DC Voltage Drop Tables and Formula for 12 V, 24 V, and 48 V Systems

At 120 V, losing 3 volts in the wiring is a rounding error. At 12 V, losing 3 volts is a quarter of your system, and it is the difference between an inverter that runs and an inverter that trips on low-voltage cutoff while the battery is still half full.

This is the single most common reason a small solar system underperforms. The wire is legal, the wire is cool to the touch, the wire is within its ampacity — and the load at the far end is being starved. Ampacity is a safety limit. Voltage drop is a functionality limit. On low-voltage DC runs of any length, voltage drop is almost always the one that decides your wire size.

The formula

For a DC circuit, current flows out on one conductor and back on the other, so the length that matters is twice the distance to the load:

V_drop = 2 × L × I × R

Where L is the one-way run length, I is the current in amps, and R is the conductor resistance per unit length. As a percentage of system voltage:

drop % = (2 × L × I × R) / V_system × 100

Rearranged to answer the question people actually have — how far can I run this wire?

L_max = (drop_fraction × V_system) / (2 × I × R_per_foot)

This is Ohm’s law, not a code rule. It is arithmetic you can check, and every number in the tables below came out of it.

Copper DC resistance (NEC Chapter 9, Table 8)

AWG	Ω per 1000 ft	Ω per km
14	3.07	10.1
12	1.93	6.34
10	1.21	3.984
8	0.764	2.506
6	0.491	1.608
4	0.308	1.01
2	0.194	0.634
1	0.154	0.505
1/0	0.122	0.399
2/0	0.0967	0.317
4/0	0.0608	0.1996

These are referenced at 75 °C. A conductor running hot has higher resistance and therefore more drop than the table predicts, which is one more reason not to design right at the limit.

How much drop is acceptable?

There is no single answer, and the sources genuinely disagree. Be careful about who is telling you what.

Circuit	Limit	Who says so	Force
Branch circuit to farthest outlet	3%	NEC 210.19(A) Informational Note	Advisory only
Feeder plus branch circuit combined	5%	NEC 210.19(A) Informational Note	Advisory only
Critical DC circuits (bilge pumps, nav lights, main feeders)	3%	ABYC E-11	Codified in the standard
Non-critical DC circuits (cabin lighting, accessories)	10%	ABYC E-11	Codified in the standard
PV array to charge controller	~3%	Industry convention	Rule of thumb, not code

The NEC’s 3 percent figure is the one everybody quotes and the one most often misrepresented. It appears in an Informational Note, which in NEC language means explanatory material that is not part of the enforceable requirements, unless a local authority having jurisdiction has specifically adopted it. ABYC’s 3 percent and 10 percent criteria, by contrast, are actual criteria within the standard, with published sizing tables to match.

For practical planning on a small off-grid system, a reasonable posture is:

• 3% or better for anything upstream of the battery, plus the battery-to-inverter run, because losses there come straight off your harvest or your surge capability.
• 3% for anything that misbehaves at low voltage — inverters, pumps, compressors, anything with a motor or a low-voltage cutoff.
• Up to 10% is tolerable for simple resistive or well-regulated loads such as LED lighting on a short branch, where the only consequence is slightly dimmer output.

Maximum one-way run length, 3% drop

Read the row for your wire size and the column for your circuit’s current. The number is the longest one-way distance to the load in feet. The tables already account for the return conductor.

12 V system, 3% maximum drop

Wire	5 A	10 A	20 A	30 A	50 A	75 A	100 A
14 AWG	11	5	2	1	—	—	—
12 AWG	18	9	4	3	1	—	—
10 AWG	29	14	7	4	2	1	—
8 AWG	47	23	11	7	4	3	2
6 AWG	73	36	18	12	7	4	3
4 AWG	116	58	29	19	11	7	5
2 AWG	185	92	46	30	18	12	9
1 AWG	233	116	58	38	23	15	11
1/0 AWG	295	147	73	49	29	19	14
2/0 AWG	372	186	93	62	37	24	18
4/0 AWG	592	296	148	98	59	39	29

Look at what this table is telling you. At 12 V and 100 A — a 1200 W inverter at full tilt — even 4/0 cable, which is nearly as thick as your thumb, only reaches 29 feet at 3 percent. This is why inverters are mounted next to batteries and not across the room.

