Generator FAQ

Return to Generator Installation

Last changed 13 September 2009

I've received considerable email regarding generator connections and decided to put together a short page with some comments and a brief FAQ.
If you have other suggestions, comments, questions, or corrections please contact me. An email link is at the bottom of the page.

Basic generator information

There are essentially two sections to a generator. An engine creates the rotational motion and an alternator makes the electricity. There are different sizes and types of both items. Which you select will depend on personal preference, your needs, and your wallet.

The engine must operate at 3600RPM to produce a 60Hz output. There must be no engine speed variations, surging, etc. It must be a constant steady rotation at 3600RPM. Anything else and you may be looking at carburetor or speed governor issues. As the electrical load changes you will notice the throttle open on the engine to apply more power. This is due to the engine speed dropping as a load is applied resulting in the governor forcing the throttle open to compensate. A sudden drop in load will cause a short term engine speed which will cause the throttle to close to compensate. When the engine returns to the proper speed the spring tension on the throttle arm and the centrifugal force coming from the governor wheel will balance.

Once you've confirmed the engine is operating properly you can look at the alternator side. Confirm the output voltage is close to 120VAC when the engine is operating at its proper speed. Anywhere between about 115 and 125 should be acceptable. Outside that range and you may want to do further investigation. If your generator uses an automatic voltage regulator the board or brushes may have problems. Beyond that, check for loose connections, faulty circuit breakers, or even outlets which do not provide adequate tension against the plug pins.

Engine types commonly used in portable gasoline generators are either side valve or overhead valve types. Side valve engines are easier to build and thus less expensive. The advantages to overhead valve engines are lower emissions, higher efficiency for longer run time, lower noise created, and longer life. They often will have a spin on oil filter similar to used on a car engine. For commercial or frequent use such as daily use at a cottage or in an off grid home I would not suggest a side valve engine. An overhead valve engine will have a cover over the valve rocker arm to the side of the spark plug. It will likely have OHV or OHVI (industrial) stamped into it.

Here is some information, originally from Generac, which explains some of the benefits of overhead valve engines.

Most gasoline engines can have their carburetor modified to allow use of propane or natural gas. US Carburetion Kit Center I would like to get a portable diesel powered generator. Engine life is longer than with gasoline, and it can be fueled with home furnace oil. This is less expensive than gasoline, and I have a 200 gallon tank of it behind the house.

On the other end of the generator is the alternator. Two basic types, brushes and brushless. In the center of the alternator is a spinning magnet. As it rotates past the windings mounted to the case it generates electricity. At an engine speed of 3600RPM it will produce 120VAC at 60Hz. A brush type alternator will use an electromagnet to create the magnetic field. By varying the field current the output voltage can be kept constant under a wide range of load conditions. There is a circuit board which monitors the output voltage and adjusts the field current to compensate for any fluctuation. A brushless alternator uses permanent magnets. The windings are designed so that at 60Hz the output voltage will be 120VAC. There is no way to compensate for high load current causing the output voltage to decrease, so output regulation is not as good. On the other hand, there is less to break down, and no brushes to replace.

Here are pictures of my Generac 5500XL. It uses an 11HP overhead valve engine and brush type alternator. I bought the generator used and these pictures are before minor repairs and maintenance items were performed.

Here are pictures of my former generator, DeVilbiss GT5000. It uses a Tecumsen 10HP side valve engine and brushless alternator. This is similar to many of the Coleman Powermate generators avaialable.

You need to determine output voltage and power requirements for your intended use. If you will be using 240VAC appliances clearly a generator with 120VAC only will not be suitable. Note that generators which supply 120/240VAC use two windings in the alternator. Each will create 120VAC independently. Devices which require 240VAC will connect to both coils. Important in this regard is that the power rating on the generator is at 240VAC. In other words, a 5kW generator is capable of supplying 2500 watts on each of the 120VAC outputs. You can not place a 4kW load on one side of the generator or the breaker will trip. You will need to balance the load equally between both sides.

Neutral and ground connection in the generator will determine how the transfer panel should be wired. In some cases you may have to do some minor rewiring work inside the electrical panel of the generator. This is mentioned on my main page as well as other links from there.

