Dispelling a Rule-of-Thumb
Wednesday, 09 December 2009 16:40

There is a rule of thumb used to size a conductor based on the length of the run. The rule of thumb is to go up one conductor size for every 100 ft of length.  The graph shows two plots; (1) the line plot is the total length of a conductor run, (2) the length gained by going up to the next larger conductor size.

The conditions used to create the graph are:

- 120 V (volts) supply voltage
- 20 A (amperes) load
- 3% voltage drop max

The vertical axis on the left is used when reading the line graph and the vertical axis on the right is used when reading the bar graph.  The horizontal axis is the conductor size.  For instance, take 12AWG conductor on the horizontal axis.

The line graph shows a value of 46 ft which means that a 12 AWG conductor can drive a 20 ampere load from a 120 V source up to 46 ft while maintaining a 3% voltage drop.  Now go up to the next larger conductor size -- 10 AWG. Note that the line graph shows a value of 72 ft.  That means that the conductor length can be 27 ft more when using 10 AWG than a 12 AWG.

The bar graph makes it easy to see the length gain of 27 ft compared to the next smaller conductor. Similarly, using an 8 AWG instead of a 10 AWG provides a 43 ft gain.  Only a jump up in conductor sizes from 6 AWG to 4 AWG, 3 AWG to 2 AWG, and 2 AWG to 1 AWG provides the 100 ft gain in length.

Therefore, calculate the voltage drop instead of the using the rule of thumb next time you are faced with a long conductor run.




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