Published · Vili Power Solutions
Safety first
A generator must only ever connect to farm wiring through a transfer switch. Backfeeding through an outlet, a welder plug, or a panel can energize the utility line and kill a lineworker, and it destroys the generator when utility power returns. Transfer switch and generator installations need a permit and a qualified electrician.
Start with the list of what has to run
Sizing starts with a list, not a catalog. Walk the dairy and write down every load you cannot do without during an outage, with the nameplate horsepower or wattage and whether it is single-phase or three-phase. On most dairies the list looks something like this:
- Milking vacuum pumps. Usually the largest motors that must run, and the ones that set the generator size.
- Milk cooling. Bulk tank compressors, plate cooler pumps, and the controls for them. Milk that does not cool is milk you cannot ship.
- Water. Well pumps, pressure pumps, and the water heater for parlor wash-up.
- Ventilation. Barn fans in summer, and enough of them to keep animals comfortable.
- Parlor equipment and controls. Detachers, gates, crowd gates, the parlor computer, and lighting.
- Feed and manure handling you would run during a long outage, such as a TMR mixer or manure pumps.
Then decide what can wait. Shop equipment, office heat, and nonessential lighting can usually be left off, and leaving them off can drop the generator size by a full step.
Motor starting decides the size
Adding up running watts is where most undersized generators come from. A motor started across the line draws several times its running current for the first moments of every start, and the generator has to supply that surge without the voltage collapsing. If it cannot, the motor stalls, contactors drop out, and the other loads ride through a deep voltage dip.
- Size for the largest motor starting with everything else already running. That moment is the worst case, and it is usually a vacuum pump or a compressor.
- Drives and soft starters help. A vacuum pump on a variable frequency drive ramps up gently and draws far less starting current, which can shrink the generator you need. Soft starters do the same for other large motors.
- Stage the starts. Starting large motors one at a time, largest first, keeps the generator from seeing two surges at once.
- Match the phase. A three-phase dairy needs a three-phase generator. A single-phase generator cannot run three-phase motors without a converter, and that is rarely a good answer in an emergency.
Generator manufacturers publish motor-starting ratings alongside the running rating. A qualified electrician or the generator supplier can run the numbers from your load list.
Standby or PTO?
- A permanent standby generator with its own engine and an automatic transfer switch starts on its own within seconds of an outage. It costs more up front, but nobody has to be home, and it can be exercised automatically every week.
- A PTO generator runs off a tractor and costs much less. It needs a tractor big enough to drive it, someone to hook it up and run it, and a tractor held at the right speed. A common rule of thumb is about two tractor horsepower for every kilowatt of generator, but check the manufacturer's figure. A tractor that bogs down drops the frequency, and motors run hot even when the voltage looks fine.
Many dairies use a PTO unit as the backup to the backup, with a second inlet on the transfer equipment so the tractor can carry the critical loads if the standby unit fails.
Picking the transfer switch
- Manual or automatic. A manual switch is simpler and cheaper but needs a person. An automatic transfer switch senses the outage, starts the generator, and transfers on its own, then transfers back when utility power is stable.
- Whole service or critical loads. A switch at the service entrance carries the entire farm, which means the generator has to be sized for everything that might be switched on. A switch feeding a separate critical-loads panel keeps the generator smaller and the load predictable.
- Rated for the job. The switch has to be rated for the available fault current and, at the service entrance, listed for service equipment use.
Test it before you need it
A generator that only starts during outages will fail during an outage. Exercise it regularly, and at least a few times a year run a real test: kill the utility power, let it transfer, and milk on it. That is when you find the dead battery, the stale fuel, the vacuum pump that will not start, or the breaker that was never labeled. Keep the fuel tank topped off before storm season and write a one-page procedure for whoever might be on the farm when the lights go out. Our power outage emergency checklist covers what to do in the first hour.
Where we come in
We build the load list with you, size the generator and transfer switch around the motors that matter, install it under permit, and test it under real load. It is a core part of our dairy and agricultural electrical work across Weld and Larimer counties. Contact us or call (970) 685-2068.
