Hello guys,

I need a quick help in understanding from you as I'm in the process of assembling a machine for use in the US.

NEMA 5-15 Outlet with 15Amp breaker - OK.
NEMA 5-20 Outlet with 20Amp breaker accepts both 15Amp and 20Amp plugs.
If now a 5-15 extension cord with multiple outlets is inserted into the 5-20 Outlet, can't the cord be overloaded? Because the breaker rating is too high?

Thanks,
Andy

In theory (and the real world), yes the cord could be overloaded, based on the rating of the cord.

Does this happen? Yes.

Any issues?? Warm/hot cord; deterioration of the cord over time.

The NEC does not address what gets plugged into the 5-15 or 5-20, or any other devices, and for all intensive purposes we stop at the device.

I think we all have stories of what the customer has plugged into outlets after we've left. That's why you're only responsible up to and including the device.
Greg

Just figured that out, did you?

Now, take a look at the cords with multiple outlets - cords that are only #16 wire ... even a single 15 amp appliance has that cord overloaded.

Look inside one of those outlet strips with integral 15 amp breakers, and you'll find the internal wire is best described as 'dental floss gauge.'

If household wiring followed the same voltage drop advice as we are given for using extension cords, we'd wire houses in #8.

When reality conflicts with theory ... and everything is working fine ... it's the theory, the basic assumptions, that need to be reconsidered.

When I mentioned what customers plug in, I had a specific case in mind. Home owner had 3 outlets in a small room and wanted a 4th. Not to worry, they bought a 6', #16 extension cord and stapled it to the wall, and plugged in a floor lamp. Then they wanted a closet light. Not to worry, they bought a 2nd #16 extension cord and plugged it into the first, drilled a hole into the closet and again stapled everything in place. Oops, how to connect the light (pull chain fixture)? They cut the 2nd extension cord and hardwired the fixture, and mounted the fixture directly to an exposed wood beam (who needs a box). They told me "it's only 2 lights so it's not overloaded". I was speechless.

OK, time for a stroll down Memory Lane...
Back in time installation of 'outlets' using zip cord, stapled to baseboards and thru walls was common. Matter of fact, Eagle/Leviton made 'devices' for 'surface wiring'.

The device was wired; forget polarity & that 'ground', Screwed to the baseboard, and extended where 'outlets' were wanted. Some installs were 'tapped' off of a duplex, not plugged in.

Light fixtures were installed like Sedesigner1 describes.

Those were the days!! ??

This was the closet light in my daughter's new house when I first saw it. The "home inspector" either missed it or thought it was fine. No switch, you just plugged the cord to an extension cord through the wall from the other room, hanging by the door.
(yeah I fixed it)

Nice! I especially like the cable support and service loop. Is that a water pipe at lower right? Even better!

Oh yeah, That goes to the water heater

I guess you can plug in a water heater?

Not legally but there was not much legal in that house when I first looked at it.
Note the Romex going into that cap too.

This "light kit" in the fan was a classic too.

Isn't that actually the gas pipe coming up from the floor on the right? Or is that hot, cold and recirculation?

Would it be legal to plug in the water heater if you used a cord with proper strain relief on the water heater? Technically I can't see any reason not to, but code isn't always strictly about technical issues - technically the electrons wouldn't care if the hot wire were green.

Tex:
That is from a 'blow-off' valve to relieve water pressure in the heater. The pipe going down extends any discharge down to the floor

My earlier, flippant response failed to recognize one important thing: The OP is right.

Indeed, for nearly every sort of circuit his reasoning is absolutely correct, and the NEC is designed specifically to prevent such situations from arising. With, of course, one exception .... this specific situation.

That is, the household 'convenience circuit' is the only circuit where the demands of the applied loads are not considered. Indeed, we are exempted from even using a token load value in deciding how many receptacles to place on one circuit.

Nope. It's 'keep adding receptacles' and common sense be cast aside.

This attitude seems based upon a model that uses a completely different set of assumptions. We assume that, since we cannot know what will be used, or where it will be, that we can rely upon chance and the breaker to give adequate protection.

Let me use a little project of mine to illustrate the point:

I've been using a "Kill-A-Watt" recently to see just how much current various household appliances actually draw. (I'll start a thread when I have more data). So far, my 'ordinary' kitchen appliances are showing current draws from 2 amps (the rice cooker) to 15+ amps (the microwave). All of these appliances have standard "15 amp" plugs.

If we applied the usual NEC rules, neither extreme would be allowed on a 15-amp circuit. We would require a much smaller fuse to protect the rice cooker, and the microwave is clearly supposed to be on a 20-amp circuit.

While the NEC has made some changes that reduce the impact of these facts - kitchens are required to have at least two 20-amp circuits, the changes do not affect the underlying assumptions.

We simply don't size receptacle circuits the same way we size everything else. It has nothing to do with whatever silly things we have seen in the past.

That's why the OP became confused.

OK, let's try to get back on track...
The OP is in Germany, designing a machine for the USA.

He has not made any comments to any of the responses here, as of yet.