You may notice mention of single or three phase outputs. Most residential connection is single phase. Voltage in use is 120/240VAC. Three phase is often used in large apartmetn buildings, industrial applications, or places which use many electric motors. Voltage in use is 120/208VAC. Usually three phase is in very large generators, seldom supplied in a portable unit.

Electrical issues

There is no impending doom for not connecting neutral and ground correctly. It is a potential safety issue, but not a guarantee of death and destruction. Like wearing the seatbelt in your car, leaving it off is not a death sentence, but wearing it is no guarantee of absolute safety either. We do things to try to ensure our safety, and proper electrical wiring and use is a good habit to follow.

I work in broadcasting, but have studied electrical systems and their operation out of personal interest. I've performed a considerable amount of residential wiring, but nothing on a full time job basis. It is surprising how many things can be done incorrectly and still work. I've seen improper wiring in commercial equipment, ground and neutral bonded or left floating in electrical panels, use of chassis ground for return current, electrical tape without proper connectors inside electrical panels, and it goes on and on. For anyone to open an electrical panel and start making changes, they should have some idea of the potential dangers, and the correct way to do the job.

Three wires carry the AC energy into the home. The bare wire is neutral/ground. Twisted around it are two black lines which are 120VAC with reference to the bare wire. Between them both is 240VAC. Light bulbs will connect between neutral and one live line for 120VAC, your oven will connect across both live lines for 240VAC. Inside the main electrical panel, a ground rod will connect to the bare wire entering from the road. This will become the common point for the home neutral and ground connections. They should connect nowhere else. The ground wire within the house is there for safety reasons only. It should never carry current. The neutral wire carries return current back to the panel for the live line.

There are three conductors in a typical home wiring - ground (bare), neutral (white), and live (black). Sometimes a four conductor line will be run. A red wire is added to carry an extra live line. Note that the black and red conductors must be on opposite sides of neutral. Otherwise there is a potential for overloading the neutral conductor, and there is no overcurrent protection provided.

Ground is used as a safety return in case of failure in a circuit. It also ensures the chassis of equipment remains at zero volts potential and is safe to touch. Neutral is used as the normal return line for current flowing on the live line. It is bonded to ground at the main service entrance, and should be isolated from ground elsewhere.

Backfeeding a panel is a potential danger area. It could lead to electrifying the grid backwards through the transformers and killing a lineman performing repairs. Linemen use grounding bars that connect to high voltage lines before working on them. One of the reasons is protection against home owners and improper generator use. You should use a proper transfer panel to eliminate possibility of backfeeding. When there is an extended outage, I also disconnect the main breaker feeding the house. This also provides my home added protection against spikes and surges when power is restored.

All generators should be labelled as either floating neutral or bonded neutral. The majority which I have seen are not labelled, and from my research are most likely neutral and ground bonded to the chassis. Using an ohmmeter to measure resistance between neutral and ground will confirm either way. Make sure the generator is not running while checking. Either way will work for home use, but the transfer panel must be wired differently in each case.

For a generator with bonded neutral, the transfer panel MUST be three pole and switch neutral for all circuits. The single connection point for neutral and ground will be in the generator. The ground wire will come back along the same cord and bond to the household ground.

For a generator with floating neutral, the transfer panel can be either two or three pole, but only TWO poles will be used. Neutral is not switched, and the connection point remains in the main service entrance panel. This is how my home has been wired. My transfer panel has three poles, but only two are used.

Using a generator with bonded neutral and ground in a system which does not switch neutral in the main panel leads to a potential safety issue. Neutral and ground will be connected at both ends of the extension cord, thus making them one conductor. As wire has a certain finite resistance, a voltage will develop when current flows through. This voltage will appear at the chassis and want to find a path to ground. Touch the chassis and you will be providing such a path. Normally this voltage is very low, but if there are problems elsewhere, it could become dangerously high. Safe operation can no longer be assured. Wired properly the neutral wire would be isolated at one end, preventing ground wire current and ensuring the chassis remained at zero volts.

