assumptions.
That combination — EXT1 as trigger, relay-switched power, correct wire sizing, sealed routing, and verified commissioning — is what keeps the system bright, stable, and serviceable long after installation day.
DVA Mechanics — Built by Grenadier owners, for Grenadier owners. Every DVA roof accessory is designed with the same philosophy this guide describes: electrical reliability and mechanical restraint are one system. If you’re building a roof light setup on your Grenadier, the engineering in these pages is the same engineering behind every product we ship.