DC Voltage Drop Calculator

DC voltage drop: formula, examples, and common mistakes

The formula

DC voltage drop is purely resistive — there is no phase angle, power factor, or reactance to account for. The drop on a two-wire DC circuit is:

VD = 2 × R × I × L ÷ 1000

Where R is the DC resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8 (uncoated, 75°C), I is the load current in amps, and L is the one-way distance in feet. The factor of 2 is the round trip — the supply and return conductors each carry full current and each drop voltage. Unlike the AC calculation, there is no √3 term for three-phase and no K-factor approximation; R comes straight off Table 8. For the same reason, AC Chapter 9, Table 9 (which adds reactance and skin-effect AC resistance) does not apply to DC.

Worked example

A 48 V PV array feeds a charge controller 60 ft away, drawing 30 A on #10 AWG copper. Find the voltage drop.

R(#10 Cu) = 1.24 Ω/1000 ft
VD = 2 × 1.24 × 30 × 60 ÷ 1000 = 4.46 V
VD% = 4.46 ÷ 48 = 9.30%
Voltage at load = 48 − 4.46 = 43.54 V

At 9.30% this is far past the 3% recommendation — typical of low-voltage DC over distance. Stepping up to #4 AWG copper (R = 0.308 Ω/1000 ft) brings the drop to 1.11 V, or 2.31% — within recommendation. The calculator runs this search automatically and reports the next size that lands under 3%.

Common mistakes

• Applying an AC formula to a DC circuit. The √3 multiplier and the K-factor are AC constructs. On DC there is no phase or power factor — use the resistive form above with Table 8 values.
• Entering round-trip distance. The ×2 round trip is already in the formula. Enter the one-way run; entering total conductor length roughly doubles the reported drop.
• Sizing by volts instead of percent on low-voltage systems. A 2 V drop is trivial at 480 V and crippling at 12 V. Always evaluate against percentage, and tighten the target for PV and battery work.
• Correcting resistance downward for temperature. Lowering R below the 75°C Table 8 basis understates drop — the unsafe direction for a tool that does not measure operating temperature. This calculator holds the published 75°C value, matching standard practice.

NEC references

NEC Chapter 9, Table 8 supplies DC resistance and circular mils per conductor size. NEC 210.19(A)(1) Informational Note No. 4 recommends a maximum 3% voltage drop on branch circuits, and NEC 215.2(A)(1) Informational Note No. 2 recommends 5% combined feeder plus branch. These notes are AC-oriented recommendations applied here as a general design benchmark; the NEC does not impose a DC voltage-drop limit on general wiring. Equipment listing and instructions remain enforceable under NEC 110.3(B).

Frequently asked questions

Why is voltage drop worse on low-voltage DC systems?

Voltage drop is an absolute value in volts; what changes between systems is how large that drop is as a percentage of the source. The same conductor at the same current loses the same volts whether the bus is 12 V or 480 V, but on a 12 V circuit a 1 V drop is 8.3%, while on 480 V it is 0.2%. Low-voltage DC systems — 12 V, 24 V, 48 V — therefore reach the 3% threshold at far shorter distances and force larger conductors than an electrician used to 120/240 V work would expect. Size DC runs by percentage, not by a volts rule of thumb.

Does the NEC require DC voltage drop calculations?

No. The 3% branch-circuit and 5% combined figures come from informational notes in NEC 210.19(A)(1) and 215.2(A)(1), which are recommendations, not enforceable requirements, and are written around AC branch circuits and feeders. The NEC does not mandate a DC voltage-drop limit for general wiring. Voltage drop is still a functional design constraint regardless of code: a charge controller, inverter, or DC motor that sees too little voltage at the load underperforms or faults. Some jurisdictions adopt the informational-note limits as mandatory by amendment, so check your local adopted code.

Should I enter one-way or total circuit length?

Enter the one-way distance — the conductor length from source to load. The calculator multiplies by 2 for the round trip, since both the supply and return conductors carry full current and drop voltage. Entering the round-trip length double-counts and roughly doubles the reported drop. This is the most common input error on any voltage-drop tool, AC or DC. If your run is 40 ft from the battery to the load, enter 40, not 80.

What voltage drop is acceptable for a solar PV system?

PV design targets are usually tighter than the NEC informational-note 3%. A common rule of thumb is no more than 2% on the DC string and array conductors and no more than 3% total from array to inverter, preserving the voltage budget for production rather than wire loss. Battery-to-inverter conductors on a 48 V bank are often held to 1–2% because the current is high and a small percentage is a large absolute drop. These are design choices, not NEC mandates — but the equipment manufacturer instructions are enforceable under NEC 110.3(B), so confirm against them.

Why doesn't this calculator use the K-factor or √3 formula?

Those terms belong to AC. The √3 multiplier accounts for three-phase line-to-line relationships, and the K-factor is an approximation of AC resistance per circular mil. DC has no phase, no power factor, and no reactance, so the calculation reduces to pure resistance: VD = 2 × R × I × L ÷ 1000, with R taken directly from NEC Chapter 9, Table 8 DC resistance. Using an AC formula on a DC circuit introduces error, and the √3 factor has no physical meaning for two-wire DC.

Can I use this for 12V automotive or marine wiring?

Yes — the resistance physics are identical — but the acceptable limits differ. Marine DC wiring is governed by ABYC E-11, not the NEC: ABYC commonly allows 3% drop for critical circuits such as panel feeds, navigation, and electronics, and up to 10% for non-critical loads. Automotive practice is looser still. This calculator reports drop against the NEC 3% / 5% informational-note figures, so for an ABYC or automotive job, read the raw volts and percentage and compare them to the standard that applies to your work rather than the pass/warn banner.

Related tools

• AC Voltage Drop Calculator — for standard 120/240/480 V AC branch circuits and feeders, using NEC Chapter 9, Table 9.
• Wire Ampacity Calculator — allowable conductor ampacity with temperature and conduit-fill adjustments per NEC 310.16.
• Wire Size Calculator — size a conductor for load, distance, and voltage drop.

For deeper NEC training on conductor sizing and voltage drop, Mike Holt's NEC courses are the industry standard.