GFCI, Ground Fault Circuit Interrupt outlets are a safety device which are now required in all outdoor or other wet locations such as bathrooms. They typically have two switches on the front, test and reset. The GFCI outlet monitors current on the live and neutral lines. If current begins to flow from the live line through ground, the neutral current will no longer equal the live line current and the circuit will open very quickly. I suspect a person would still get a nasty jolt, but the likelihood of major injury or death is greatly reduced. Note that a GFCI outlet will not provide protection if someone gets between the live and neutral lines. Current then will be equal in both lines and the circuit will not open. GFCI adapters for extension cords are available also. I've added a GFCI outlet in my front porch as sometimes this outlet is used for the lawnmower when it has to be used where the normal outdoor outlets will not reach.

Here are some other generator FAQ sites.
Steve Dunlop
Survival Monkey

On to generator questions

Q. If I was to keep the tie between ground and neutral in the generator during standby use, would I have to break the neutral tie coming into home from grid?

A. Never disconnect the neutral line as it enters your house. It is held very well at ground potential. Each transformer is grounded, and every home supplies ground as well. The neutral coming in is the reference to both live lines, and without it, anything connected to either live line could see up to 240VAC depending on what is across the other side. Install a proper transfer panel, and the neutral issue will be taken care of.

Q. Where would I ground the generator at this time in this situation?

A. The generator will be grounded through the extension cord plugging into the transfer panel. Your home should have one ground connection only, adding a ground rod at the generator could create other unwanted safety issues.

Q. At this time is it safe to run an extension cord off generator direct to power a drill for example?

A. Yes. The transfer panel will take care of any neutral and ground bonding issues, provided things are wired correctly.

Q. If I was to lift the neutral to ground in generator and tag generator as floating neutral, then would I leave neutral unbroken in service panel?

A. Yes. The common point for neutral and ground will be provided in the main electrical panel.

Q. Could I use the 15 amp outlets on generator safely in this position to run a drill outside? I see generators with floating neutral have a 15 amp duplex outlet on cabinet also. In this situation would I install a ground at the generator?

A. If you were connected to the house with the four conductor extension cord nothing else will be needed.

Q. If you have a floating neutral generator and use it in a stand alone mode, nothing to do with a home, do you place a ground rod down? What happens with the neutral and ground connections? What about a generator on a truck or trailer?

A. In stand alone use, you should connect to a ground rod. Neutral and ground should also be bonded in the generator. The easiest way to take care of this is with a dummy plug inserted into one of the duplex outlets or the twistlock outlet. Jumper ground and neutral in the plug. A generator on a truck or trailer will follow the same rules for neutral and ground bonding, ground rods, etc.

Q. If the generator is a grounded neutral type and you only run the two live lines to the house in an illegal back feed situation where main disconnect is off but neutral still connected will I have 120 volts available in house between each hot leg and the utility grid system neutral? Would someone get shocked if standing next to generator at that time and touched it creating a path from ground to the generator?

A. The only time I would consider backfeeding a house is if the main lines were torn down and laying on the ground. I did this once after Hurricane Juan caused a tree to tear the lines off a neighbors house. The lines were still connected at the pole and were live once commercial power was restored, but there was no possibility of the generator backfeeding to the street. It took almost two weeks before the power company could get them reconnected. We still turned off the main breaker and installed a padlock to prevent it from being turned back on. The house was totally isolated from the grid so I felt it safe to connect in this manner. The generator connected via a 20 amp two pole breaker in the main panel, and there was power available for everything in the house - well pump, electric stove, all lights, etc. Generator neutral was floating as described above. Power supply was limited, they were careful not to overload and trip the generator breaker.

Q. At the same time I went to a friends home and after killing the main breaker and all 240 breakers ran a 3 wire cord from the generator 240 volt outlet direct to his pump, now isolated from the house panel using both 120 volt lines and ground. I then ran a 12 gauge 20 amp extension cord from the generator duplex outlet to a double male pig tail and back fed a kitchen outlet so he could have lights in the house. 1/2 the house to be exact. This was also a grounded neutral generator but we didn't have a problem. I did however drive a ground rod at the generator.

A. I think you were lucky this time. Electricity can be potentially dangerous and can kill and burn if not used correctly. Double male plugs should never be used as invariably there is exposed 120V on one end. If you miswire one end you can send 120V onto the neutral. At best this will trip a circuit breaker, but not knowing what else is done, could easily create a lethal voltage on the chassis of certain equipment. It gets back to doing something properly or not doing it at all. Why run the risk of personal injury or death?
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