Over the years, I have attended many electrical inspector meetings, served on their Code Panels and gave presentations. There were always "loads" (no pun intended) of questions about conductors. Seems as though everyone enjoyed the discussions outside of the regular meeting sessions. Coffee breaks, etc. You know the routine. Many times the discussions ventured beyond what is found in the National Electrical Code^{®}.
Those of us in the electrical industry are pretty familiar with what a conductor looks like. They are either solid or stranded... insulated or bare. Stranded is more flexible than solid, and easier to work with. You know what the insulation is for. But there's more to it than that.
In my last blog, we discussed how each "layer" of conductor stranding has 6 more strands than the "layer" that it surrounds. This is referred to as concentric stranding.
Manufacturers also use compact stranding. Run through dies, the conductors are compressed. The spaces between conductor strands are eliminated. The advantage of this is that for a given circular mil area, the overall physical cross-sectional area is reduced as compared to conventional concentric stranding. See Table 5A in Chapter 9 of the NEC^{®}. There are other variations of conductor stranding. The conductor might have a larger size center conductor. This can result in consecutive layers having 7, 8, 9, or more strands. For example:
What follows are a few more interesting facts about conductors that make life a little more enjoyable.
We find in the NEC^{®}:
110.6 Conductor Sizes. Conductor sizes are expressed in American Wire Gage (AWG) or in circular mils.
Conductor sizes found in Chapter 9, Table 8 of the National Electrical Code^{®} start with 18 AWG (the smallest size) and end up at 2000 circular mils (the largest size). Small sizes of conductors actually start long before 18 AWG, even though we do not find these in the NEC^{®}. Most conductor tables start at 40 AWG. That's pretty small! Small conductors are found in extension cords, bell wire, thermostat wire, and the windings of solenoids, coils, relays, small transformers, and small motor windings.
The smaller the AWG number, the larger the conductor. Example: An 8 AWG conductor is larger than a 14 AWG conductor.
On the other hand, the larger the AWG number, the smaller the conductor. Example: A 12 AWG conductor is smaller than a 6 AWG conductor.
In Table 8, after we reach 1 AWG, the next four sizes have a different numbering system: 1/0... 2/0... 3/0... 4/0. These are sometimes shown as 0, 00, 000, and 0000. We pronounce these sizes as "one ought"... "two ought"... "three ought"... and "four ought."
Following 4/0, conductor sizes are shown in their actual cross-sectional area in circular mils. Examples: 250 kcmils (250,000 circular mils)... 500 kcmils (500,000 circular mils)... and so on. The letter "k" in the metric world means 1000. Quite a few Code cycles ago, the callout was MCM, where the M was the old Roman numeral that represented 1000 and the CM meant circular mils. About the only time we see Roman numerals these days is for the Super Bowl game.
There are some interesting relationships between conductor sizes. Bear in mind that the examples and calculations that follow depend upon how far you wish to go beyond the decimal point. Conductor circular mil areas found in the NEC^{®} have been slightly rounded off for simplicity. The examples that follow are accurate enough for what we are trying to accomplish. As we said before, we are not trying to be rocket scientists. We're just trying to enjoy life and get a better understanding of "things" we work with every day.
Relationship One: Not common knowledge is that AWG sizes are successive. Table 8, Chapter 9, of the National Electrical Code^{®} does not show all possible sizes.
Let's have a look. In the following table, all numbers are listed in succession. Note that AWG 17, 15, 13, 11, 9, 7, and 5 are shown, yet these sizes are not found in Chapter 9, Table 8 of the NEC^{®}. If you were to look at a conductor table starting with AWG 40, you would find all numbers listed. i.e. 40, 39, 38, 37.......... 4, 3, 2, 1. The missing numbers are important to understanding conductor sizing.
AWG
CIRCULAR MIL AREA
18
1620
17
2048
16
2580
15
3257
14
4110
13
5178
12
6530
11
8234
10
10380
9
13,090
8
16510
7
20,820
6
26240
5
33,100
4
41740
3
52,630
2
66360
1
83690
Relationship Two: For wire sizes through 4/0, every consecutive wire size is approximately 1.26 times larger or smaller than the preceding wire size.
Example: A 3 AWG conductor is approximately 1.26 times larger than a 4 AWG conductor:
41,740 circular mils x 1.26 = 52,592 circular mils
Example: A 2 AWG conductor is approximately 1.26 times smaller than a 1 AWG conductor:
83,690 circular mils ¸ 1.26 = 66,420 circular mils.
Relationship Three: For wire sizes through 4/0, every third size approximately doubles or halves in circular mil area. The same relationship holds true for the weight of the conductor.
52620
0
105600
00
133100
000
167800
0000
211600
Example: A 1 AWG conductor is approximately 2 times larger than a 4 AWG conductor:
Counting the conductors from 1 AWG, the 1st is 2 AWG... the 2nd is 3 AWG... the 3rd is 4 AWG.
83,690 circular mils versus 41,470 circular mils.
Let's prove this by applying the 1.26 relationship.
(That's close enough to 2 times for our purpose.)
Example: A 1/0 conductor is approximately one-half the size of 4/0 conductor:
Counting the conductors from 4/0 AWG, the 1st is 3/0 AWG... the 2nd is 2/0 AWG... the 3rd is 1/0 AWG.
105,600 circular mils versus 211,600 circular mils.
(That's close enough to one-half for our purpose.)
Some important NEC^{®} references are 110.14 and Tables 5A, 8, 9, and 10 in Chapter 9.
Want to learn more about conductors? Check out the companies that make wire. To name a few... www.okonite.com and www.southwire.com. Southwire bought American Insulated Wire Corporation in 2010.
Having fun yet? More to come next time.