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What Size Alternator Does Your Work Truck Need?
Short answer: add up everything that runs continuously while the truck is working, multiply by 1.5 for headroom, and buy an alternator that produces that number at idle — not at 2,000 RPM. For most plow trucks that lands between 220 and 320 amps. The peak draw of the plow motor itself, which can hit 200 amps, is not what you size for. Your battery covers that. What kills work-truck charging systems is the boring sustained load — lights, blower, heated glass, spreader, beacons — running for hours at low RPM in a parking lot.
That distinction is the whole article, and it's the one nearly every "alternator sizing" guide gets wrong.
Why sizing for peak draw is the wrong approach
A snow plow's hydraulic pump is one of the largest electrical loads a light truck will ever see. Operators measuring real systems on Plowsite report roughly 157 to 165 amps in normal operation on a Fisher/Western setup, with about 200 amps at maximum — the pump held against its pressure relief with the blade fully raised and the control still activated. A Blizzard unit in the same thread draws 55 to 65 amps to angle and 150 to 165 amps to lift.
Those are terrifying numbers until you notice how long they last: about a second and a half. You bump the blade up, the motor spikes, and it's over.
A battery handles that. An alternator can't and doesn't need to. Lead-acid batteries are very good at dumping several hundred amps for a moment and being recharged afterward. That's what a starting battery is designed to do. Sizing your alternator to cover a 200-amp momentary spike is like sizing your home's electrical service around the inrush current of the air conditioner compressor.
The load that actually matters is the one that never stops.
What actually runs continuously on a working truck
Here is where the amps really go. Every figure below is sourced from a manufacturer spec sheet or a measured report, not estimated.
| Load | Draw | Duty | Source |
|---|---|---|---|
| Plow hydraulic pump (Fisher/Western) | 157–200 A | Momentary, ~1–2 sec | Plowsite measured |
| Plow pump (Blizzard, lift) | 150–165 A | Momentary | Plowsite measured |
| Plow pump (Blizzard, angle) | 55–65 A | Momentary | Plowsite measured |
| Liftgate pump motor | up to 205 A | Momentary, ~10–20 sec | Tommy Gate Railgate wiring instructions |
| Electric tailgate spreader | ~70 A | Continuous while spreading | Plowsite reported |
| LED work light | 2.9–16.5 A each | Continuous | Peterson Manufacturing specs |
| LED beacon / strobe | 0.33–0.79 A each | Continuous | Peterson Manufacturing specs |
| LED marker lights | 0.01–0.03 A each | Continuous | Peterson Manufacturing specs |
| Headlights, blower on high, wipers, heated seats and mirrors, rear defrost | 60–90 A combined | Continuous | Varies by truck — measure yours |
| ECM, fuel system, glow plugs / grid heater (diesel) | 20–40 A, grid heater far higher in cycles | Continuous + cyclic | Varies by platform |
Add the continuous column for a typical plow truck on a January night: spreader running at 70, exterior and cab electrical around 80, beacons and work lights another 20, engine management around 30. That's 200 amps of sustained draw before the plow blade moves at all.
Now the important part.
Why your alternator's rated amps are not the amps you get
A published alternator rating is measured at roughly 2,000 RPM, not at idle. This is the single most misunderstood fact in work-truck electrical systems, and an experienced contributor on Plowsite put it more plainly than most manufacturers do:
"This is the rating of amps it puts out at 2000 rpms. What you need is an alternator that puts out high amps at LOW rpms."
Plowing does not happen at 2,000 RPM. It happens at idle and just above it, in a parking lot, for six hours. The same truck whose 160-amp alternator is perfectly adequate on the highway is running a significant deficit in the lot — pulling the difference out of the batteries all night and never fully replacing it.
That deficit is why plow operators burn through alternators and batteries on a schedule. It isn't a defect. It's an undersized charging system operating outside the RPM band it was rated in.
So the real sizing question is not "how many amps is this alternator?" It's "how many amps does it make at 700 RPM?" — and almost nobody in the industry publishes that number.
Idle output data: Most of the industry publishes one peak number. We publish measured output at idle where we have it, and we say so where we don't. See the idle output data →
How to size your alternator, step by step
1. Total your continuous loads. Everything that runs the whole time the truck is working. Use the table above, or better, put a clamp meter on the charge wire and measure your own truck under working conditions.
2. Ignore the momentary loads. Plow pump, liftgate, starter — the battery covers these. Do make sure your batteries are up to it; more on that below.
3. Add 50% headroom. An alternator run continuously at 100% of its capability gets hot, and heat is what kills alternators — it's what cooks stator windings and rectifier diodes. Headroom is thermal margin, not wasted capacity.
4. Buy for output at idle, not rated output. This is the step that separates a charging system that works from one that limps.
