Emergency (standby) generator

01

Read standby and prime ratings correctly

AGG Power's specification for its C850D6 generator set (Cummins KTA38-G engine, 1,800 rpm, 60 Hz) lists a standby (ESP) rating of 850 kVA / 680 kW and a prime (PRP) rating of 775 kVA / 620 kW at 0.8 power factor, with 1,022 A at 480/277 V. Citing ISO 8528-1, it defines prime power as the maximum power available for unlimited hours per year in a variable load sequence, with the 24-hour average not exceeding 80% of prime power and 10% overload only for governing, and standby power as the maximum power available for up to 500 hours a year (no more than 300 hours continuously) with no overload capability. [1]

Editorial calculation: the prime rating is about 91% of the standby rating (620 ÷ 680 kW); by the 80% rule the average prime load over 24 hours should not exceed about 496 kW. 680 kW ÷ (√3 × 480 V × 0.8) gives about 1,022 A, matching the stated current. Size an emergency set so the critical load, including motor starting, sits within the standby rating. [1]

Cummins' QST30-G5 engine sheet (EPA NSPS Stationary Emergency Tier 2, 30.5 L V12, 1,800 rpm) gives gross engine output of 1,112 kWm standby, 1,007 kWm prime and 832 kWm base load, and generator set output of 1,000 kWe / 1,250 kVA standby (ESP), 910 kWe / 1,138 kVA prime (PRP) and 752 kWe / 940 kVA base load (COP). It defines ESP as power for varying load for the duration of an interruption of a reliable utility source, PRP as varying load for unlimited hours with 10% overload to ISO 3046, limited-time running power (LTP) as constant load for limited hours, and COP as constant load for unlimited hours, all to ISO 8528. [2]

Cummins' 2022 product guide lists the DQFAD set on the same QST30-G5 engine (EPA Tier 2, HC6K alternator, PC 3.3 control) at 1,250 kVA / 1,000 kWe standby and 1,125 kVA / 900 kWe prime. [3]

Editorial calculation: Cummins' prime rating is 91% of standby (910 ÷ 1,000 kWe), the same ratio as AGG's 620 ÷ 680 kW; base load is 75% of standby (752 ÷ 1,000). The 1,000 kWe set output is about 90% of the 1,112 kWm gross engine output, after the 42.4 kWm fan and alternator losses. The engine sheet's 910 kWe prime and the product guide's 900 kWe show that the set model and document date matter. [2] [3] [1]

Editorial note: ratings are stated at reference conditions; ask the supplier for derating at your site's altitude and air temperature.

Give the supplier the critical load (kW, kVA, power factor), largest motor and starting method, voltage, phases and frequency, required runtime, standby or prime duty, fuel type and storage, start time and transfer switching, enclosure and noise limit, emission standard, site altitude and temperature, installation (indoor, outdoor, container) and quantities.

02

Check fuel use, runtime, size and weight

The specification gives fuel consumption of 173 L/h at 100% standby power and 154, 118 and 84 L/h at 100%, 75% and 50% of prime power, at reference conditions of 25 °C air inlet, 100 m altitude and 30% relative humidity with diesel of 0.85 specific gravity. The open set measures 4,315 × 2,020 × 2,235 mm and weighs 6,868 kg; the silent version measures 6,058 × 2,438 × 2,591 mm, weighs 12,500 kg and has a 1,000 L fuel tank. [1]

Editorial calculation: a 1,000 L tank lasts about 5.8 hours at full standby load (1,000 ÷ 173) and about 12 hours at 50% prime load (1,000 ÷ 84). At full prime load, 154 L/h × 0.85 kg/L ≈ 131 kg/h of fuel for 620 kW, or about 211 g/kWh. Plan bulk fuel for the outage duration the site must ride through. [1]

The Cummins QST30-G5 sheet gives 275 L/h at 100% standby power (1,112 kWm), 248, 185, 126 and 69 L/h at 100%, 75%, 50% and 25% of prime power, and 246 L/h at continuous power, with a dry engine weight of 3,822 kg, 202 L of coolant and a 50 °C limiting ambient temperature for its cooling package. [2]

Editorial calculation: at full standby the Cummins-engined set makes about 3.6 kWh of electricity per litre (1,000 kWe ÷ 275 L/h) and the AGG set about 3.9 kWh per litre (680 kW ÷ 173 L/h). For NFPA 110 Class 48 the Cummins set needs about 13,200 L at full standby load (275 × 48) and the AGG set about 8,300 L (173 × 48); the AGG 1,000 L tank lasts about 5.8 hours, so it falls short of Class 6 at full load. [2] [1] [4]

Editorial note: the silent enclosure nearly doubles the weight in this example; check crane, floor loading and access before choosing an enclosure.

