Trying to choose the extension cords to carry? Maybe this will help.

I found an old Belden catalog and it has ratings in it for the “common” extension cords that they made. I checked the packaging on several at the local Wally World the other day, and the ratings match the old catalog. I think most of us throw away the package and use the cord. Some of us forget what was on the package… So I thought I’d post the information. Since the CFB code uses proportional fonts (and spacing), I can’t do a good table, but will try to make it readable.

The table is wire gauge, the length, the maximum current and the maximum wattage (they use 125 volts, the maximum voltage rating, to compute the wattage).

There is also a 10 gauge, 25 foot cable with the common household plugs that is rated at 15 amps being sold by RV supply places. The common plugs are ANSI/NEMA 5-15, and are rated for 15 amps maximum and thus the rating. The same cord with the TT-30 connectors (the common 30 amp RV connectors) is rated for 30 amps.
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Voltage drop in extension cords.

I decided to attempt to provide an estimate of the voltage drops in the various extension cords. The reference data I’ve chosen may not exactly fit the cords, but the results should be pretty close. To do this, I got the resistance per 1,000 feet for each of the gauges from Table 8. Conductor Properties, in Chapter 9 of my old 1993 NEC book. The resistance is for coated wires, 7 strands, at 75 degrees C, or 167 degrees F. I have calculated the voltage drop for 15 amps – in all cases – through 200 feet (100 feet for the hot lead, 100 feet for the neutral lead) of the gauge wire.

The “table” is wire size, resistance per 1,000 feet, voltage drop for 15 amps in 100 feet of extension cord.

16 gauge, 5.29 ohms, 15.9 volts

14 gauge, 3.14 ohms, 9.4 volts

12 gauge, 2.05 ohms, 6.0 volts

10 gauge, 1.29 ohms, 3.9 volts

8 gauge, 0.809 ohms, 2.4 volts

6 gauge, 0.510 ohms, 1.5 volts

Notes:

1. For a 50 foot cord, divide the voltage drop will be half, for a 25 foot cord, one fourth.

2. 10 gauge wire is used in the cables in RV’s with 30 amp service.

3. 6 gauge wire is used in the cables in RV’s with 50 amp service.

4. This does not include any losses in the connectors on the cables.

5. The 15 amps exceeds the maximum current rating for the 100 foot 16 and 14 gauge cables.
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I chose 15 amps as that is the approximate current of the 13,500 btu air conditioners – the one thing we are always trying to run when the electrical power is a problem and we’ve had to dig out extension cords.

This is only the voltage loss in the extension cord, and does not include the loss in the connectors and RV wiring.

I hope this helps in your choices.

Mel & Mary Ann; Mo'Be (More Behave...) and Bella
"If you have an RV, you don't need another hobby." Comment from a friend...

Interesting data! the theoretical definition of 10 AWG is 1 ohm per thousand feet. Your table shows the effect of insulation and strands. Also 3 AWG sizes should be a factor of two in resistance, theoretically.

You also mention the connection resistance as another factor. Connectors are usually the first to melt in an under rated system, in my experience.

The two things I think people need to watch out for in load and cable length are (1) delivered voltage and (2) cable heating.

Delivered voltage also has to consider that the voltage at the plug will likely drop as its feed is also at the end of a line and it may depend upon what your neighbors are doing. Anyone running motors (e.g. A/C) needs to make sure they don't have below spec voltage.

Cable heating is why you don't leave an extension cord all bundled up in a cabinet if used for a short run. A 5 volt drop at 15 amps is 75 watts that goes into heating the cable.

I have been puting some of my ideas in this area at SierraNevadaAirstreams/rvtech/energy/electricity.html

You can use pure resistance values on AC circuits as a reference however, the resistance to AC current is more complex and often refered to as impedance. Why not just go to an electrical code book and pick the wire size.

Are you doing that site, or part of it? I'll have to go "browse" a little - some of the topics looked interesting as I tried to find the site you mentioned.

There have been some posts on extension cords in the past, and I just decided to assemble some data for future threads. Sometimes folks don't remember the cord maximum ratings and discussions take longer. I added the voltage drop as a piece of data folks could use to choose the cords they want to carry.

And yes, at lower temperatures, the resistance of the 10 gauge cable drops to about 1 ohm, the coating of the stranded wires has little effect.

If you go to the Beginning RVing forum, to the Frequently Asked Questions on the Open Roads Forum topic, you'll find links to previous threads and some outside sites on a number of topics. I'm sure this one will end up in the electrical section so it'll be easy to find.

Wow a lot of info for me to take up. I need a simple sulution for an extension cord 25 feet to add onto my popup which has about 20 feet already. I have a 13000btu air condition, lights,frig,heat and the hook up is 30amp. Question what gauge do I need for the 25 feet I need to buy. Reason because where Iam going the hook up is about 50 feet away.

Great post, MELM! The watts ratings helps greatly.

Perhaps a little off topic, but I would like to add that I dont think most realize what appliances really draw or perhaps dont think about what the wattage/current really mean.

As a rule of thumb 100 watts is approx 1 amp (little less but easy to calculate). So that suggests you could tie 15 - 100 watt bulbs to most common outlets.

The A/C units are approx 12 amps by themselves. This always seems to be mentioned as the 'heavy' load device.

Heres a gotcha - how many women (not picking on women) realize that a single blow dryer will draw as much or more than the A/C unit?

When people start tripping breakers or overheating extension cords, it is usually the result of just the wrong combination of appliances being used at the same time.

The appliances that seem to be the biggest concerns are any form of heating element (blow dryers, small room furnaces, hot plates, etc) and you need to be selective with what else you try to use concurrently with these appliances.

I think you will find most are 'temporarily or briefly used' so its not an ongoing problem.

*This Message was edited on 19-Apr-03 06:11 AM by wittmeba*