"Short Cord Instructions": Do extension cords exist that are safe to use with high wattage appliances (e.g., a microwave)?

Factual Questions
1

I’ve always heard that it’s risky to use extension cords with high wattage appliances like vacuum cleaners, space heaters, and microwave ovens. But does this conventional wisdom apply to all extension cords, or only the cheap thin ones that look like lamp plugs? Is it safe to use one of those thick, grounded, heavy duty cords?

The reason this issue has come up is that we need to replace our over-the-range microwave, which came with the house and has recently died the True Death. Shortly after we moved in, we had most of the kitchen rewired, because we were tripping the fuses so often; among other things the microwave oven got its own circuit. That’s really the standard, but somebody had decided to cut a lot of corners, at some point.

If we can’t find a replacement microwave that will fit into the same space, which wouldn’t be unheard of in a manufactured house, then we’ll have to buy a countertop model. It goes without saying that the power cord to a countertop microwave will be three feet long, if that…and hence my question about extension cords. If it’s safely feasible, my idea is that I could run the extension cord up behind and above the old microwave*, to the microwave outlet. Otherwise, with a countertop microwave, we’ll be blowing fuses again, and the dedicated two-pole microwave circuit will be wasted.

In case you’re wondering why I wouldn’t be able to find a microwave that fits, it’s because this is a manufactured house. Manufactured houses have this one peculiarity where some standard things tend to be just a tiny bit shorter or narrower than in other houses; you don’t notice it until you try to install or replace something. We couldn’t use our pet door** because its shortest adjusted height was about a quarter inch too tall. We had to send a new range back to Home Depot because it didn’t quite fit. And now some of the microwaves I’m seeing online are too wide or too high.

*In this scenario, the old microwave would have to stay in place. Otherwise, we’d have to fix the gap in the decorative backsplash, and I don’t want to make this into more of a project than necessary. We can always use the old one for storage or something.

** This turned out to be a good thing, because when we left doors open, neighborhood cats would come in and eat our girls’ food. The previous owners must have been feeding them.

2

Look for an Appliance & Specialty Extension Cord.

(They (in my experience) always look like the above, right-angle plug and flat wire.)

3

Yes.

Heavy duty cords tell you the load they can carry-- just make sure it matches or surpasses the draw of the microwave. If the microwave has a shutoff built into the wall end to protect it from surges or overheating, then buy an extension cord with a breaker switch at the wall end, since the extension cord will defeat the built-in safety measure the oven comes with.

I needed a few more feet in the cable for level 3 charging of my electric car, and I found one online. Just looked for the numbers that matched the numbers on the cord that came with the car, and specified “breaker” in my search terms. The cable I bought is four feet long, because that’s all I needed, but there are longer ones. I don’t remember what it cost-- it was less than $20, but close to that, I think. For one 6-10 ft., I’m guessing $25.

4

When the outlet for our AC went out back in the day, I specifically bought a long extension cord that was rated for the highest used wattage, which is around 1500W. It was a godsend when we had to move to an apartment for a bit.

Just look for the wattage ratings for the particular cord. The ones I got were 100 ft and indoor/outdoor, so they weren’t flat. But they were designed for that load.

I have more problems with outlets wearing out than I do our extension cords.

5

I’m not al electrician, but this part confuses me.

IMO …

If their existing outlet is e.g. a GFCI, it protected the old downstream microwave, it will protect the new microwave if plugged in directly, and it will protect the new microwave if plugged in via a completely passive extension cord of suitable wattage / ampacity.

If instead what you mean is the case where the new microwave comes with a GFCI or overcurrent breaker built into its male plug, then …

A passive extension cord of suitable ampacity and the downstream oven will still be protected from any faults in the microwave itself or in its own cord. You’re correct that the extension cord won’t be protected from faults solely within the extension cord; that’ll be up to the building’s breaker / fuse. But as long as the extension cord’s capacity meets or exceeds the microwave’s I don’t understand how any protections are “defeated” by inserting a passive extension cord into the circuit.

What am I missing or misunderstanding?

6

That the length of the cord can defeat the fault-interrupt in the microwave’s cord. The extension can catch fire, and a surge can arc the connection between the two cords if they are not REALLY tight.

If the wall outlet has a breaker, see what it is rated-- it probably is above what the microwave needs, and you are fine. If it doesn’t, then you need to know whether the microwave cable has one. If it doesn’t, then one isn’t necessary-- just buy a cable that matches or surpasses the rating of the microwave.

Also, do NOT use a grounded cable to ungrounded outlet adapter. If you have both kinds of outlets in your place, the microwave needs to be plugged into the one that matches it. If you don’t have any grounded outlets at all, your landlord, or you, if you own, needs to install some.

7

Well, but, how? The interrupt was already dozens or maybe even hundreds of feet downstream from the panel, via the wiring in the wall, and so on. What has length got to do with it?

8

Way back in my youth when I was still renting, I had a fairly hefty window air conditioner and that was exactly the extension cable I used with it. Never had a problem. You can see the large size of the individual conductors, which is why it’s common to line them up in a flat configuration. The right-angle plug is because the big conductors are relatively stiff.