03

Specify NFPA 110 Class, Type and Level

NFPA 110 (2016 edition, as reprinted in a Cummins seminar) defines Class as the minimum time, in hours, that the emergency power supply system (EPSS) is designed to run at rated load without refuelling or recharging: Class 0.083 (5 min), 0.25 (15 min), 2, 6, 48, or X for another time set by the application, code or user. Type is the maximum time, in seconds, that the transfer switch load terminals may be without acceptable power: Type U (basically uninterruptible), 10, 60, 120, or M (manual). Level 1 systems are installed where failure could result in loss of human life or serious injury; Level 2 where failure is less critical to life and safety. [4] [5]

Cummins' Type 10 white paper notes that NFPA 70 (NEC) Article 700.12 requires emergency power within the time the application needs and not more than 10 seconds, and that NFPA 99 classes healthcare essential electrical system sources as Type 10, Class X. The 10 seconds run from loss of power at the load side of the transfer switch until acceptable power is restored there, and include the start time-delay (NFPA 110 annex A.6.2.5 suggests a nominal one second), sensing and transfer; generator and load recovery time after the load step is not included. [5]

Testing in NFPA 110-2016: generator sets in service must be exercised at least monthly for at least 30 minutes, either at a load that keeps the manufacturer's minimum exhaust temperature or at no less than 30% of the standby nameplate kW. Diesel sets that cannot do this must be exercised monthly with available load and annually with supplemental load at no less than 50% for 30 minutes and 75% for 1 hour, at least 1.5 hours in total. Transfer switches must be operated monthly. Level 1 systems must be tested at least once every 36 months for the duration of their class, which may stop after 4 continuous hours where the class is longer. Fuel tanks must be sized so fuel is used within its storage life, and fuel quality must be tested at least annually. [4]

Editorial note: local codes may adopt an edition of NFPA 110 other than the latest; confirm the edition, class, type and level with the authority having jurisdiction, and ask the supplier for a tested start-to-ready time and a load-bank connection.

04

Apply EPA 40 CFR 60 Subpart IIII to US emergency engines

Under 40 CFR 60.4211(f), an emergency stationary engine may run without time limit in emergencies, and for up to 100 hours per calendar year for maintenance checks and readiness testing; up to 50 of those hours may be used in non-emergency situations, but not for peak shaving, non-emergency demand response or supplying power under a financial arrangement except under listed conditions. Any other operation means the engine is not an emergency engine and must meet the non-emergency requirements. [6]

Section 60.4202(a) requires manufacturers to certify 2007 and later emergency stationary diesel engines up to 2,237 kW (3,000 HP) with less than 10 litres per cylinder, other than fire pump engines, to the Tier 2 or Tier 3 nonroad standards for their rated power where that power is 37 kW (50 HP) or more. Owners of 2007 and later emergency engines below 30 litres per cylinder must meet those standards (60.4205(b)), by buying a certified engine and installing it to the manufacturer's emission-related instructions (60.4211(c)). Section 60.4219 defines an emergency stationary engine as one used for power during an emergency, such as when utility power is interrupted. [6]

Editorial calculation: the QST30-G5 has 30.5 L over 12 cylinders, about 2.5 L per cylinder, and a gross standby output of 1,112 kWm, below 2,237 kW, so it falls under 60.4202(a); Cummins labels it EPA NSPS Stationary Emergency Tier 2. [2] [6]

Editorial note: the ISO 8528 standby limit of 500 hours a year is a rating condition from the generator maker; the EPA 100-hour limit governs non-emergency running of a US emergency engine. A set that will run for peak shaving or non-emergency demand response must meet the non-emergency engine requirements.

05

Compare two published generator sets

Values are as printed in each maker's document at 60 Hz, 1,800 rpm and 0.8 power factor.

06

Compare what suppliers offer

Suppliers linked to this page list standby generators; each card links to the page reviewed.

Listings include home standby generators alongside diesel, gasoline and inverter generators.

07

Trace the generator set components

This is a functional reference built from the sources cited; it is not one maker's design.

08

Fill the specification fields with values for your site

A generator set quotation needs values for your site. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

09

Send the load, runtime and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Load: [kW; kVA; largest motor]
Rating: [standby or prime; ISO 8528-1]
Electrical: [voltage; phases; frequency]
Runtime: [hours; fuel storage]
Site: [altitude; temperature; noise; enclosure]
Supply: [transfer switch; emissions; quantity]

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