24 V system, 3% maximum drop

Wire	5 A	10 A	20 A	30 A	50 A	75 A	100 A
14 AWG	23	11	5	3	2	1	—
12 AWG	37	18	9	6	3	2	1
10 AWG	59	29	14	9	5	3	2
8 AWG	94	47	23	15	9	6	4
6 AWG	146	73	36	24	14	9	7
4 AWG	233	116	58	38	23	15	11
2 AWG	371	185	92	61	37	24	18
1 AWG	467	233	116	77	46	31	23
1/0 AWG	590	295	147	98	59	39	29
2/0 AWG	744	372	186	124	74	49	37
4/0 AWG	1184	592	296	197	118	78	59

48 V system, 3% maximum drop

Wire	5 A	10 A	20 A	30 A	50 A	75 A	100 A
14 AWG	46	23	11	7	4	3	2
12 AWG	74	37	18	12	7	4	3
10 AWG	119	59	29	19	11	7	5
8 AWG	188	94	47	31	18	12	9
6 AWG	293	146	73	48	29	19	14
4 AWG	467	233	116	77	46	31	23
2 AWG	742	371	185	123	74	49	37
1 AWG	935	467	233	155	93	62	46
1/0 AWG	1180	590	295	196	118	78	59
2/0 AWG	1489	744	372	248	148	99	74
4/0 AWG	2368	1184	592	394	236	157	118

Maximum one-way run length, 10% drop, 12 V

For non-critical branch circuits where ABYC’s 10 percent allowance applies.

Wire	5 A	10 A	20 A	30 A	50 A	75 A	100 A
14 AWG	39	19	9	6	3	2	1
12 AWG	62	31	15	10	6	4	3
10 AWG	99	49	24	16	9	6	4
8 AWG	157	78	39	26	15	10	7
6 AWG	244	122	61	40	24	16	12
4 AWG	389	194	97	64	38	25	19
2 AWG	618	309	154	103	61	41	30
1 AWG	779	389	194	129	77	51	38
1/0 AWG	983	491	245	163	98	65	49
2/0 AWG	1240	620	310	206	124	82	62
4/0 AWG	1973	986	493	328	197	131	98

Remember that a 10 percent allowance still has to clear the ampacity requirement. Loosening the drop budget never lets you go below the safe current-carrying size.

Worked example, by hand

A 12 V shed system runs an 18 A load 22 feet from the distribution panel. What size wire?

Start with ampacity. 18 A continuous requires the conductor to be rated for at least 18 A after derating; 12 AWG at 25 A (75 °C, NEC) clears it comfortably. So ampacity says 12 AWG.

Now check drop. 12 AWG is 1.93 Ω per 1000 ft, so 0.00193 Ω per foot:

V_drop = 2 × 22 ft × 18 A × 0.00193 Ω/ft = 1.53 V
drop % = 1.53 / 12 × 100 = 12.7%

Nearly 13 percent — well past even the loose 10 percent allowance. Step up and retry with 8 AWG at 0.000764 Ω per foot:

V_drop = 2 × 22 × 18 × 0.000764 = 0.605 V
drop % = 0.605 / 12 × 100 = 5.0%

Better, but still above 3 percent. 6 AWG gives 3.2 percent; 4 AWG gives 2.0 percent.

Ampacity said 12 AWG. Voltage drop says 6 AWG at minimum and 4 AWG if you want margin — three to four sizes larger. That gap is the entire lesson of this page.

Three ways to reduce drop, ranked by effectiveness

Raise the system voltage. Drop percentage scales with the square of voltage for the same delivered power, because doubling voltage halves the current and doubles the reference voltage. A run that fails badly at 12 V may pass comfortably at 24 V with the same wire. This is why 24 V and 48 V dominate above about 1500 W.

Shorten the run. Drop is linear in length. Moving the inverter three feet closer to the battery is free; upsizing 100 feet of 4/0 cable is not.

Increase the wire size. Effective, but the most expensive lever, and it hits diminishing returns as cable becomes hard to bend, hard to terminate, and hard to route.

Limits of these tables

They assume copper, DC, and the resistance values at the 75 °C reference in NEC Chapter 9 Table 8. They do not account for connection and termination resistance, which is real and adds up across crimps, lugs, busbars, fuse holders, and switches. They do not cover aluminum conductors. They do not account for a conductor running hotter than 75 °C, which raises resistance further.

Design with margin. If a calculation lands within a few percent of your limit, take the next size up — the real installation will be worse than the arithmetic.