WE all have to remember that the NEC is not a design tool, but the minimum (or maximum, in some cases).

To the 'microwave' comment from Reno...is it a dedicated circuit for the micro, on #12 & a 20 amp OCP, with a single 20 amp, 120 volt receptacle??

Greg:
I saw the NM going to the cord cap, and bit my tongue on that.

I think we beat that up in another thread, along with the fan light.

Dedicated circuit? Who knows? Who cares?

First, let's be sure we understand the source of my number. I took a new, ordinary, "1000 Watt" microwave, plugged it into the Kill-A-Watt, then plugged the meter in to a convenient receptacle. Put a cup of tea in the micro, push the 'cook' button, and take the reading as it ran.

I just repeated the test, and read 14.86 amps at 119,8 volts. Note that I (still) have no idea as to what the nameplate reads.

There's no code requirement for a dedicated circuit, or a 20-amp circuit, for this microwave. Anyone can plug it in anywhere. John, you're making the same error as the OP.

More germane to my post is that all those code rules about 'plug connected loads' go right out the window for convenience circuits. The rules apply to the circuit for your clothes dryer but not for any convenience circuit. Only when the circuit is dedicated to a use do they apply.

This, of course, is necessary, as we have no way of knowing what will actually go into the receptacle. Thus the NEC has us run convenience circuits in the same manner as a farmer feeds his chickens ... a few over here, some over there, and hope for the best. Lots of chickens, we say he can use a larger "20 amp" handfull

Add to this the various 'engineering assumptions' of the rating labs, and that 15-a plug can very well be deliberately overloaded for brief periods. It's a safe bet that requiring 20-amp plugs on the most common microwave size would very quickly result in the 1000-watt microwave becoming extinct.

Reno:
My error is that I thought you were talking about a 'fixed' micro, the over the range type. Sorry for that!!

Yes, 100% on people plugging in 'appliances' and reaching or exceeding the circuit capacity.

The discrepancy between the nameplate info, and your actual reading may be a marketing move by the mfg. Single items cannot exceed a percentage of the circuit capacity, without being factory marked that a dedicated circuit is required.
(think window air conditioners)

Ah, thought the fittings didn't really look like gas!

In case of the rice cooker the rationale is probably similar to what we have in Europe - if the appliance needs a fuse, it has to be built into the appliance itself and if it doesn't the appliance will limit the current under normal operating conditions in order to protect the cord. The household breaker/fuse only needs to protect the cord in case of a short. That's why you can connect a table lamp with 0.75 mm2 flex and Euro plug to a 230 V/16 A circuit.

Of course power strips with Euro plugs are illegal because they could draw well over 2.5 A from a socket only designed for that current.

Doesn't work in the UK since 32 A isn't suitable for short-circuit protection of 0.75 mm2 and BS1363 power strips aren't designed for 32 A.

The NEC deals with that in the sections that deal with cords and fixture wires.

The basic rule is that 18ga (0.823 mm2) is big enough to operate the 15 or 20a overcurrent device with a bolted fault. The cord still has to be matched to the load as part of the listing.

Just think about a blender with a 14 or 16 ga cord plugged into a 20 amp kitchen outlet. Same difference and legal.

Tex, Remember in the UK the BS1363 power strip is fed through a fused plug - fuse is usually 13A but occasionally less - the one feeding my computer right now says 10A. That light's fine cable would also be fed through a BS1363 plug fitted (hopefully!)with a 3A - or less- fuse.

I think that was actually my point, you need the fuse in the plug not only for overload protection but also for short-circuit protection of the appliance flex and power strip. In case of a Euro table lamp or similar device you still don't need a fuse in the plug, even though the flex isn't suitable for carrying 16 amps at all.

Guys, thank you very much for all your responses. I must admit that after reading the first few answers some time ago, and seeing that it's an unclear situation, i forgot about this thread.
I am surprised it's really like that.
So you can plug a 15 amp rated power strip into a 20 amp outlet, plug in some devices and have it running at 20 amp summarized load with breaker not moving and the wire overloaded by 33% ??
For the machine (designed to be ran on a nema 5-15 or 5-20 outlet), we decided to use #14 wire and put a 15 amp fuse right at our infeed, so we protect the cord even when plugged into a 20 amp.

But doesn't this topic lead to melting cheap power strips used on 20 amp outlets all the time? What's the US standard gauge for an average 5 or 6way residental power strip?

And what is your maximum permitted current for a non-fixed appliance on 5-15 (for example a vacuum cleaner, electric bbq or whatever)??

We also do have power strips with thermal problems, but usually they only melt when constantly overloaded. Our standard outlet is 16 amp, but only few people know it's officially "16 amp intermittent, 10 amp permanent" so they use a power strip to run two 2000 watt appliances, which is just under 20 amp and might never trip the 16a breaker.

This is a flaw in the listing of the power strip, not a problem with the receptacle you plugged it into.
Most will have a breaker in them and in a commercial installation it is required. Most places will have a fire marshal/life safety officer, enforcing that.