20 amp extension cord?

I was in my local home improvement store and noticed a 25 ft extension cord made with 12 gauge wire for around $25. I thought because this one was using 12 gauge wire it must me a 20 amp cord. It was labeled 15 amps.

This same store has a 25 ft, 12 gauge cord rated at 20 amps for over twice the price. Why would one on the cords to be rated at 15 amp and the other at 20 amp when they both use the same size wire?



I took another look at the 20 amp cord and it is for 250 volts so maybe my comparison was not valid.

I have a popup camper with an air conditioner that is rated at 125 volts and 20 amps if I am not mistaken. I would like to have an extension cord to be able to test the air conditioner at home on a 20 amp circuit that I do have. I’ll keep looking.

Usually, the gauge of the wire determines current and the insulation material/thickness determines voltage. So that could explain the price difference. However, I would also think a 12 gauge wire would be rated 20 amps unless the cord was only submitted to UL for 15 amps. That is the only reason I could think of for a lower rating. Hard to know without seeing the cords.

Generally, the current carrying criteria of a wire are based upon the composition of the wire (example, aluminum vrs copper, etc), the length of the wire, whether stranded or solid wire (surface area of wire) and gauge of the wire.

The insulation comes into play with heat dissapation and insulating capacity of the voltage between the wires.

I agree with OU812 that perhaps the company submitted the sample to Underwriters Labs for 15 amp or perhaps because of how it is made, that it only qualified at 15 amp and was not appropriate for a 20 amp rating.

Then again, maybe they are just being overly conservative on the rating.

Lotsa good info so far. The basics of AC household current are that the amount of current, (expressed in amps,) is limited by the size, (the total cross-sectional area,) and the length, of the conductor, (the extension cord.)

That is to say: If you want to support a 20-amp load in your RV from your house. You need, at a minimum, a 10-gauge, 25ft maximum length extension cord. And keep in mind that your 20-amp breaker is designed to handle 80% (16 amps) of current. A 25 or 30-amp breaker with appropriate wiring would be better.

The the thing most people will do is just plug it in and see if it works or trips the breaker. That’s fine as long as nobody has already pushed the limits in wiring your house. THAT combination is responsible for lotsa house fires.

I am looking at a manual for a Dometic roof top, RV air conditioner. The amps for the compressor and fan are 12.5 and 3.5 for a total of 16 amps. The breaker specified is 20 amps. The Minimum Wire Size is 12 AWG Copper for up to 24’. The manual says “For wire length over 24 ft., consult the National Electric Code for proper sizing.”

So maybe the 25 foot 15 amp extension cord could have been rated at 20 amps if were a foot shorter?

It depends: It sounds like the manual may be referring to a permanent installation. And there’s a difference in how much a solid-wire 12 AWG will carry vs the multi-stranded wire used in an extension cord. .

There’s too many unknowns to be able to answer your question. I recommend you check with a source that sells portable electrical cords for RVs. Like the ones they use at RV parks for overnight hookups. Or just go one size larger wire, or one that says it’s for the load you anticipate.

Length has no bearing on current rating. The issue with length is the voltage drop due to resistance in the wire. You want less than 3% voltage drop and this only becomes an issue over about 50 feet. This a good guide: https://www.cerrowire.com/products/resources/tables-calculators/voltage-drop-tables/
Notice up to 50 feet, it recommends 12 gauge for 20 amps.

Low voltage is a big issue in appliances like air conditioners. But for 25 feet, this should not be an issue with 12 gauge. Stranded wire only has a little more resistance than solid so I doubt it would make a difference in 25 feet. This is why I didn’t get into all the details in my initial post. I don’t think all those variables would make any difference in 25 feet. I still think it has to do with UL rating. But go with 10 gauge if you are still worried.

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Perhaps the lower amp rating on the less expensive model might have to to with the connecting hardware on either end. It might not have correct metal content to accomodate higher amp loads. BTW, I’m not an electrical professional, nor do I play one on TV.

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The plugs are different for 20A and 15A for commercial ratings. It could be that they wanted to use a 15A plug. If it was for 220-250V there is another wire inside and different plugs. The wire being used is probably rated for 600v. The extra wire would run the cost up as well. But most likely it has more to do with the costs and the UL ratings. Most likely the more expensive one has better insulating values, maybe even wet area ratings. But, yes 12 gauge wire is for 20A ratings, but the drop cord has a lot more features than just the wire.

