How big do you go when sizing a portable generator for field work?

[CobraSorrow]

I’ve been getting more calls for short-term power setups lately and I’m curious how everyone’s sizing their portable generators for these kinds of jobs. I usually estimate based on total load and then bump the generator size up a notch for startup surges, but sometimes it still feels like the equipment strains more than it should. Do you guys go by a rule of thumb when figuring out what size gen to bring, or do you run quick load calcs for every job? I’ve been tempted to keep a mid-sized one ready for general use, but I’m wondering if that’s just asking for headaches when the gear doesn’t match the demand.

 

[FoxHound]

If I’m going into something blind, I default to bigger. Too many times I’ve shown up thinking I only needed a couple kilowatts and then someone plugs in an extra tool nobody mentioned. I still try to match it properly, but field work is messy enough that oversizing feels like the safe move.

 

[PGB1]

The main circuit breakers on portable generators are often notoriously oversized.

If the load is non-critical and can tolerate slight dips if everyone starts equipment, heaters & such at once- this is my method:
A)
For "gonna get abused" scenarios where people plug directly into the generator or L14-30 break-out boxes, I size 30% above the unit's Kva rating (Not the Kw rating) of what the job calls for.

And, when my company did rentals, I'd get a signed Certificate of Damage Compensation (Yiou wreck it, you pay to fix it) and
Release of Liability Due To Power Loss signed by the renter. (They mess up or overload the generator & lose power = We don't pay for job downtime or downtime for idled workers.)

B)
For semi-permanent, when there is going to be a load center or panelboard downstream (such as at long-term constructiion projects), I size between zero and 10% over the amperage of the main circuit breaker in the panelboard or load center. On those jobs, the initial start-of-day load can be large with 20+ stories of string lights, heaters, construction elevators... all starting up at once.

C)
For "Who Knows?" jobs, I would size double what the person thinks they need. And, again with the damage waiver.

Recommendation:
Install Hour Meters on your construction site & other "periodic use" machines so you know when preventative maintenance is needed. (Even the little putt-putts)

I've sent units out for a "week", but the customer had them for months. Other times, they went our for "tool use" and ended up powering not only tools, but dewatering pumps operating 24/7. There is a big difference between 10 hrs : 5 days a week and 24/7 use.

The hour meter told me when to do PM on the units.

 

[Alan]

Startup surges are where most portables struggle, even if the continuous numbers look fine on paper. Some crews solve it by keeping two smaller units instead of one big one just to split the load. Have you ever tried pairing generators or do you prefer keeping everything on a single unit?

 

[PGB1]

Have you ever tried pairing generators or do you prefer keeping everything on a single unit?

I often parallel generator sets. When paralleling, the generators are fitted with software and data cables from one to the next which ensure the phases stay exactly in cycle, no matter the dips or surges. Not doing so would likely result in destroyed alternators. ( Or one, big explosion)

By syncing & paralleling, there are certain benefits. One is, naturally, sharing the load. Another benefit is if there is a start-up surge, that is also shared. Also, if one unit fails during use, the others fill in for it. If the load is scheduled, for instance heavy use during work hours and light use at night, I can shut down all but one or two machines- saving the customer fuel and maintenance charge hours on the meters.

And, I can break out of one, or all, generators. To explain better, if the site would need 1,500 feet of feeder to reach a panelboard or switch at the far end, I can spread the generators around and break out of the farthest unit- or any unit(s). Its load is shared by all. The negative aspect is the fuel delivery guy gets crabby when he has to drive all over a site to re-fuel.

Conversely, the machines can be placed close together and I can use jumper feeders with tees, ending with one set of cabling whose ampacity is good for the entire load.

I did this not long ago at a big city's city hall & court when the main bus bars melted. The building needed 2,000 megawatts at 480 vac for minimal power while repairing the service. But the city would not allow a 2 Mw unit on the street. (Weight) I grouped smaller units in a plaza out front and had a truss bridge built to cross the road. I then back-fed the building with one set of feeder. (Many paralleled cables per phase for the feeder.)
Bonus? If one generator died, the rest would pick up the slack.

For sites with small generators & separate loads or "plug into the machine" loads, putting in several small ones is surely workable. But if one fails, that section is down. Many generators from 25 to 300 Kva working independently is how music festivals are usually wired.
Downside? More towing costs. And, again with the crabby fuel guy.
Up Side? Less cabling & one can wire the loads that have to be powered first earlier than the others.

 

[Ortega_Aliahan]

I usually do a quick load calc, but I still bring a generator that’s one size bigger than what the numbers say. Portable units don’t love running near their limit, especially with tools that spike on startup. A mid-sized one can cover a lot, but only if your loads are predictable.