Portable Generator and Voltage Drop Considerations

Sometimes it gets confusing for the average consumer to keep things straight when talking about electricity and more specifically voltage, power, and watts.

In this short article we are going to address the issue with voltage and the difference that are expressed by manufactures and specifications as well as take a brief look at how wire size and length can affect voltage.

Voltage

In practical terms 110V, 115V and 120V all refer to the standard voltage that you will encounter with your household and your appliances.

But, why the difference in numbers?

In the USA, the electric utility company has a requirement to provide a split phase 240V feed to your home which are two legs of 120VAC of opposite polarity. Because I want to keep this simple and practical I’m going to avoid the discussion of frequency for 60 Hz and oscillating AC power because it just adds to the confusion. If you’re truly interested you can look that up elsewhere.

To keep it simple the utility feed uses two 120V circuits to power your 240V appliances such as your dryer or range. The majority of your home appliances work on a single phase 120V power. “3 phase” power is used primarily in industrial applications and rarely found in your home.

115V and 110V are derived from the line losses in voltage associated with the resistance of power flow in the wires and connections. Line voltage loss is around 3 to 5% (at 4% the loss is .04 x 120V = 4.8V), which will bring the voltage down to about 115V which is what most appliances are rated to run.

Often times you’ll see appliances rated at 115V/110V. This so that long house wire runs or a long extension cord can drop the voltage to 110V.  These lower voltages are called utilization voltages. It is why your appliance will be rated for use at these voltages.

What of 240V?

The same scenario is true also for 240V where 230V and 220V are commonly associated with the utilization voltage.

3 phase voltage is stated as 208V which is a SRSS vector addition of three 120V legs. There are some good explanations and analogies for describing this, but for the portable generator market it only has specific applications. You should not be using 3 phase power and equipment unless you are well versed in electrical applications.

Voltage Drop and Wire Size

Voltage drops in long extension cords. Wire size and length have a direct impact on how much power you will have available for use.  See our article on wire size considerations for insuring you have the proper size of wire to minimize overload conditions. Let’s address the basic concept here.

For a 30 amp outlet, 120 volts and a 100 foot extension cord a 6 gauge wire is appropriate. At 150 foot you need a 4 gauge wire.  At 10 amps and 120V your length of wire for a #10 is 250 ft and for #16 it is 50 feet. That can really restrict your use of power if not properly sized. For 240V output the 100 and 150 foot appropriate wire gauges are 8 and 6 respectively.

Note: The higher the wire gauge the smaller the wire. Go here: American Wire Gauge at Wikipedia at Wikipedia to learn more about the wire gauges.

What you may notice is that the higher voltage requires a larger wire size (smaller number) as it can provide more watts with half as many amps (less current or resistance). The bigger the wire sizes the less resistance. This is simplistic, but you should get the idea.

You can search for wire gauge charts to be sure of your purchase, but most extension cord manufactures are going to tell you what the cord is rated for in the way of amps and watts. As an example a L14-30 plug will be rated for 30 amps at 240V. A duplex or standard 3 prong plug and cord will typically be rated for 20 watts at the length supplied.

Here are some examples found on Amazon for power cords.

20 Amp Service Extension Cords

Coleman Cable 09208 12/3 SJTW Twist To Lock Extension Cord, 20-Amp, 50-Foot, Yellow

Yellow Jacket 2885 12/3 Heavy-Duty 15-Amp SJTW Contractor Extension Cord with Lighted Ends, 100-Feet

30 Amp Service Extension Cords

Coleman Cable 01912 25-Feet 10/3 Generator Power Cord with L5-30P Plug and 3-Outlets

Generac 6388 100-Foot 30-Amp Generator Cord with NEMA L14-30 Ends for Maximum 7,500 Watt Generators

Conntek Marine Shore Power 10/3 30 Amp Cordset with Light Indicator (Yellow, 50-Feet)

50 Amp Service Power Cords

Camco 55195 50 AMP 30′ Extension Cord with PowerGrip Handle

Conntek 1450SS2-15 15-Foot Temporary Power Cord, 50-Amp 125/250-Volt, NEMA 14-50P Generator Plug to CS6364 Locking Connector

 Summary

Here’s what you have to know. Most portable generators are going to provide 120V or 240V although they may indicate a rating of 250V.  That means you’ll be able to run the vast majority of your appliances or tools on long extension cords or through your house wiring without any issues.

The only thing you should consider is that if you’re using a long cord to connect to your transfer box in your house or using multiply long extensions cords you could easily get to a low voltage situation if you get too long or don’t have a large enough wire size.

In general you usually don’t have too much to worry about if you’re not pushing the limits of length and are using appropriately rated power cords.

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