SECTION 5 - Fuel

5-1. Gasoline

For gasoline egine generators, Unleaded or Regular gasoline with a lower octane rating than 85((RON/MON)/2) may cause pre-detonation (knocking) which can damage the engine. Regular gasoline can be used, however, unleaded gasoline is preferred because it reduces pollution and combustion chamber deposits. See the generator's engine owner's manual for fuel information or contact the generator set's manufacturer.
ENGINE FUEL can cause fire or explosion.

POOR QUALITY, LOW OCTANE FUEL can damage the engine.


5-2. Gaseous Fuels

Typically on LP Gaseous or Natural Gas powered unit the unit will run on either vaporous fuel source. Most gaseous fuel units are set up for a specific number of ounces of line pressure (a.k.a. as a corresponding number Inch Water Column) with some type of threaded pipe hook up. Note: Use of gaseous fuels decrease the amount of total power the generator is capable of producing by as much as 20 percent, dependent upon thermal content of fuel in your specific area. Refer to Section 8 for Gaseous Fuel Systems.

5-3. Diesel Fuel Piping and Filtration

The basic requirements of a diesel engine fuel system are adequate piping, proper selection of filters for the application and completely air-tight joints, with a minimum number of fittings to prevent air from entering the fuel lines, especially in installations where the fuel tank is lower than the pump.

5-3. A. Piping to the Tank

Vacuum at the transfer pump inlet must be avoided. If an auxiliary pump is used, pressure at the transfer pump must not be less than 0 p.s.i. nor greater than 5 p.s.i.(typical). Pressure after the return line connector assembly must not exceed 5 p.s.i. unless called for in the pump or system specification.

NOTE: The return line must never be piped back to the transfer pump inlet side. Both supply and return line should be connected to standpipes in the tank with the opening for each 2 inches minimum from the bottom of the tank to allow space for water and sediments to settle and to eliminate siphoning problems.

5-3.B. Filter Requirements

  1. A pleated paper type filter with large area and minimum pressure drop capable of filtering out 75 - 80% of five micron particles should be used. A Master Filter is recommended since it provides sufficient area for long life and two stage filtration for maximum protection.
  2. Where water in the fuel is known to be a problem, a Master Separator is recommended.  Contact the generator manufacturer for a recommended filter.

5-3.C. Maximum Pressure Drop

Pressure drop across clean filters should not be more than 2.5 inches of mercury (1.2 p.s.i.) at full load. Pressure drop in the supply system exceeding 10 inches of mercury (4.9 p.s.i.) because of dirty filters or other restriction usually will affect pump and engine performance, (erratic operation, low power, engine stall).

5-4. Fuel Consumption

Fuel consumption is typically specified in the generator's user manual and is specified in a quantity of fuel consumed per hour based on a specified load. Refer to the generator's user manual for expected fuel consumption, which is based upon a specific load. While the generator's manufacturer may not be able to predict the consumption for your site because of the differences in the typical load, by site, an estimate a full load is typically given.

A simple fuel consumption model that is a "ball park" predictor of fuel consumption is as follows:

Based on experience, a generator at no load typically uses about half of the fuel of a generator at full load. The ratio of output power from a generator to the amount of fuel consumed is almost linear. Consumption on diesel fueled generators is somewhat less than gasoline.

Fuel Consumption = (Estimated Load/Maximum Generator Power Output) X .5 X Maximum Fuel Consumption + 50% of Maximum Fuel Consumption

Assuming Maximum fuel consumption = 1 Gallon/Hour

Minimum fuel consumption = .5 Gallon/Hour

At a 50% load, Fuel Consumption = (((50 Amps/100 Amps) X .5 Gallon) + .5 Gallon) = (((.5) X .5) + .5) = .75 Gal./Hr.

Other factors that need to be taken into account are the temperatures of the areas where the fuels are stored are as follows:

SECTION 6 - Electrical System

There are a number of different generator systems and typical loads in the context of electrical systems.  Most systems, unless they contain automated swtich gear, have a means of disconnect between the generator and the load.  This is typically a transfer switch or disconnect.  Ensure the contacts on the switch are rated for the size of your system.  System schematics are beyond the context of this tutorial at this time.

6-1. General (Electrical System)

Generators are rated for a maximum current in Amps and power output in Kilowatts. Typically power outputs can vary between different models. The output is dependent upon fuel type, ambient temperature and altitude of the installation.   

Of same model types using different fuels, Gasoline units will have the highest output followed by gaseous fuels (Natural Gas and Liquid Petroleum(Vapor withdrawal). LP is about 95% of that of gasoline and natural gas at about 85% of gasoline. These units typically derate at about 3% per 1,000 ft (334 meters) starting at about 3,000 feet (1,000 meters) and an additional 1% for every 10 degrees over 78 degrees F. This is fairly common for all asperating engines.

Diesel units derate more for temperature and altitude. Diesels typically derate at about 4% per 1,000 ft (334 meters) starting at sea level and an additional 1% for every 10 degrees over 78 degrees F. At higher elevations, this power loss can be significant.

Circuits to carry power from the generator needs to be sized accordingly. Distance of the generator to the load (Typically a UPS) will also effect wire size. The conduit entryway for the generator is typically specified for a nominal size and may need to be increased in size if you are required to go to the maximum wire size. Flexible liquid tight metallic conduit should be used.

Direct Current (D.C.) Generators typically require a significantly larger cable size from the generator to the load (typically UPS batteries) in order to compensate for voltage drop.  Voltage drop is a function of the resistance of the wire over the distance from the source to the load.  The typical recommedation for D.C. generators is to keep the generator as close to the load as possible.  Refer to the generators manufacturer's recommendations.  Refer to N.F.P.A. 70, also known as the National Electric Code (NEC) for cable sizing tables.

When mounting electrical panels, a 3 foot clearance is required and the use of an emergency light to illuminate the unit during operation is typically required.  Power for the emergency light should be from both the primary utility and the generator. This is highly recommend so that in the event of a malfunction there is a light source to see to work on the unit.  Refer to your local building and electrical codes to ensure compliance.

Use of powered exhaust fans and powered louvers for ventilation is typical for indoor installations.  Emergency power will be needed operate the auxillary devices.  Make sure the generator is sized large enough to cover the load and the auxillary equipment.

6-2. Conductor Sizing Connection

This information is dependent upon your generator output and intended load. When connecting cables to the generator, unless instructed differently by the equipment manufacture, make connections at the generator first. Make the connections at the load last. Failure to do so may constitute a fire or safety hazard.

All ampacities are typically calculated at 75 o C (Celsius)(167 o F(Fahrenheit) in the conductor size charts. Building wire conductors should be rated at 90oC(194oF) to allow for different ambient temperatures that these conductors may pass through.

All conductors are typically required by electrical code to be copper. The recommended conductor sizes are based on maximum current. Ampacities are found in NEC Article 310, Table 310-16. Conductor resistances are found in NEC Table 8 "Conductor Properties".

Direct Current (D.C.) generators require larger output power cables than comparable A.C. generators due to voltage drops in the cable caused by increased resistance.

Most A.C. Generators require the use of transfer switches. Refer to manufacturer's installation instructions and recommendations.

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Go to Section 7

Tips on Hooking up a Generator (genset) to a Uninterruptible Power System (UPS)

Dexter Hansen

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