This has been educational for me. I bought the cord and on the back of the labeling there is a chart: “Choose the right cord.” The chart lists 12 gauge, 25 to 50 feet as 20 max amps. This does not seem to agree with their own specification for this cord.

Anyway, I have been using a 16 gauge extension cord for a 15 amp miter saw. The chart says 13 max amps for a 16 gauge cord. At least I can replace the undersized cord with the new 12 gauge cord I just bought.

Thank you to everyone for your help.

I’m pretty sure you don’t have a 15 Amp miter saw. 15 Amps at 110 Volts is 2 HP. At best it’s a 1 HP (~7Amps) miter saw, and most likely less than that. If the miter saw really drew 15 Amps, you would pop the 15 amp breaker it’s plugged into since breakers are de-rated by 20%, so they pop at ~12 Amps.

I always laugh when I see shop vacs at Lowes or other places that say “Generates 5 HP” and plug into 15 amp circuits. All marketing BS.

So, continue to use your miter saw with the 16 gauge extension cord. If the cord was really long or you had a few plugged into each other you would start to see degradation in performance such as slower starting.

So we can not believe the labels on extension cords or on saws? The saw is a Ryobi, Model TSS103. It is a smaller, less expensive saw. I took a look at the seller’s Internet site and they are saying “Heavy duty 15 Amp motor to easily cut through larger material.” Is there a way that I could test the amperage of the saw motor? My testing has been limited to voltage with a cheap multimeter.

The 15 amps refers to the startup current that occurs for a very short time until the motor is running at full rpm. Following startup the running current will drop to 5 amps or less. Most house breakers have a delay in tripping to handle short term high current loads like this.

You could also draw high current if you jam the saw and keep it from running. This could throw the 15 amp breaker and possibly ruin your saw if it lasts long enough.

If you want to see the current draw for your saw, you can get devices that plug into your wall outlet that you would plug the saw into and will tell you the current draw. It won’t be exactly correct due to things like power factor and such, but it will give you an idea of how much it draws in steady state. I checked one of these out from the library once.

I looked up your saw and see the “15 Amp…” right in the title. It sounds great, but it’s very deceiving. If this was truly a saw that ran at 15 amps steady state, the motor would have to be much heavier duty which would make this saw much less portable.

Not lies, they’re just not telling you the whole story. Like I said, this is probably the Startup current, which only lasts for milliseconds and then starts to diminish quickly as the motor rpm increases. So, did the saw draw 15 amps? You bet, just not for very long.

You could make some assumptions that if the saw is capable of drawing high current for short periods of time then it can probably handle some short term high loads, such as cutting through tougher materials that would slow the rpm and increase the current draw. Thus, a 15 amp saw would still be much better than a 5 amp saw provided they’re referring to the same thing with the amp rating.

You can also apply this to extension cords. You can exceed their rating for short periods of time (just like when your saw is plugged into the 16 gauge extension cord). It can handle the startup current of the saw since the time is so short, but draw 15 amps for extended periods and it will definitely get warm and possibly burn up.

Thank you for the explanation.

My saw is on a circuit with a 15 amp beaker in a sub-panel in a garage. I took a look at the main panel in my 40 year old house. The light and receptacle circuits have 20 amp breakers. I went to the attic to check the wire gauge for the lights and found it to be 14 gauge. If I find that the receptacles are also on 14 gauge wire, do I need to replace all of the 20 amp breakers with 15 amp?

If the wire gauge in the breaker box that is connected to the 20 Amp breaker is 14 gauge then that’s a problem and the breaker needs to be replaced with a 15 Amp breaker. There are some cases where you can have 14 gauge wire connected to a receptacle on a 20 amp circuit. Such as if you have a 12 gauge wire running to the box that has a 15 amp receptacle (the usual ones we buy with the two vertical holes and one round hole in them) then the receptacle could have a 14 gauge pig tail connected to it. Thus, you may have to investigate more deeply to really see how it’s wired if you find a 14 gauge wire on a receptacle.

Does your house have aluminum wiring? If so, it could also have additional potential safety risks when subjected to overloading a circuit.

Back in the 1960s and 1970s, many homes were built with aluminum electrical wiring. Due to a copper shortage linked with the Vietnam war, using aluminum as a viable alternative made sense at the time because of this material’s availability and lower cost over copper wiring.Ma

I have replaced light fixtures, receptacles, and switches and I have never seen any aluminum. I plan to check out the wiring under the house for gauge and will make sure it is copper.

The lack of knowledge that I have about electricity is troubling.