7:02 on a Monday morning. Three tower cranes on the same job site swung their hoists up at the same moment — first lift of the day, three crews racing the schedule. The 320 kVA genset feeding them choked, the breaker threw, and the whole site went dead quiet. Fifty guys on day rates, standing around, doing nothing. That was my introduction to tower-crane power.
Twelve years later, I size generators for crane sites differently than for almost any other job. Cranes don’t kill generators with their average load. They kill them with timing.
Here’s the thing most people get wrong about a tower crane’s electrical appetite. They add up the motor nameplates — hoist 24 kW, slewing a couple of 5.5 kW, trolleying 3.7 kW, a little extra for the lights and the cab — and come out around 40 to 45 kW installed. Then they figure a 100 kVA genset covers it with room to spare.
It doesn’t. Because motors don’t draw nameplate current when they start. A hoist motor slammed on full-voltage start pulls six, seven times its rated current for a fraction of a second. That’s locked-rotor kVA — the number that actually sizes your generator. Modern cranes soften the blow with VFDs (variable frequency drives) that ramp the motor up instead of hitting it all at once. Honestly, that helps a lot. But it doesn’t erase the hit. It just turns a punch into a hard shove. I’ve stood next to a genset and watched the frequency meter sag from 50 Hz toward 46 when a hoist caught a heavy load mid-ramp. The machine recovered. The one next to it, on a different site, didn’t.
So rule one of crane power: size for the worst second of the day, not the average hour. The running load is almost irrelevant. It’s the starting kVA — and how much voltage dip your other loads can swallow while it happens — that decides the genset size.
The real killer on a crane site isn’t steady-state load. It’s coincidence.
Think about how a construction site actually works. Shift starts. Three crane operators climb into their cabs within five minutes of each other. The foreman waves. Three hoists go up, three slews swing, and somewhere a welder fires up his machine on the same bus. Every motor on that site just decided to start within the same sixty seconds. I watched exactly this happen on a site outside Wuhu — I’ve been on enough of these projects now that the morning startup sequence feels like a ritual. Three TC5610-class cranes, one 320 kVA set, all three hoists lifting at once. The breaker’s thermal trip doesn’t care about your schedule.
The fix wasn’t a bigger generator. Not at first, anyway. We made the foreman stagger the morning starts — crane one at 7:00, crane two at 7:02, crane three at 7:04. Ninety seconds of discipline, printed on the site rules and enforced by a guy with a whistle. The trips stopped. Then we upsized the genset anyway, because I don’t trust human discipline on a rainy Monday when the schedule is already two weeks late. Both halves of that solution matter: stagger the starts, and buy enough iron that a bad Monday doesn’t end the shift.
Let me walk through the math the way I do it on site, with a notebook and no software.
Take one TC5510-class crane: installed motor load around 40 kW. Running, it draws maybe 35–45 kVA depending on what the hoist is doing. Starting the hoist — even through a VFD — briefly demands on the order of 120–180 kVA from the supply, because the VFD still has to deliver the torque and the input stage draws current to do it. (Those are rule-of-thumb ranges from cranes I’ve measured, not lab figures — treat them as planning numbers.) Now add a second and third crane on the same genset, plus site lighting, welders, and the batching plant that always gets added later without telling anyone.
For three cranes sharing one set, I end up in the 400–500 kVA range, prime power — and I want the largest motor’s starting kVA covered with the voltage dip held under about 20%. Dip deeper than that and the other cranes’ contactors start dropping out, which turns one electrical event into three mechanical ones. If you’re not sure how voltage dip interacts with your other loads, read my piece on motor starting methods and generator sizing — star-delta versus VFD makes a bigger difference than most buyers expect.
One more sizing rule I never bend: a crane site runs the generator every single day, all day. That’s prime duty, not standby duty. If you’re fuzzy on why that distinction exists, the nameplate will punish you — I wrote about prime versus standby ratings after watching a “standby” set die two years into daily site work. And if the whole site-sizing question feels new, start with the sizing guide I wrote for contractors before you talk to any sales rep.
Almost every crane-site quote I see prices the generator and forgets the cable run. On a real site the genset sits fifty, eighty meters from the crane base — sometimes more, because the set has to stay clear of the swing radius. That distance means voltage drop, and voltage drop on top of a starting dip is how you get a crane that stalls mid-hoist with a load in the air. Upsize the cable. It costs a fraction of what one dropped load costs.
Two more things that never make it into the quote but always make it into my notebook. First, grounding the mast. A tower crane is the tallest lightning rod on site, and its foundation ground has to be bonded and real — not a rod somebody hammered in during a rainstorm and forgot. Second, dust. Crane sites are dust factories, and a clogged air filter derates your engine silently until the day you need full power. I tell every site electrician the same thing: check the restriction gauge weekly, swap the filter before it hits the red zone, and don’t let “it ran fine last month” be your maintenance plan. The construction-site piece I wrote earlier goes deeper on dust and site abuse if you want the full list.
Before you sign off on a generator for a crane site, ask the supplier one question: can this set take three hoists starting within a minute of each other on a cold Monday morning?
If the answer is “probably,” it can’t. If the answer comes with a voltage-dip calculation attached, you’re talking to the right person.
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