Farm electrical loads are different from residential loads. A house has a predictable mix of lighting, HVAC, and appliance loads that an electrician can size from a standard NEC load calculation. A farm has grain dryers that draw 200 amps for six weeks in October, irrigation pumps that run 24 hours a day in July, welders in the shop, and motors on every bin, auger, and conveyor. The loads are seasonal, intermittent, and often scattered across buildings a quarter mile from the meter.
The PTO side of farm power is equally important. Tractor-driven equipment like grain augers, PTO generators, and manure spreaders place mechanical loads on the tractor engine that are easy to underestimate. A PTO-driven generator sized for a grain dryer needs to account for the starting surge of the dryer fan motor, and the tractor needs enough engine horsepower to drive it at full load without bogging. This guide covers electrical service sizing, feeder calculations, PTO power conversions, and generator sizing for grain drying operations.
Sizing the Farm Service Panel
Farm service sizing starts with NEC Article 220, Part V (Farm Load Calculations). The farm calculation method differs from residential and commercial methods because it accounts for the low likelihood that all farm loads operate simultaneously. The NEC farm calculation separates loads into two categories: the dwelling load (calculated using the standard residential method) and the farm building loads (calculated with demand factors that reflect agricultural use patterns).
For the farm building loads, the NEC allows the following demand factors: the largest single load at 100%, the second largest at 100%, the third largest at 65%, and all remaining loads at 50%. If you have four buildings with loads of 60A, 40A, 30A, and 20A, the farm demand is: 60 + 40 + (30 × 0.65) + (20 × 0.50) = 129.5 amps. Add the dwelling load to get the total farm service demand.
Many farms have grown their electrical loads over decades without upgrading the service. A farm that was adequate with a 200-amp service in 1990 may need 400 or 600 amps today after adding a grain dryer, a second bin site, and a heated shop. The symptom of an undersized service is the main breaker tripping during harvest when the grain dryer, aeration fans, and shop loads run simultaneously.
Three-phase power, if available from the utility, is a significant advantage for farms with large motors (above 10 HP). Three-phase motors are smaller, more efficient, and less expensive than equivalent single-phase motors. Three-phase grain dryer fans and irrigation pumps draw about 80% of the current of single-phase equivalents. For farms with 50+ HP of motor load, three-phase typically pays for itself within 5 to 10 years.
Largest load: 100%
Second largest: 100%
Third largest: 65%
All remaining: 50%
Example: 60A + 40A + 30A + 20A connected
Demand = 60 + 40 + 19.5 + 10 = 129.5A
Electrical Load Panel Planner - Farm/Ag
Size electrical service for farm shops, grain handling, and livestock buildings using NEC Article 220 demand factors. Includes motor FLA lookup, conductor sizing, and transformer kVA recommendation.
PTO Horsepower vs Drawbar Power
Tractor power is rated at two points: the PTO (power take-off) and the drawbar. PTO horsepower is the power available at the rear PTO shaft, measured on a dynamometer with the tractor stationary. Drawbar horsepower is the pulling power at the hitch, measured while the tractor is moving and pulling against a known resistance. Drawbar power is always less than PTO power because some engine power is consumed by drivetrain friction, tire slip, and rolling resistance.
The relationship depends on the surface and tractor type. On firm soil, drawbar HP is about 85% to 90% of PTO HP for 2WD tractors and 87% to 93% for MFWD tractors. On soft soil or mud, drawbar efficiency drops to 65% to 75% because of increased tire slip. A tractor rated at 150 PTO HP delivers roughly 128 to 135 drawbar HP on firm ground and as little as 98 to 113 HP on soft soil.
For PTO-driven equipment, use PTO horsepower ratings. An auger rated for 30 PTO HP needs a tractor with at least 30 PTO HP available, and preferably 40 to 50 PTO HP for headroom. Equipment manufacturers publish minimum and recommended PTO HP. Using the minimum PTO HP rating leads to overloading, belt slippage, shear bolt failures, and excessive fuel consumption.
For drawbar implements (plows, discs, chisels, planters), use drawbar HP or draft force. Draft (lb) = specific draft (lb/ft of width) × implement width (ft). The drawbar HP needed is: HP = Draft × Speed (mph) ÷ 375. A 12-foot chisel plow in medium soil at 5 mph: HP = 9,600 × 5 ÷ 375 = 128 drawbar HP.
Drawbar HP = PTO HP × Efficiency
Firm soil: Eff = 0.85–0.90 (2WD), 0.87–0.93 (MFWD)
Soft soil: Eff = 0.65–0.75
Drawbar HP = Draft (lb) × Speed (mph) ÷ 375
1 PTO HP = 745.7 watts = 33,000 ft-lb/min
Generator Sizing for Grain Dryers
Grain dryers are the largest seasonal electrical load on most grain farms. A typical continuous-flow grain dryer has one or two large fans (5 to 25 HP each), an auger drive (3 to 5 HP), and controls/ignition electronics. The electrical demand is dominated by the fan motors, which draw locked-rotor starting current when they start and full-load current while running.
Example: a dryer with a 20 HP fan motor and a 3 HP auger motor on single-phase 240V. Running loads: fan = 24,000W, auger = 4,080W, controls = 500W. Total running: 28,580W. Fan starting surge: 100A × 6 × 240V = 144,000W for 5 to 8 seconds. You need a generator with at least 144,000 surge watts and 28,580 continuous watts. That points to a 50 to 60 kW generator minimum, even though the running load is only 29 kW.
PTO-driven generators are common on farms because the tractor already provides the prime mover. A 50 kW PTO generator requires approximately 75 to 80 engine HP at the PTO. The tractor must be dedicated to generator duty for the entire drying period, which may be days or weeks. Fuel consumption is substantial: a 100 HP diesel engine at 80% load burns roughly 5 to 6 gallons per hour.
The alternative is a standby engine-generator set permanently installed at the dryer site with an automatic transfer switch. The upfront cost is higher ($15,000 to $40,000 for a 50 to 75 kW unit), but the per-hour operating cost is lower than PTO because the generator engine is optimized for its load. For farms that dry more than 50,000 bushels per year, a dedicated generator often pays for itself within 5 to 8 years.
Feeder Sizing and Voltage Drop for Farm Buildings
Farm buildings are often hundreds or thousands of feet from the service entrance. A bin site 1,000 feet from the main panel with a 30 HP fan motor requires a feeder that handles the load and keeps the voltage drop within NEC recommendations. The NEC recommends no more than 3% voltage drop on the feeder and 2% on the branch circuits, for a total of 5% from the meter to the load.
Voltage drop calculation for single-phase feeders: VD = (2 × L × I × R) ÷ 1000, where VD is voltage drop (volts), L is one-way distance (feet), I is current (amps), and R is resistance per 1,000 feet of wire. For a 30 HP motor at 150A running, 1,000 feet, #2/0 copper (R = 0.0967): VD = 29 volts = 12.1% drop. That is way too high. You need 250 kcmil or larger wire for that run.
The cost of large copper feeders for long runs drives many farms to use aluminum instead. Aluminum wire costs about 60% of copper per ampere of capacity, but requires wire two sizes larger. The connections must use anti-oxidant compound and connectors rated for aluminum. The upfront savings are significant for runs over 500 feet.
Another option for very long runs is stepping up the voltage. A 480V feeder has one-quarter the current (and one-quarter the voltage drop) of a 240V feeder for the same power. A step-up transformer at the main panel and a step-down transformer at the bin site add cost ($2,000 to $5,000 for a transformer pair) but can eliminate tens of thousands of dollars in large wire for runs over 1,500 feet.