Are power cables under rated and circuit/load breakers question?

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canidoit

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Are home power cables under rated. Eg. If you have power cables in a house that is rated at 10amps, is that really the maximum of its loading capabilities, or is it like 15 amps but the companies just say its 10 as a safety margin?

Also, is this true, that the circuit breakers or safety switch will shut off first before power cables could burn if the cables capacity is too low than the load that is running.
eg. 10 amps cable, and there is load that is greater than it that could make the power cable hot. A device in the power cabinet(safety switch, circuit breaker, something??) of a house will shut the power off before the power cable gets too hot to the point of damage??

I am not familiar with the device names in the power cabinet, but if you know what their called that support this argument, please tell me.

Thanks.
 
It seems like a simple question, but boy it's complicated!  If you asked this question on the electrician's rules forum it could get hundreds of posts.

From one point of view, the codes add a huge safety factor. A 15 Amp wire will handle 29 Amps for 3 hours safely. But remember to reduce the rating if there are a large number of wires in a small conduit or if the wires are in a hot area.

In another situation, a clock with a very small cord is permitted to be plugged into a 20 Amp circuit. Only when the clock's cord totally fails and shorts will the 20 Amp circuit's fuse blow.
 
That depends on what you mean by under rated?

It all comes down to temperature rise and voltage drop due to resistive losses.

Very small cross section area wire strands, like inside fuses, literally melt when they over heat and I recall in a crunch making  replacement fuses from single strands pulled from multi strand wire.  When troubleshooting TO-3 power devices it was not unusual to see the fine base wire melted inside the can from too much current.

At temperatures below the wire actually melting, the next concerns are integrity of the wire insulation jacketing, and ignition of flammable materials near or in contact with wiring.

Wiring is sized to deliver power with reasonable power wasted to heat and not threaten human safety by causing fires. Breakers or fuses in electrical panels are designed to remove the mains voltage if it detects too much current draw.

JR


 
In the UK, it has been the custom to use extra high amps in the wall, and fuses IN the plug to protect smaller cords. The cord on Andy's UK laptop was frighteningly small (to my US eyes), but plug-fused at 0.63A it was quite safe.

In the USA..... 10A circuits are a distant memory, 15A was common for lighting and incidental circuits, but then 20A was mandated for kitchen appliances and it became simpler to use 20A everywhere.

Remember it is not a "20 Amp" circuit. Not until the fuse blows. The dead-short current is typically 70 to 700 Amps.

A dead-short should(*) blow the fuse real fast. At 700A, a single cycle. At 70A, a few seconds.

Experience (mostly lab) says that SPT-2 lamp cord on 20A fuse will get very hot but blows the fuse before anything catches fire.

You should be using PC-type cord for most stuff. Sold in the USA it is good for 10 13 or 15 Amps continuously, and will reliably blow 15A 20A breakers.

(*) 500 feet of #18 is 8 ohms. (Note that you could not pull even 1.5 Amps without serious voltage drop; but it would be electrically fine for a few holiday lamps at the street.) At 120V, the dead-short current is 15 Amps. A 20A even 15A fuse will never blow, but the cord is well over its continuous rating. At 3.6 Watts per foot, it sure will be hot. 0.5 Watts per square inch will be about 50 deg C temp rise.
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> It all comes down to temperature rise and voltage drop due to resistive losses

You'd think. Although temp-rise is a dubious thing. It's never about the copper, insulation is everything. Is 65 deg C for 10 seconds OK? How about 50 deg C rise for 100,000 hours?

In fact the UL and NEC tables have thin underpinnings. The 1987 specs are a slight improvement on the older work (which was maybe-valid for Code-grade rubber in black iron pipe, utterly obsolete). The theory (from 1957!) is very tedious, 67 equations in 88 terms. The simplified 1987 specs had to cover 35 new kinds of insulation. Many of the "better" insulations can't take advantage of high temp limit because devices (especially wall outlets) are not rated to run hot (most wall duplexes are all thermoplastic; one brand is thermoset back with tp front). Cord connectors are tested for pull in several ways, but not for heat.
 
Now for a little TMI, some of us remember when aluminum wiring was used as a cheap replacement for copper wiring in low cost housing (think trailer parks). Aluminum had a nasty habit of either oxidizing or getting loose around screw terminals (perhaps due to thermal cycling). When the resistance of the wire connection at a light switch or outlet became significant, even the modest current from a light circuit could cause enough IxR temperature rise to ignite the cheesy tinder boxes.

JR
 
For a very clear explanation of the 1960s-1970s aluminum AC power wiring situation, see this Wikipedia page.

http://en.wikipedia.org/wiki/Aluminum_wire
 
> I am not familiar with the device names in the power cabinet

"Fuses".

In fact they have not been fuses in a long time. Circuit Breakers arrived in 1922 and became common in new-work by 1960. But a circuit-breaker is (mostly) a resettable fuse.

In the UK they have a LOT of very strange names for the things that they put where fuses/breakers go. The overload-blowout function is almost incidental to the additional protections available. You probably have some of these. Names like Residual Current, only abbreviated.

> rated at 10amps, is that really the maximum of its loading capabilities, or is it like 15 amps but the companies just say its 10 as a safety margin?

Does it matter what they say? Nobody (except a few here) could ever add-up the actual load.

"Maximum"? Wires and fuses don't drop dead instantly on 101% load.

"In General".....  the electric business picks a reasonable size of load for one circuit. A couple thousand Watts is good. Much smaller means too many circuits, too much labor. Much larger, and the whole house goes dark when one fuse pops.

With the local voltage, this gives the Amps.

They design a fuse or breaker which will carry that load but not much more. You always have to allow some extra because lamps and motors draw a starting surge. A few-Second surge does not heat the wire right away.

A "20A" breaker will pass 200A for 1 second, 100A for 10 seconds, 30A for 100 seconds, and 20A for over 1,000 seconds (~~15 minutes). It will blow instantly (0.025 seconds) at 300 Amps. (Other time-curves are available.)

By test or guess they pick a wire-size which will handle these currents "safely". The raw copper in open air can handle a lot of current, it is the rubber (now plastic) which is the limit, influenced by conductors per cable and conduit-fill and connector ratings.

This is usually conservative. Hot wires is wasted power. It is generally better to use a plenty big-enough wire. It wastes less, runs cooler. In the US, #12 wire can be rated 25A, but in general outlet use it is always fused at 20A. #12 wire can carry over 900A for some seconds. Likewise #14 will carry 20A steady at 60C, but must be fused at 15A.

So yes. If you plugged 13 Amps into a "10A" circuit, it will work, for a while. Possibly minutes.

They used to do this at work. One heater or tea-pot is 13 Amps. Two such loads is 26 Amps. On a 20A breaker, you can often get toasty toes and a pot of tea before the breaker blows. Or maybe not. They got good at running to the breaker box and re-setting for more abuse.

Constantly re-setting the breakers to hold an excessive load WILL melt the insulation and burn your house down. May take a while, but you really shouldn't.

There is a special problem in the US. Older wiring was mostly #14, which should be protected at 15A. It is too easy to change the breaker to 20A. Now instead of tripping the breaker, the wire runs hot for long periods of time. We often find old rubber-coat #14 toasted to death.
 

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