Three worked examples using the sourced figures above:
| Truck profile | Continuous load | +50% headroom | Recommended |
|---|---|---|---|
| Plow only, basic lighting | ~130 A | ~195 A | 220–250 A |
| Plow + spreader + beacons + work lights | ~200 A | ~300 A | 320 A, or dual alternators |
| Service truck: liftgate, 2,000 W inverter, compressor, lighting | ~270 A | ~405 A | Dual alternators |
A 2,000-watt inverter is the accessory that quietly wrecks the math. Renogy publishes a maximum continuous battery input current of 210 amps for its 2,000 W 12 V unit, and roughly 310 amps for the 3,000 W — considerably more than the naive watts-divided-by-volts estimate, because the rating is set at the low-voltage cutout rather than at charging voltage. That single accessory can exceed everything else on the truck combined, which is why inverter-equipped service trucks almost always end up on a dual-alternator setup. Full inverter sizing guide →
Do you need dual alternators?
Reach for a second alternator when any of these is true:
- Your continuous load plus headroom exceeds roughly 320 amps
- You run an inverter above 2,000 watts
- You need redundancy because the truck can't be down
Below that threshold, a single high-output unit with strong idle output is simpler, cheaper, and one less belt path to maintain. Don't buy complexity you don't need.
If your truck already has two alternators from the factory — many 6.0L Power Stroke trucks shipped that way — you don't need a bracket at all. You need the right alternator for the mount you're replacing, and the mounts are not interchangeable. That's a different decision, and we cover it separately.
What else has to be upgraded
An alternator is the source. If you don't upgrade what carries and stores the current, you've moved the bottleneck rather than removed it.
Charge wire. Factory charge wiring is sized for the factory alternator. Doubling output through undersized cable produces voltage drop, heat, and a real fire risk. A 250-amp-plus alternator generally wants 1/0 gauge on both the charge wire and the ground. Big 3 wiring upgrade guide →
Batteries. The battery is what absorbs every plow-motor spike. Operators consistently report needing 800+ CCA per battery in plow service, and a weak battery makes an oversized alternator look defective — voltage sags on every blade lift regardless of what's charging it.
Grounds. Engine-to-frame and frame-to-body straps corrode and are the most commonly overlooked cause of charging complaints on work trucks.
Belt. A high-output alternator loads the accessory drive harder than stock. Belt slip under load presents exactly like a failing alternator. What belt do you need? →
Frequently asked questions
How many amps does a snow plow draw?
A Fisher or Western hydraulic plow pump draws roughly 157 to 165 amps in normal operation, with peaks near 200 amps when the pump is held against its pressure relief. A Blizzard unit draws 55 to 65 amps to angle and 150 to 165 amps to lift. These are momentary draws lasting one to two seconds, not continuous loads — your battery supplies them, not your alternator.
What size alternator do I need for a plow truck?
Most plow trucks are properly served by 220 to 320 amps, depending on what else is running. Size for your continuous load — lighting, blower, spreader, beacons, engine management — plus 50% headroom, and choose based on output at idle rather than rated output. A truck running a spreader plus full lighting typically needs 320 amps.
Will a bigger alternator fix my voltage drop when plowing?
Usually, but not always. If voltage sags briefly each time the blade lifts and recovers immediately, the issue is battery capacity or cable size, not alternator output. If voltage sits low continuously while you're working in the lot, that's an undersized alternator or poor idle output, and a high-output unit will fix it.
Do I need a high-output alternator for a liftgate?
Often yes. A Tommy Gate Railgate pump motor can require up to 205 amps at 12 volts. That draw is momentary, but liftgate trucks typically cycle repeatedly during a delivery route with the engine at idle, giving the charging system little chance to recover between cycles. Batteries drift down over a shift and never fully recharge.
What size alternator do I need to run a 2,000-watt inverter?
More than the simple arithmetic suggests. Renogy publishes a maximum continuous battery input current of 210 amps for its 2,000-watt 12-volt inverter, because the figure is set at the low-voltage cutout rather than at charging voltage. That is the inverter alone, before every other load on the truck. Most service trucks running a 2,000-watt inverter or larger end up on a dual-alternator setup.
Why does my alternator keep failing on my plow truck?
Almost always sustained overload at low RPM. Alternator ratings are measured near 2,000 RPM, while plowing happens at idle where output is far lower. Running near maximum capability for hours generates heat, and heat is what destroys stator windings and rectifier diodes. The fix is more capacity at idle, not another replacement of the same unit.
Can I just add a second battery instead of a bigger alternator?
A second battery helps with momentary loads like plow lifts and liftgate cycles, and it's often the correct first upgrade. It does not help with sustained deficit — if your continuous draw exceeds alternator output at working RPM, more batteries only means more capacity to slowly drain. Batteries buffer; alternators supply.
Does a high-output alternator hurt fuel economy or engine power?
Only in proportion to what you actually draw. An alternator consumes engine power based on current produced, not on its rating. A 320-amp alternator producing 80 amps costs the same as a 160-amp alternator producing 80 amps. Larger capacity does not mean larger parasitic loss. ---
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