Replies

1. 
   DanH | Jul 22, 2007 12:47am | #1

   Because they don't have #9 and #11?

   So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
2. 
   MSA1 | Jul 22, 2007 03:25am | #2

   Cause #10 is Orange and everything goes with orange. :>)

3. 
   pm22 | Jul 22, 2007 06:48am | #3

   It would be easier if you could give the Table or Article number of this chart. I seem to recall that #8 is used for a lot of these sizes instead of #10.

   In general, the NEC is a rather rough approximation of physical reality. It is written by committee and enforced by bureaucrats. They take a conservative view on these things. That is, if you follow the Code, then you are pretty certain that you will not have fires or electrocutions. However it is not a design manual and is often treated as a maximum requirement.

   The EGC is green [or bare copper] and not orange. Thus it is eco-friendly. #9 and #11 are rare but may be used in motors. Incidently, the numbering system derives from the number of times the wire is drawn through the dies, each die being progressively smaller.

   The purpose of the Equipment Grounding Conductor [should be bonding?] is mainly to carry abnormal currents, especially short circuits. For instance, a #10 wire can handle 1000 amps for several seconds. This is enough to cause the instantaneous trip section of the breaker to trip with-in one cycle or 1/60th of a second.

   ~Peter

   Summer is the season of fleas, flies and mosquitos.

1. 
   tab1 | Jul 22, 2007 11:16am | #4

   The table is 250-122.If I add a column of cmil areas you get:------ #18 -- 1620
   ------ #16 -- 2580
   15A - #14 -- 4110
   20A - #12 -- 6530
   30A - #10 - 10,380
   40A - #10
   60A - #10
   100A - #8 - 16,500
   200A - #6 - 26,240
   300A - #4 - 41,7--
   400A - #3 - 52,6--
   500A - #2 - 66,3--
   600A - #1 - 83, 7--That would have led me to think that, for instance, a 30A circuit would need 33% of what a 100A circuit would, i.e. 5,500 cmils, or #12 AWG, not #10.200 through 600A numbers are all fairly proportional, while 100A and below, aren't?Thanks.

1. 
   BillHartmann | Jul 22, 2007 12:09pm | #5

   I suspect that a couple of things come into play here.One these ratings (as are all current ratings) come down to I**2t (and other factors). That is the amount of heating is proportion to the square of the current and time for any given wire.Assuming that the larger breakers trip in the same amount of time as smaller ones the corresponding larger wires have more mass and thus heat up less proportionally.Also the disconntinuity of in the smaller wire sizes probably also is related to strange way that #14,12, and 10 wire is rated.If you look at table 310.16 you will see that those wires have an ampacity larger than you can protect them with over current protection.Why that is is another discussion..
   .
   A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

2. 
   DanH | Jul 22, 2007 02:53pm | #6

   For larger circuits the EGC is undersized (on the assumption that it only handles brief fault currents). For physical strength and because a continuous fault current is more likely (and also because the savings in copper of a smaller wire wouldn't amount to much), smaller circuits have an EGC consistent with the other conductors.
   So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin

4. 
   renosteinke | Jul 22, 2007 05:09pm | #7

   The "ground" wire is sized only to carry current for a very brief period - just until the breaker trips. The other wires are sized to carry this same current, over an extended period, without getting so hot as to damage the insulation.

   The net effect of this is that the 'ground' wire is allowed to be smaller for the larger circuits.

   Please note: there are some special circumstances where other requirements apply to the "ground" wire. In some cases it must be 'full size,' and in other it must be insulated. A good example is a hot tub; you are not allowed to feed the tub with Romex, because Romex has a reduced, and bare, ground wire- both problems for the application.