Generator Safety Frequently Asked Questions

What size generator do I need?

This depends on the number and size of the loads that you want to run at the same time. If a house uses natural gas (or propane) for heat and hot water, a 4,000 to 6,000 watt generator can maintain a high level of comfort (assuming natural gas or propane is available). A house using an electric furnace and electric water heat may need a generator in the 20,000 – 35,000 watt range.

Generator size can be estimated by determining the major loads to be supplied and allowing some overhead for convenience appliances (television, computer, etc). Some of the most common major loads are listed below (these are average numbers and can vary):

Furnace blower motor (propane/natural gas heat): 1,500-2,000 watts
Refrigerator/freezer 1,500-2,000 watts
Well pump (1 horsepower) 3,500-5,000 watts
Water heater (electric) 4,000-5,000 watts
Electric furnace 11,000+
Heat pump 10,000+

By manually switching loads off and on the owner can ensure that only one major load is running at a time. This then allows a smaller generator to be used.

What is a manual transfer switch?

A manual transfer switch is a panel wired into the house electrical distribution system that allows a generator to be used to supply power. It prevents the generator from back-feeding the utility and reduces the potential for lineman injury.

How do I connect my loads directly to a generator?

The simplest and lowest cost method is to run extension cords from the generator to the loads to be supplied, i.e. refrigerator, freezer, lamps, space heaters, etc. A properly sized extension cord must be used when connecting loads to the generator.

How do I connect a generator to my house?

When a generator is to be connected to household wiring, an approved method of isolating the house electrical system from the utility must be used. THE MAIN BREAKER OF THE HOUSEHOLD PANEL IS NOT AN APPROVED DISCONNECT DEVICE. NEVER CONNECT A GENERATOR TO HOUSEHOLD WIRING BY EXTENSION CORDS. A manual or automatic transfer switch must be wired into the distribution system to allow the use of existing household wiring and receptacles. This is to prevent the generator from back-feeding and injuring line workers trying to restore power. Significant damage can result from the generator being on-line when utility power is restored.

How much does it cost to install a manual transfer switch?

This can vary depending on many factors. Typical installations are between $500 – $900 for a surface mounted switch located near the main distribution panel. Most transfer switches come with clear documentation and can be installed by the owner for the cost of a permit in a couple hours. Some factors that can affect cost:

  • Distance from the contractor: Travel time and mileage costs money. The farther you are from the contractor the higher the cost.
  • Type of transfer switch: Transfer switches come in a variety of makes, models and sizes. They can be surface mounted, flush mounted, or designed for outdoor use.
  • Type of installation: Surface mounted switches take less time to install than flush mounted switches, resulting in a lower cost of installation.
  • Distance from the main panel: Most manual transfer switches are designed to be installed within a couple of feet of the main panel. Additional materials and labor would be necessary to install the switch away from the main panel and would raise the installation cost.
  • Remote generator receptacles: Most manual transfer switches have a receptacle to allow the generator to be plugged in to. In some cases it is more convenient to have the receptacle located closer to where the generator will be located when it is running.
  • Load identification: In most cases the manual transfer switch will not be supplying all of the circuits on the main panel. The more time the electrical contractor spends identifying the circuits to be connected, the higher the installation cost.

Can I open my main disconnect breakers and connect my generator to a receptacle?

No. It is not allowed for the following reasons:

  • The national electric code and most city, state, and county ordinances require approved means of isolating generator power from utility power. These requirements are designed to protect the lives of both you and the utility workers and to protect your equipment.
  • The main disconnect breakers are not designed to isolate power from two sources.
  • The main disconnect breakers can fail without visible indication. The breaker may feel l like it is open but may be electrically connected internally due to a spring or other failure. This can go unnoticed until utility power returns.
  • When both sources of power are available it is possible to connect both sources together. Accidentally operating the wrong breaker can have severe consequences. Approved means of isolation usually require 3-position switches or breakers that prevent connecting both sources of power.

What type of fuel is best?

This depends on many factors – shelf life, cost, storage locations, availability, etc. See question #8 for more information on the different fuels.

What are the advantages and disadvantages of the different fuel types?

Gasoline

Advantages:

  • Common fuel source – easily obtained
  • Increases portability of smaller generators

Disadvantages:

  • Highly flammable
  • Short shelf life (approximately 12 months)
  • Storing large quantities is hazardous
  • May not be available during power outages

Propane

Advantages:

  • Long shelf life
  • Clean burning
  • Easily stored in both large tanks or in smaller 5 – 10 gallon cylinders
  • Obtainable during localized power outages – suppliers may be unable to pump fuel during widespread outages
  • Home delivery available for larger tanks

Disadvantages:

  • Pressurized cylinder of flammable gas
  • Fuel system is more complicated (increased possibility of failure)
  • Larger tanks are not aesthetically pleasing (unsightly)
  • Fuel system plumbing results in higher installation cost

Diesel

Advantages:

  • Least flammable fuel source
  • Easily obtained
  • On-site fuel delivery available

Disadvantages:

  • 18 – 24 month shelf life
  • Installing large storage tanks raises cost of system
  • May not be available during power outages

What size propane tank do I need to run my portable generator?

That depends on a couple of factors. Larger tanks offer longer run times between refueling and on-site delivery of fuel may be available for tanks larger than 100 gallons. Small tanks like those used on barbecue grills allow several hours of operation and are easy to transport for filling at fuel stations.

How long will a generator run on a tank of fuel?

This depends on the size of the tank and amount of load on the generator. Higher loads require more fuel. A conservative rule of thumb is 1 gallon of fuel per hour for a 5,000 watt generator.

Propane fueled portables use a slightly larger amount of fuel than their gasoline counterparts.

How long can a generator be continuously operated?

Unfortunately, there is no set answer for this question. Most portables are designed for long life, heavy duty operation, and long run times. Gasoline units should be shut down during refueling to reduce the potential for starting a fire by spilling gasoline on hot exhaust components. The oil level should be checked and the unit inspected for signs of fatigue or abnormalities (cracked receptacles, etc) before restarting.

How often should I shut the generator down?

The answer to this question is similar to #11. Generators should be shut down and inspected daily or when refueling. If long periods of no load or light load operation are expected (such as during the night when everyone is sleeping) the generator could be shut down to extend the useful life of the generator and reduce fuel costs.

Is it possible to run my portable in the garage?

This is not recommended. Portable generators are designed for outdoor use. Running them indoors presents the following problems:

  • Increased risk of carbon monoxide being admitted to living spaces.
  • Potential for fire. Garages can have combustible material near the generator that can catch fire when in contact with hot exhaust components.
  • Potential equipment damage. Indoor operation of the generator may restrict cooling airflow to the engine.

Is it okay to build an enclosure around my portable generator?

Yes, as long as the generator is taken out of the enclosure prior to operating it. Access to cooling air is vital to the proper operation of the engine. Portable generators do not have large cooling fans and are unable to circulate air in confined spaces. They rely on the natural circulation of heat resulting when the hot air rises away from the engine and is replaced by cool air.

How large a pump can a portable generator start?

When starting loads on a portable generator, the larger loads should be started first. A 4,000 watt portable can easily start a one horsepower pump. The 5,000 and 6,000 watt generators are capable of starting up to a 3 horsepower pump.