How to Choose Wire Size for a Long DC Run

Long DC circuits often fail at the load even when the conductor appears large enough for the current. The usual problem is voltage drop: resistance in both the outgoing and return conductors reduces the voltage available at the equipment.

For a DC circuit, the voltage-drop calculation is:

\(\displaystyle V_{\text{drop}} = I \times R_{\text{loop}}\)

Where:

\(V_{\text{drop}}\) = voltage lost in the conductors
(I) = load current in amperes
\(R_{\text{loop}}\) = total resistance of the positive and negative conductors

The number to focus on is the calculated voltage drop in volts and as a percentage of the source voltage:

\(\displaystyle \text{Voltage Drop \%} = \frac{V_{\text{drop}}}{V_{\text{source}}} \times 100\)

That result is used to compare candidate AWG sizes, verify the voltage arriving at the load, and identify whether a larger conductor, shorter routing path, higher system voltage, or different equipment arrangement is needed.

A conductor can have adequate ampacity and still be too small for a long low-voltage DC run.

Two-Conductor Loop Resistance

DC voltage-drop calculations use the complete circuit path. Current leaves the source on the positive conductor and returns on the negative conductor, so both conductors add resistance.

For equal-size conductors:

\(\displaystyle R_{\text{loop}} = 2 \times L_{\text{one-way}} \times R_{\text{conductor per foot}}\)

Where:

\(L_{\text{one-way}}\) = one-way distance from source to load
\(R_{\text{conductor per foot}}\) = resistance of one conductor at the selected AWG size

The factor of two is frequently missed. A load located 100 feet from a battery, power supply, or DC distribution panel has approximately 200 feet of conductor in the electrical loop when the positive and negative conductors follow the same route.

Do not use only the physical distance to the load as the resistance length unless the calculator specifically asks for total circuit length. For a typical two-wire DC branch circuit, enter the one-way run where required and allow the calculation to account for the return path.

The DC Voltage Drop Calculator can be used to evaluate voltage drop from circuit voltage, load current, one-way length, and conductor resistance or AWG selection. The AWG Wire Resistance Calculator is useful when comparing the resistance values behind each candidate conductor size.

Voltage at the Load

The voltage remaining at the equipment is:

\(\displaystyle V_{\text{load}} = V_{\text{source}} - V_{\text{drop}}\)

This is often more useful than voltage-drop percentage alone. A 2 V drop has very different consequences on a 48 V circuit than on a 12 V circuit.

For example, a 12 V source with a 2 V conductor drop supplies only:

\(\displaystyle 12\text{ V} - 2\text{ V} = 10\text{ V}\)

at the load terminals. Motors, solenoids, LED drivers, controls, pumps, communication equipment, and electronic DC devices may not operate properly at that reduced voltage even if the branch-circuit overcurrent device has not tripped.

The acceptable voltage-drop limit should be based on the equipment’s required operating-voltage range, system design criteria, and the actual source voltage under load. Battery systems add another consideration: source voltage may decline with state of charge, temperature, cable connections, and simultaneous loads. A circuit that works with a freshly charged battery may become unreliable as source voltage falls.

AWG Comparison Example

Assume a 24 V DC load draws 3 A and is located 100 feet from the DC source. The loop length is 200 feet.

The calculation for each conductor is:

\(\displaystyle V_{\text{drop}} = I \times \left( R_{\text{per 1,000 ft}} \times \frac{200}{1,000} \right)\)

The following comparison uses representative copper conductor resistance values for illustrating the method. Actual resistance varies with conductor material, stranding, temperature, and product construction.

Candidate conductor	Resistance, \(\Omega/1{,}000\text{ ft}\)	Loop resistance at 200 ft	Voltage drop at 3 A	Voltage at 24 V load	Voltage drop
20 AWG	10.15	2.03 \(\Omega\)	6.09 V	17.91 V	25.4%
18 AWG	6.385	1.277 \(\Omega\)	3.83 V	20.17 V	16.0%
16 AWG	4.016	0.803 \(\Omega\)	2.41 V	21.59 V	10.0%
14 AWG	2.525	0.505 \(\Omega\)	1.52 V	22.48 V	6.3%
12 AWG	1.588	0.318 \(\Omega\)	0.95 V	23.05 V	4.0%

The 20 AWG conductor has a calculated drop of 6.09 V. Even though 3 A may appear modest, the load would receive only 17.91 V from a nominal 24 V source. That candidate should be rejected if the equipment needs a higher operating voltage.

