Generator Fuel at Part Load and Minimum-Load Check

ScopeEmergency generators are nearly always oversized for the load they carry, and a lightly loaded diesel burns more fuel per kilowatt-hour and wet-stacks. The fixed losses of turning the engine over do not shrink with the load, so the fuel curve is not proportional. This calculator reads the manufacturer's four published points -- fuel at 1/4, 1/2, 3/4, and full load of the rating in use -- interpolates in straight lines at the real load, and flags any load below the 1/4 point as extrapolated. It reports kWh per gallon at the load and at full load, the load as a share of the standby nameplate, and the NFPA 110 Section 8.4.2 check of not less than 30% of standby nameplate, with the shortfall in kW as the load bank to rent. A Cummins C60D6R on its 55 kW prime rating (1.5, 2.5, 3.4, 4.6 gph; 60 kW standby) carrying a 20 kW shelter load burns 1.955 gph at 10.23 kWh per gallon, 14% less than the 11.96 at full load, and at 33.3% of standby needs no load bank. At night on 12 kW it burns an extrapolated 1.373 gph, sits at 20%, and needs a 6 kW load bank. The load fraction uses the standby nameplate because that is what the code states. It does not derate for altitude or temperature, compute tank runtime (the Generator Fuel Runtime and Backup Duration calculator), or size a generator. The manufacturer's data sheet and minimum-load guidance, NFPA 110 as adopted, and the AHJ govern.

Formula or tablefuel at load P = straight-line interpolation of the published gph at 1/4, 1/2, 3/4, and full load of the rating in use (first segment extended and flagged below 1/4); kWh per gallon = P / gph; load share = P / standby nameplate kW; minimum load = 0.30 x standby kW; supplemental load bank = 0.30 x standby kW - P when positive.

Source and editionThe manufacturer's published fuel consumption at 1/4, 1/2, 3/4, and full load (worked example from the Cummins C60D6R rental generator data sheet D-6567) named as the data; NFPA 110 Section 8.4.2 (exercise at not less than 30% of the EPS standby nameplate kW, supplemental load bank permitted) cited by number.

Free accessInterpolation on four points the user enters from the unit's data sheet and a 30% comparison stated in the text; no fuel map or code text is reproduced.

What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

API field namesstandby_kw, rating_kw, fuel_quarter_gph, fuel_half_gph, fuel_three_quarter_gph, fuel_full_gph, load_kw, fuel_gph, kwh_per_gal, full_load_kwh_per_gal, load_pct_of_standby, load_bank_kw

• Four-point fuel curve straight-line interpolation between the published 1/4, 1/2, 3/4, and full-load points of the rating in usemanufacturer data sheet
• 30% of standby nameplate the NFPA 110 exercise floor, reached with a supplemental load bank where neededNFPA 110 Section 8.4.2
• Extrapolation flagged below the first published point the first segment is extended and markedproject method