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#21709 - 02/08/03 02:26 PM Fairy Tale
electric-ed Offline

Registered: 07/08/02
Posts: 184
Loc: Canada
I have had many questions about the origin of the so-called "80% rule"

Here is my understanding of it’s history, in the form of a fairy tale.

Once upon a time, long ago and far away, when a loyal subject purchased a piece of electrical equipment, it would carry its rated current, indefinitely.
A 100 amp fused switch, or circuit breaker, would carry 100 amps.
Even when a panel was full of circuit breakers, each breaker would safely carry its rated current.

Then the wicked equipment engineers got their little heads together and said to each other "Many times the load will not be continuous." "Why can’t we cut back on the quality here and there to save on the cost of production." "We could make more money."

And so they did.

Soon, there were cases where the load actually was continuous, and the result was overheated panelboards and other equipment, resulting in fires. The engineers said "What can we do now".

Then one of them had a bright idea. "Let's get the king to make a new rule." "When the load is continuous, the load must be reduced to 80% of the circuit rating." "That way, we can continue to sell a 80 amp breaker as a 100 amp breaker."

The king did as they suggested, and they all lived happily ever after.


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#21710 - 02/08/03 02:45 PM Re: Fairy Tale
sparky Offline

Registered: 10/18/00
Posts: 5545
best bedtime story a sparky could ask for Ed

[This message has been edited by sparky (edited 02-08-2003).]

#21711 - 02/08/03 04:06 PM Re: Fairy Tale
CTwireman Offline

Registered: 02/07/02
Posts: 839
Loc: Connecticut, USA
LOL! That's great! I'll remember that one if I have kids someday!

#21712 - 02/08/03 04:07 PM Re: Fairy Tale
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
I like the fairy story approach!

So when was the 80% rule introduced?

#21713 - 02/21/03 11:43 AM Re: Fairy Tale
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
May I suggest a different version of this fairy tale?

Conductors ratings were and still are for the continuous load. A breaker would carry it's rated current with some margin forever. There was just one little problem: Thermal breakers act on heat. When you put a lot of breakers side by side, the breakers warm each other. Nice and cosy, but this made them trip at just over 80% of the load they are rated for. It was expensive making breakers that understood to compensate for the warm and cozy environment, so instead an 80% rule was introduced in the code.

Europeans who have always preferred undersizing hid the information about the shortcomings of breakers in the back in the breaker's information sheet. As long as there are "cold" breakers between the "warm", it doesn't matter.

Now there are temperature compensating breakers which means that 20A is always 20A. In the US you can sell them as 100% rated breakers.

#21714 - 02/24/03 04:21 PM Re: Fairy Tale
pwood Offline

Registered: 02/04/03
Posts: 64
Loc: castella,calif usa
the 1901 nec has the 80% rule.class d, article 53,(b)states"must be stamped with about eighty percent of the maximum current they can carry indefinately,thus allowing about twenty-five percent overload before fuse melts."
a fine print note states that the maximum current which will melt them in about five minutes may be taken safely as the melting point,as the fuse practically reaches its maximum temperature in this had to ask!


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