Moving to 12 AWG reduces the calculated drop to about 0.95 V, leaving approximately 23.05 V at the load. Whether that conductor is acceptable still depends on the equipment voltage tolerance, conductor ampacity, overcurrent protection, installation method, terminals, and the conditions of the installed circuit.

Use the Wire Size Calculator to review conductor sizing from the load-current side, then compare the voltage-drop performance of the resulting candidate sizes.

Selecting a Voltage-Drop Limit

Set the allowable voltage drop before selecting the conductor. Start with the actual electrical requirement: the lowest voltage that the equipment can receive while operating normally.

A practical review includes:

Source voltage at expected operating conditions, not just nominal system voltage
Continuous and peak current, including starting or inrush current where applicable
One-way routing distance and the complete two-conductor loop
Minimum equipment operating voltage
Voltage-sensitive loads such as electronic controls, relays, solenoids, LED systems, pumps, and motors
Future load additions or longer-than-planned field routing

Motor and actuator circuits need additional attention because current can increase during starting, stall, or high mechanical load. The voltage drop calculated at normal running current may understate the voltage loss during those conditions. Low terminal voltage can also affect motor starting performance and may lengthen acceleration time.

For control circuits, verify the pickup and holding voltage requirements of relays, contactors, valves, and other coils. A coil may energize during a no-load test but drop out when the source voltage declines or other DC loads operate at the same time.

Conductor Resistance and Temperature

AWG is a conductor-size designation, but voltage drop is driven by resistance. Larger conductors have lower resistance per foot, which reduces voltage loss over the same route.

Copper conductor resistance rises as conductor temperature rises. A calculation using a base resistance value may therefore be optimistic when conductors operate in a warm enclosure, rooftop raceway, engine compartment, bundled cable assembly, or other elevated-temperature location.

Conductor material also affects the result. Do not substitute copper resistance values for aluminum conductors, copper-clad products, or specialized flexible cable without confirming the appropriate resistance data.

For larger conductors, compare resistance in kcmil or in ohms per 1,000 feet rather than relying only on AWG labels. The same loop-resistance method applies:

\(\displaystyle V_{\text{drop}} = I \times \left( 2 \times L_{\text{one-way}} \times R_{\text{per foot}} \right)\)

The Voltage Drop Calculator can help compare circuit voltage loss where the conductor resistance, circuit length, and load current are known.

Ampacity and Installation Review

Voltage-drop arithmetic does not establish final conductor suitability. After identifying an AWG size that meets the load-voltage requirement, perform a separate ampacity and installation review.

Confirm the final design against:

Load current and whether the load is continuous
Overcurrent protective device rating
Conductor insulation temperature rating
Terminal rating and equipment listing requirements
Ambient-temperature correction factors
Adjustment factors for bundled or grouped current-carrying conductors
Raceway fill and conduit-routing constraints
Termination size, lug range, and conductor flexibility
Equipment grounding and bonding requirements where applicable
Local amendments and AHJ requirements

A larger conductor selected for voltage drop can affect the rest of the installation. It may require larger raceway capacity, larger fittings, different lugs, more bending space, and greater pulling tension. Conduit layout should account for pull points, offsets, bend radius, conductor insulation protection, and available working space at enclosures.

The final conductor must satisfy both sides of the decision:

1. It must carry the calculated load safely under the applicable installation conditions.

2. It must keep voltage at the load within the equipment’s required operating range.

Field Verification

Verify voltage at the actual load terminals with the circuit operating under representative conditions. Measure source voltage and load-terminal voltage at the same time when possible.

The field voltage difference includes more than the calculated conductor resistance. It can reveal losses from loose terminals, undersized connectors, corroded splices, damaged conductors, undersized disconnects, fuse holders, breakers, switches, or battery connections.

If measured voltage drop is significantly higher than calculated, inspect the complete current path, including both the positive and negative conductors. In DC systems, a poor return conductor or negative-bus connection can produce the same operating problems as an undersized positive conductor.