Arbitrary Numbers?

[bphgravity]

I am trying to understand the reasoning and rational of certain requirements within the NEC. Does anyone know the history or particular substantiation for these values?

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• <font size="2" face="Verdana, Helvetica, sans-serif">Article 225.33(A) and 230.71(A) - maximum number of disconnects at 6.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Article 404.8(A) - maximum height of switchs at 6 ft. 7 in.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Article 408.15 - maximum of 42 overcurrent devices</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Article 550.10(D) - Mobile Home cord lengths not less than 21 ft. and not to exceed 36.5 ft.</font>

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If you guys are aware of any others, please list and comment. Thanks!

 

[Guest]

Re: Arbitrary Numbers?

1. They had to pick a number. More than six and it takes too long to flip all the handles in an emergency.

2. Maximum height of switches: To keep them within reach.

3. To prevent overload of the bussing.

4. That's what works.

 

[pierre]

Re: Arbitrary Numbers?

408.15 was installed as a requirement after a large hotel in NYC had a devastating fire due to overfusing of the bus. The CMP decided to limit the bus to 42 circuits.

Pierre

 

[charlie]

Re: Arbitrary Numbers?

A lot of times, the numbers are indeed arbitrary when a section is first proposed. The person that proposed the change asked for a number that he wanted that would do the job for him. After the change was in the Code, other people wanted it to be a larger or smaller number because of needing more utility or it was too dangerous the way it was written. Over time, a number was settled upon that was palatable for nearly all concerned.

A current example is the evolution of the GFCI receptacle requirements. In 1975, it was required for the bathrooms and outside. In 1978, bathroom was defined because no one knew what a bathroom was anymore. At each Code cycle, proposals were made to expand the use. The 2008 Code will require them at laundry sinks. Generally, they are required wherever water is present.

 

[al hildenbrand]

Re: Arbitrary Numbers?

LOL

In 1978, bathroom was defined because no one knew what a bathroom was anymore.

Ain't language a be-a-u-tiful thang?!!!

There's always room for more.

 

[charlie b]

Re: Arbitrary Numbers?

It?s like the ?Ham Bone? story. A man notices that before his wife puts a ham into the pan, she cuts off the last three inches of the ham bone, and throws it away. He asks why. She says that that is the way her mother did it. So they decide to call her mother. The mother says that that is the way her mother did it. So they decide to call the grandmother. The grandmother says that that type of ham was always to big for her small pan.

Sometimes a present-day requirement goes back so far into antiquity that its basis is long forgotten. I once heard, for instance, that the distance between the rails on all modern-day passenger and freight train lines was initially selected on the basis of having two horses walking side-by-side while pulling a cart along the rails. Also, long ago, someone figured out, on average, how much work a horse is capable of doing (i.e., ?work? in the sense of energy expended per unit of time). When they translated that amount of work into the American system of units, the result was that one ?horsepower? has the definition of ?550 foot-pounds per second.?

 

[charlie b]

Re: Arbitrary Numbers?

Originally posted by al hildenbrand: Ain't language a be-a-u-tiful thang?!!!

Only in such a strange language as ?English? could you transport a package via a car, and call it a ?shipment,? but if you transport it on a ship, it becomes ?cargo.?

 

[amptech]

Re: Arbitrary Numbers?

I can't resist one more of these tidbits of "standards". The gate man in the guard shack at a steelmill had the responsibility of blowing the noon whistle every day. He was training a new guard and instructed the trainee to call the bank every day at 11:45AM to make sure his clock was correct before blowing the whistle at noon. The trainee called the bank the next day at 11:45 to check his clock and asked the lady at the bank, just out of curiosity, how she verified the accuracy of her clock. "I go by the noon whistle at the steelmill", she replied.

 

[eprice]

Re: Arbitrary Numbers?

Originally posted by charlie b:
...I once heard, for instance, that the distance between the rails on all modern-day passenger and freight train lines was initially selected on the basis of having two horses walking side-by-side while pulling a cart along the rails...

I heard, and I can't vouch for the story's accuracy, that roman war chariots were built with wheels just wide enough apart to allow two war horses' patoots between them. Early british roads were rutted by the wheels of roman war chariots. Later wagons and carts were built with wheel spacing to match the ruts because doing otherwise resulted in broken wheels and axels. The same gage was used for railroad cars. This same gage was brought to America by settlers from england. railway tunnels are built just barely wider than the rails. The designers of the space shuttle boosters would have liked to make the boosters a bit larger in diameter, but since they were to be made in Utah and transported by rail through the railway tunnels, they were constrained to a size that would fit. In other words, the standard determining the width of the shuttle boosters is the width of two horses' patoots

 

[mule821]

Re: Arbitrary Numbers?

Originally posted by charlie b:

Originally posted by al hildenbrand: Ain't language a be-a-u-tiful thang?!!!

Only in such a strange language as ?English? could you transport a package via a car, and call it a ?shipment,? but if you transport it on a ship, it becomes ?cargo.?

Another, We drive on parkways & park on driveways.

 

[steve66]

Re: Arbitrary Numbers?

eprice:

I have heard a similar story before, only it claimed that the distance between railroad rails are based on the width of two horses rumps. Being the skeptic I am, I measured the distance between the rails. If I remember correctly, it was exactly 5 or 6 feet. I considered it proof that the story was bunk.

Most people take anything that winds up in their inbox as fact. Of course, NEC forum members are smarter than that

 

[peter]

Re: Arbitrary Numbers?

Steve,
"it was exactly 5 or 6 feet". The distance between rails should be 4' 8 1/" or 2.942 cubits.
[A cubit is 19.2" and is marked on some tape measures with little black diamonds.]
~Peter

 

[stew]

Re: Arbitrary Numbers?

Peter. Of course I was curious. My tape has little black dots every cubit no BS. That makes a stick of conduit 6.25 cubits to be exact. golllleeee

[ May 13, 2004, 09:18 PM: Message edited by: stew ]

 

[ryan_618]

Re: Arbitrary Numbers?

Stew: 19 3/16 is a very common on-center spacing for rafters and joists. Works really nice if you are overspanned at 24", but don't want to spend the money going to 16" or 12".

 

[bennie]

Re: Arbitrary Numbers?

Maximum number of disconnects;
Six was established for continuity of service.The number is not for turning power off , it is for keeping power on.

The phrase "6 movements of the handle, is to emphasise that six trip devices are permitted.

If a handle tie comes off are you going to leave one single pole switch turned on?

The six mains are typically, Range, Water well,water heater,furnace, a-c unit, lighting main,and dryer.

The six mains were primariy for residential wiring methods.

Six foot, seven inches, Maximum switch height fo loadcenter.

6'7", is close to 2 metres. 6'7'', is near the maximum reach of an average adult person.

42 space panels will occupy a minimum of 24 inches for branch breakers, plus 8 inches on top, and 6 inches on the bottom. This leaves approx. one and a half foot at both the top and bottom. This makes uniform Logical spacing for panelboards.

A mobile home not for occupancy requires no specal methods for wiring.

An occupied mobile home must have special cable and the length for available fault current containment. The maximum length is to insure short and ground faults will open the device.

Now tell me where did a 4 and 11/16" box came from?

[ May 13, 2004, 10:58 PM: Message edited by: bennie ]