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Everything You Ever (or never) Wanted to Know about Emergency Power, Lights, and Generators (Part 2)

06 Jan, 1996

This is Part 2 of a 2-part series on Emergency Power, Lights, and Generators.

Have You Decided You Need a Generator?

If you have decided that you need a generator, we're sure you have a few questions:

What Will It Cost?

From the the data gathered in Part 1 of this series and from personal experience:

  • Quiet is expensive! Loud is cheap!
  • Long run time means low power output. High power means short run time!
  • 230 volt output means 8 HP or larger with most brands, and short run time (small hours per gallon).

Notes from the first part in this series (Emergency Power, Lights & Generators - Part 1):

  • The smaller the better. Run time is a major concern. If you don't have power in your neighborhood, your local gas station won't either. No power, no way to pump gas at the gas station! The gas supply on hand must last as long as possible. The key is to evaluate what you really need to power in an emergency.
  • All generator manufacturers rate their generator's fuel consumption based on running with a load that is 1/2 the rated capacity. It can be misleading, but they all do it! Remember the fuel usage numbers shown below are for 1/2 the rated continuous load!
  • To get a generator that has 240 volt output requires an 8 horse power motor in most cases. Try to limit your generator choice to a 5 horse power or smaller, which implies a 120 volt only unit.
  • Generators with 12 V DC output in addition to 120 V AC cost about $100-$150 more than AC only units. What you're really paying for is an extra set of coils in the generator, plus the support electronics to rectify and regulate the DC power. Is it worth the money to have a gas powered battery charger?

How To Hook Up a Generator

generator-power-receptacleFirst, if you have ever taken a look at a generator, you will find an outlet like shown. It looks exactly like your wall outlet doesn't it? In fact, it is!

If the generator is a 240 volt model, it will also have a another outlet for the 240 volt power. The generator will also have a circuit breaker with a push-button reset.

Notice that the plug on the generator is intended to have devices plugged into it. Also notice that it is a socket, meaning that the connections are protected from accidental contact.

The rule is, that if power can be supplied at a connection, the supplying connector is a socket. Notice that on your appliances the connector is a plug (since it doesn't supply the power).

How Do You Get the Power from the Generator to Items in the House?

There are only two safe ways I know of to get the power into your house:

  1. Use a heavy gauge extension cord.
  2. Have a transfer switch and sub panel installed.

The easiest way is to use an extension cord. But there are two critical notes you need to know about using extension cords:

  1. Wire has resistance. The resistance per foot depends on the construction of the wire (number and size of strands). The maximum amount of current that can be provided by an extension cord depends on the type of insulation used in the construction of the cord, and the number of conductors in the cord.
  2. Because current is passed through the resistance in the wire, voltage across the wire drops. This voltage drop should be kept to under 2%. If the voltage drop is greater than 2%, efficiency of the appliance is severely decreased (and can also result in damage to some equipment). Larger wire size, or decreased current requirements can reduce the voltage drop.

Let's forget the details for a moment to examine the tables below:

Maximum Length of Wire (in feet) for 120 Volt, 2% max voltage drop allowed
Current and Power for 120 Volt Max Extension Cord Length (for given wire size)
Amps Watts #14 #12 #10 #8 #6
1 120 450 700 1100 1800 2800
5 600 90 140 225 360 575
10 1200 45 70 115 180 285
15 1800 30 47 75 120 190
20 2400 NO! 36 57 90 140
Maximum Length of Wire (in feet) for 240 Volt, 2% max voltage drop allowed
Current and power for 240 Vol Max Extension Cord Length (for given wire size)
Amps Watts #14 #12 #10 #8 #6
1 240 900 1400 2200 3600 5600
5 1200 180 285 455 720 1020
10 2400 90 140 225 360 525
15 3600 60 95 150 240 350
20 4800 NO! 70 110 180 265

From the above tables, a few things should be pointed out:

Voltage drop can be calculated using Ohm's Law which is given by: Voltage Drop = Current (amps) x Resistance (ohms).

Let's take an example where you might want to use a 200 foot, 14 gauge extension cord to run a 1000 watt flood lamp:

  • Current = 1000 watts / 120 volts = 8.33 amps Resistance of # 14 wire = 0.00258 ohms/foot x 200 feet = 0.516 ohms
  • Voltage drop = 8.33 amps x 0.516 ohms = 4.29 volts
  • Percent drop = 4.29 volts / 120 volts = 3.6 %

A 3.6% voltage drop is above our recommended 2% maximum voltage drop given in the table above (for 120 volt). Either the wattage of the lamp must be reduced, or a larger size wire (12 gauge) must be used. The larger size wire has less resistance per foot, so the voltage drop will be less.

