Question 1

How do I read the Voltage Drop Chart: 10 AWG Copper at 20 A?

Accepted Answer

Find your one-way run length in the first column and read across. At 25 ft the voltage dropped is 1.24 V; at 800 ft it is 39.77 V. The table runs from 25 ft to 800 ft in 19 rows.

Question 2

How much does the voltage dropped change per row?

Accepted Answer

By 1.24 V for every 25 ft of one-way run length, and by the same amount everywhere in the table — the relationship is a straight line, so halfway between two rows is halfway between their two answers. That is what makes reading between the rows safe here.

Question 3

What if my one-way run length is outside the table?

Accepted Answer

The chart covers 25 ft to 800 ft, which is the range people actually look up. Anything above or below goes into the calculator, which takes any figure and shows each step of the working rather than only the result.

Question 4

Where do these numbers come from?

Accepted Answer

Every row is computed at build time from the formula published by NFPA 70, by the same code behind the calculator — not transcribed from another table, which is how a rounding error in one chart ends up in twenty.

One-way run length (ft)	Voltage dropped	Drop as a percentage	Voltage reaching the load
25	1.24 V	0.52 %	238.76 V
50	2.49 V	1.04 %	237.51 V
75	3.73 V	1.55 %	236.27 V
100	4.97 V	2.07 %	235.03 V
125	6.21 V	2.59 %	233.79 V
150	7.46 V	3.11 %	232.54 V
175	8.70 V	3.62 %	231.3 V
200	9.94 V	4.14 %	230.06 V
225	11.18 V	4.66 %	228.82 V
250	12.43 V	5.18 %	227.57 V
275	13.67 V	5.7 %	226.33 V
300	14.91 V	6.21 %	225.09 V
350	17.40 V	7.25 %	222.6 V
400	19.88 V	8.29 %	220.12 V
450	22.37 V	9.32 %	217.63 V
500	24.86 V	10.36 %	215.14 V
600	29.83 V	12.43 %	210.17 V
700	34.80 V	14.5 %	205.2 V
800	39.77 V	16.57 %	200.23 V

Voltage Drop Chart: 10 AWG Copper at 20 A

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