Length Table for Copper Wire


The table is based upon a 2 volt "drop" across the conductor.

Voltage length and current are linearly proportional. Half the current results in half the voltage drop, half the length results in half the voltage drop.

NEC limits consider temperature and are based on a steady loading of 80%. There are actually several tables and correction factors.

Do NOT use 10 feet current levels except for very short durations. Large amounts of heat will melt insulation and possible start a fire!



This table does NOT take into account the parallel current path of the rail. With large buss wires it is negligable.

18% Nickle-Silver has a conductivity of only 5.5% that of copper. (I don't know the alloy fo nickle-silver rail but believe that it is 64% copper, 18% zinc and 18% nickel). Brass has a conductivity 28% that of copper. At an approximate guess, code 100 rail would have a similar cross sectional area to AWG 16 wire. So it should carry only 5% of 5 amps over 100 feet, or 250 milliamps with a two volt drop. This is not enough to run even a single locomotive. Code 83 probably has a CSA close to that of 20 gauge. Now you know why we use a copper buss and multiple track feeders to large layouts.


What is the right size of wire for your layout?

From the table, it can be clearly seen that 16 gauge wire is sufficient for even a fifty foot length of track running several locomotives. Why then does everyone insist on using #10 or #12 busses? Perhaps they have very large layouts. (Mine has 100 foot long main lines with looped busses). Perhaps they run huge numbers of locomotives and lighted cars. Perhaps they don't like even a one or two volt drop to the furthest point.

At today's cost of over 25 cents a foot for a single strand of #12 THHN wire, careful consideration of your own needs is more important than adhereance to an overly cautious standard. What size do I use? #12 looped for the 100 foot main lines, #14 looped for the 110 foot mountain line (fewer engines), and #16 to the reverse loop sections. Everything works just fine. the layout would work just as well with one size smaller wire. Luckily I had several spools of "pre-war" copper sitting in the shop.

Wire size vs Maximum Current Capacity


AWG
 Diameter in Inches Ohms per 1000 feet Theoretical Maximum
Amps at 100 feet		 Theoretical Maximum
Amps at 10 feet		 National Electrical
Code Limits*
2
 0.2576
 0.1563
 128.0
 1279.6
 115
3
 0.2294
 0.1970
 101.5
 1015.2
4
 0.2043
 0.2485
 80.5
 804.8
 85
6
 0.1620
 0.3951
 50.6
 506.2
 65
8
 0.1285
 0.6282
 31.8
 318.4
 50
10
 0.1019
 0.9989
 20.0
 200.2
 30
12
 0.0808
 1.588
 12.6
 125.9
 20
14
 0.0641
 2.525
 7.9
 79.2
 15
16
 0.0508
 4.016
 5.0
 49.8
 10
18
 0.0403
 6.385
 3.1
 31.3
20
 0.0320
 10.15
 2.0
 19.7
22
 0.0254
 16.14
 1.24
 12.4
24
 0.0201
 25.67
 0.78
 7.8
26
 0.0159
 40.81
 0.49
 4.9
28
 0.0126
 64.90
 0.31
 3.1
30
 0.0100
 103.2
 0.19
 1.9

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