The House Wiring and Generator Connection

House WiringThe only safe way to power items in your house without using an extension cord is to have a Double Pole Double Throw (DPDT) transfer switch and sub panel installed. It's an expensive proposition, but is the only legal way to connect a generator to your house wiring!

The diagram shows the addition of the DPDT transfer switch. Note that the loads you have determined to be necessary in an emergency must be moved from the main breaker to the sub panel. When the transfer switch is in the normal position, power is routed through the switch to the circuits attached to the sub panel.

When power is lost and generator power is to be used, notice that the switch must be moved to the "generator as source" position. Once the switch is moved, the desired loads may get power from the generator connection. Even if main power is restored, the desired loads are isolated, and will continue to receive power from the generator until the switch is returned to the normal position.

Notice that there is a special connector shown on the diagram. This is the only place that you will find a plug with exposed contacts mounted on a piece of equipment in your house (except your computer). It is a connector that is panel mounted with a recessed set of male contacts (like a plug, not a socket). Remember from the text above that anything that can be the source of power has a socket. Since the opposite ends of the transfer switch are wire to connector and the source in the main breaker panel, the connector can never be delivered power from the main.

A QUICK MENTION regarding a very dangerous method for connecting your generator to items in your house. Our intent is to discourage you from this practice! : It's called "back feeding", and is very dangerous for three reasons:

  • It requires that the home owner remembers to turn off the main breaker. Little details like that can easily be forgotten when an emergency occurs. More on this later.
  • It requires use of a very dangerous plug to plug cable! As mentioned above, a plug has exposed contacts, and the second it is inserted into a powered outlet, the other end of the cable would have exposed "live" contacts.
  • If the home owner forgets to kill the main breaker, the power will back feed to the pole-mounted transformer and will be stepped up to lethal voltages. A line man who wouldn't expect the line to be powered if he were working on line is sure to be killed by your generator! In addition, if power is returned while the main breaker is on and the generator is connected......ZAPPO!

EpicenterBryan's Baby Generator

500 Watt Honda GeneratorThis unit, from my personal emergency supplies, is a 500 Watt Honda. It's an oldie, but a goodie. This is a VERY quiet model and it eats like a bird. The power output is low, but the operating hours per gallon of gas makes it look like a perpetual motion machine!

Now, I want to tell you what I paid for this generator. I bought it from a guy who had it sitting in his scrap pile behind his house. He told me that a friend gave it to him because it needed a new motor. He priced a new motor and decided it was going to cost too much. I bought it from him for $10, and took it home to determine what could be salvaged from it. I found that it had a large rust hole in the gas tank, the carburetor jets were clogged, and the float bowl was full of rust chunks. A few hours later, it was up and running!

This little generator is nothing fancy, and requires the user to adjust the output frequency when it is first started, but I think the inconvenience is worth the dollar savings!

EpicenterBryan's Other Generator

GeneratorThis is what I think is a "great buy", from the table above. It's a Coleman 2250, and runs for about 1 hour per gallon of gas. Several years ago I bought this for about $350 on sale, so it provides big bang for the buck. However, I use the small "baby" generator whenever possible. Fuel is a major concern, but sometimes you really need the juice. I sized this generator so I could run a 7-inch circular saw if needed for making emergency repairs to my house.

Notice the modifications that were necessary to keep the neighbors happy. This generator is VERY loud. You pay a premium price for silence, and although this is a great value ($/Watt), the stock unit will cause you (and your neighbors) to go deaf! What?

I added some pipes, and a small car muffler to help reduce the noise. Also note the addition of a set of wheels and "move handles." These additions added up to another $75 or so but were worth it!

EpicenterBryan's Used Generator Story (or two)

  • Several years ago, I was in a store looking at the Coleman 2250 Generator, when an old man mentioned that he had a generator he wanted to sell. He was a Ham radio operator, and had used an old Heath 1000 Watt generator for emergency power for his Ham gear. He said he wanted $150 for it and I went over to take a look. I eventually decided that I wanted the 2250 Watt generator for several reasons, but the point is that you CAN find a deal on a used one if you try.
  • I remember sanding in the gas line in Princeville on the island of Kauai the morning that Hurricane Iniki was about to hit. A man pulled up in a truck and was trying to sell a generator to the folks in line for gas. There were no takers! After Iniki, the island was without power for 3 months. I wonder if that man sold his generator that morning and if he regretted it?