The concrete trucks were already lined up when the pump died. 2 a.m., a foundation pour, and our rental set started hunting — revving, sagging, revving again — while the pump operator stared at me like I’d personally betrayed him. I’d signed off on that set myself. On paper, it had plenty of headroom.
That night taught me the lesson this whole article is about: a construction site is not a load list. It’s a living thing, and it lies to you about how much power it needs.
Contractors ask me three questions more than any others. Not the same questions homeowners ask, not what data center people ask. Site questions. Here they are, with the answers I actually give.
No. And this is the most expensive misunderstanding in the site-power business.
A motor’s nameplate tells you what it draws while running. What it doesn’t tell you — at least not in a way most buyers read — is what it draws in the three seconds after you push the start button. A three-phase motor pulls roughly five to seven times its running current at start. That 90 kW crane hoist wants maybe 600 kVA for a few seconds before it settles down.
The generator doesn’t care that it’s “only a few seconds.” The alternator sees a wall of demand, the voltage dips, and if it dips too far the contactors on every other piece of equipment start chattering and dropping out. That’s what happened on my 2 a.m. pour: the pump and the crane started within a minute of each other, the voltage sagged, and the pump’s control relay gave up. The set was sized for running load. Nobody had sized for starting load.
So here’s how I size site sets now. Take the starting kVA of your largest motor — nameplate kW times roughly six, or read the locked-rotor code off the motor plate if you want to be precise — then add the running load of everything else that’s likely on at the same time. Not everything on the site. Everything that could realistically be running together. On most sites that’s the crane, the pump or the hoist, plus lighting and the site cabins.
Honestly? On a mid-size commercial site with a tower crane and a concrete pump, the honest answer usually lands around 250 to 350 kVA prime power — for equipment whose running draw might be half that. If that feels like overkill, price out one dead pour. It’ll feel cheap again.
And the trick nobody tells you: soft starters. A soft starter or a variable frequency drive on the crane and the pump tames that inrush dramatically — sometimes cutting the starting demand by more than half. They’re the best money nobody spends on a site. If your equipment has them, say so up front, because it changes the whole calculation. I ran through the full sizing arithmetic in my generator sizing guide — worth a read before you sign anything.
Depends on what happens if the power stops. If the honest answer is “the pour fails” or “the crane hangs a load mid-air,” get two. I don’t care that it costs more. One dead generator and a half-set foundation will cost you more than the second set ever will, and I say that having watched it happen at 2 a.m. from the wrong side of the night.
There’s a second reason, less dramatic but just as real: sites don’t run at one load all day. Day shift, you’re at 80%. Night shift, it’s floodlights, a hoist, and the cabins. A single big set loafing along at 25% load all night is burning fuel badly and — worse — asking for wet stacking. That’s when a diesel runs so light for so long that unburned fuel glazes the cylinders and the exhaust starts weeping. Two sets in parallel let you drop one offline when the load falls, and the one that’s left runs where diesels are happy, around 60 to 80%.
That said, two sets aren’t free. You need paralleling controllers, proper sync protection, and an electrician who actually understands load sharing — not just a guy who can wire a breaker panel. On a small site running cabins and hand tools, one well-sized set is simpler, cheaper, and honestly the right call. Don’t let anyone sell you redundancy you don’t need.
One more thing that matters here: rate the set for prime power, not standby. Construction is prime-power work by definition — long, continuous, variable load, day after day. A standby-rated set run like a prime set will age fast. I wrote about the two ratings that decide how long your generator lives a while back; on a site, this one bites harder than anywhere else.
Construction sites are the worst operating environment a generator will ever see. Worse than a factory. Worse than a data center by a mile. Everything that can hurt a diesel — dust, heat, bad fuel handling, nobody checking the oil — happens on sites, usually all at once.
Dust is the quiet killer. Fine construction dust packs air filters solid in weeks, not months, and a starved engine doesn’t politely tell you — it just loses power gradually until the crane feels sluggish and everyone blames the crane. In one project I handled, a set on an earthworks site was down to maybe 70% of rated output before anyone thought to look at the filter. Now I tell site clients: cut the air filter inspection interval to weekly during dusty phases, and keep a spare filter on site. It’s a fifty-dollar part that protects a fifty-thousand-dollar machine. The math is not hard.
Then there’s the practical stuff nobody puts in brochures. Sites move. The set that sat by the gate in phase one needs to be by the tower in phase three, so a towable mobile set or a skid that a telehandler can actually lift beats a set that needs a crane to relocate. Fuel delivery has to reach it — I’ve seen sets installed where the fuel bowser couldn’t get within hose distance, and the “solution” was jerry cans. Don’t be that site. And if you’re in a city, check the noise curfew before you buy: an open skid running at 95 dB will get you a visit from the neighbors and then the council. Enclosed canopy sets knock that down to the 70s at seven meters, which is the difference between working nights and not.
Oh, and grounding. Site distribution boards, long cable runs, wet ground — get a proper earth, test it, and don’t assume the last contractor’s earth rod is still doing anything. I’d put site electrical safety one rung below sizing in the things that actually hurt people.
Size for starting current, not running load. Read the motor nameplates, add the big inrush honestly, and if there’s a crane and a pump, budget 250 to 350 kVA prime before you let the quote sheet talk you down.
Buy two smaller sets when downtime costs more than hardware — pours, lifts, anything with a schedule penalty. Buy one when the worst case is a dark cabin.
And spec for the site you have, not the site in the catalog: canopy if there’s a curfew, towable if the site moves, weekly filter checks if there’s dust, prime rating always.
Get those four right and the set disappears into the background, which is exactly what a good site generator should do. Nobody remembers the pour where the power worked. They only remember the one at 2 a.m. where it didn’t.
If you’re staring at a load list and a quote sheet that don’t agree with each other, talk to someone who’s read a few thousand motor nameplates before the money moves.
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