I’m pretty sure the reason that makers of microwaves, air conditioners, and other high-current appliances flatly say “no extension cords” is because there’s bound to be some idiot who thinks it’s just fine to use a light-duty lamp cord, so from a liability standpoint it’s useful to be able to point to the “no extension cords” warning when said idiot burns their house down.

9

An extension cord made with 12 gauge wire of reasonable length will handle up to 20 Amp load. This should be fine for a micro wave oven or A/C unit. Your outlet may likely be 15 Amp.

10

My first few days in my new (to me) home was spent doing this. The bedrooms had a LOT of three-prong outlets. The combined living/dining room and the kitchen had ONE EACH.

Fortunately, my outlet boxes were metal with metal casing for the wiring, so I didn’t need to get an electrician to install a ground wire, I just needed to swap out the outlets.

Anyway, it could prove to be a non-trivial task if whatever is installed can’t readily accommodate a ground outlet.

Disclaimer: I am not an electrician.

11

The OP says the kitchen was newly remodeled so I assume the microwave outlet is properly grounded. And I once used a high-voltage extension cord with a toaster oven that had a cord too short to reach the outlet. I had no problems doing so.

12

How long are those appliance cords? And can they be used outdoors?

My wife and I have an ongoing debate about how to wind our 100ft outdoor extension cord on the storage spool. She says when I start winding, I should leave about a foot of cord hanging off the end so that she doesn’t have to unwind the entire spool in order to plug it in. My problem is that if I leave a foot of cord hanging out, it keeps trying to whap whap me in the face as I’m trying to wind the rest of the cord onto the spool. I thought that a good compromise would be to look for a short outdoor cord that I could use between the spool and the outlet, but nobody makes outdoor cords in 6-8’ lengths. But something like that appliance cord looks like it could work.

13

You’re in luck, usually 6ft!
(I don’t see any real problem using them outdoors.)

14

True but not really relevant as according to modern building code, a circuit capable of supplying 20A must use one of several special receptacles that have either two horizontal blades, or one horizontal and one vertical. The latter is commonly found in newer kitchens, intended for high-current appliances, where one of the slots is the normal vertical configuration and the other is T-shaped, accommodating both normal and 20A plugs (I believe the latter is designated NEMA 5-20P). Many or most extension cords advertising 12 AWG and 20A capacity have those unique 20A plugs, so they’re useless in most normal applications.

I am definitely not an electrician , but if an appliance really does draw more than 15A, it would be really dangerous to replace the intended 15A breaker with a 20A one, because the house wiring on that circuit may well be 14 AWG and not the required 12 AWG. The classic symptom of wire gauge being too small for the current is it starts to overheat. That’s not something you want happening inside your walls.

That said, extension cords rated for 15A are usually 14 AWG, but occasionally 12 AWG. Sometimes they’re just 16 AWG, which is generally only safe for up to 13A.

15

It can also cause a non-trivial fire if you use the adapter when you shouldn’t.

If you check the outlet case behind the faceplate, and the whole thing is a ground, so that the screw that holds the faceplate in is a ground, and you put the screw through the flange on the adapter with the hole (for the screw), then the adapter is grounded. But it still has to be able to handle the load flowing through. If the package shows that it is too small, or doesn’t say, don’t use it.

Keep in mind, my caution about not using a non-grounded extension cord, was about having the adapter plugged in between the extension cord, and the appliance cord. I see now that this was not at all clear-- but I still do caution against using the adapters at the wall if the outlet cannot be grounded.

And yes, the main reason the instructions say not to do this is that people will use too light a cord, but there is another reason: people are prone to trip over long extension cords and pull appliances onto the ground.

Rather than say “If you trip over the correctly-rated extension cord that you have safely installed, and the microwave crashes on the ground STOP USING IT, even if it appears to be operating correctly. The seals may be slightly out of adjustment, and it may leak radiation,” companies just say “Don’t use an extension cord,” and make sure the power cord is short enough that there is no chance it will be one the ground.

16

This is not accurate. 20A circuits require a 20A receptacle only if the circuit has just one simplex receptacle. If the circuit has multiple receptacles (even a single standard duplex receptacle), 15A receptacles are perfectly legal on a 20A circuit. NEC 210.21(B)(3). 15A duplex receptacles must be capable of passing through 20A to be UL listed, and UL-listed devices with 15A plugs must be safe on a 20A circuit.

There is little point to putting 20A receptacles on general-purpose 20A circuits (like kitchens and bathrooms, which mandate 20A) because very few devices have 20A plugs.

17

This is true, and in fact I’d be hard-pressed to think of any kitchen appliances that use the special 20A receptacles. Nevertheless, it’s a fact that around here all kitchen outlets in newer houses use the new 20A receptacles, and that many extension cords advertised as 20A capable have the specialized plugs – like this one – which would be useless for any normal application.

18

It’s not obvious to me that the US NEC is identical to what’s required in Canada.

I’d not be surprised to find some corner-case differences.

Heck, the US NEC isn’t even universal law across the US: it’s a recommendation to state legislatures. Most of whom have followed it mostly.

19

I didn’t know how to ask for what I need. Thanks!

20

For the purposes of this discussion, that’s correct. The previous owners replaced most of the appliances in the kitchen, while we only had the wiring brought up to code, and had a few more outlets added on the far side of the counter, facing the dining area.