Remote DC Load Voltage Budget: Cable Length, Contacts, and Temperature

Start at the Load Pins, Not at the Cable Reel

The Wire Resistance and Gauge Selection Guide develops conductor sizing from resistivity, current, and length. This guide begins with a system requirement: what is the lowest DC voltage the load may see while doing its job? Record the load's minimum operating voltage, maximum relevant steady current, and whether it draws more current as voltage falls. Then take the source's minimum credible voltage at its terminals under that condition. Battery state of charge, regulator tolerance, upstream harness drop, and load simultaneity may all lower the available input.

A quoted 5 V rail is not a guarantee of 5 V at every operating corner. A remotely powered module needing at least 4.5 V cannot spend the entire nominal 0.5 V difference in cable if the source can already sag to 4.75 V. Before choosing AWG, reserve voltage for every non-wire series element and for uncertainty. If the remainder is zero or negative, no positive cable length solves the requirement; change the source, load requirement, architecture, or series hardware first.

Allocate the Steady-State Voltage Budget

The Maximum Wire Length Calculator solves the equal-wire term from an allowed drop, current, area, and reference resistivity. Feed it the *remaining cable-only drop*, not the entire source-to-load voltage budget. It does not subtract connectors, switches, or reserves internally, and its resistivity preset is a reference-temperature value. For a critical design, convert the desired hot resistance back to an adjusted input or verify the result separately with a hot-resistance model.

Worked Remote-Module Example

Suppose a module requires 4.50 V minimum. The source can supply only 4.75 V at the design corner, and the module draws 2.0 A. The hot resistance budget for connectors, switch, and fuse is 40 mΩ total, so their drop is 0.080 V. Reserve 0.030 V for measurement and production uncertainty. The cable receives 4.75 - 4.50 - 0.080 - 0.030 = 0.140 V. With equal 0.5 mm² copper conductors and reference resistivity 1.724 × 10^-8 Ω·m, the reference-temperature wire-only maximum is 0.140 × 0.5 × 10^-6 / (2 × 1.724 × 10^-8 × 2) ≈ 1.015 m one-way. This is not an installed hot-length approval.

If the conductor resistivity at the actual operating temperature is 20% above that reference value, the same budget allows about 1.015/1.2 ≈ 0.846 m. That 20% is a scenario, not a universal copper correction. Use the measured or specified conductor temperature and material coefficient in the Wire Resistance vs Temperature Calculator. The non-wire 40 mΩ may also vary with temperature, insertion cycles, contamination, and assembly quality. Record it as a test or supplier-derived limit, not an assumed constant.

Treat Every Path Through a Branched Harness Separately

A single Lmax equation describes one uniform two-wire path with one load current. In a branched harness, a common trunk carries the sum of downstream branch currents; each branch segment carries only its own load current. For a given load, sum Isegment Rsegment from the source terminals through its supply path and return path, plus that load's applicable contacts and protection elements. Two loads can have different voltage drops even at equal physical distance because they share different current-carrying segments. The Wire Voltage Drop Calculator can evaluate each known segment, but the designer must assemble the network totals and simultaneous-load scenario. Do not give every branch the full global drop allowance independently.

A pulsed load may require a separate transient check. Cable inductance, source response, local capacitance, connector dynamics, and current step shape can produce an instantaneous voltage dip that the DC resistive formula does not predict. Conversely, a very brief peak may not justify treating that peak as a continuous thermal current. State both steady and transient requirements, and measure load-pin voltage during the actual mode transitions.

Remote Sensing Is an Architecture Choice

A regulator with properly designed remote sense can raise its output to maintain the far-end voltage while remaining within its allowed output range and stability limits. It still dissipates I²R in cable and contacts, and it can create overvoltage at the load if a sense lead opens unless the system has the required fault behavior. If remote sensing is unavailable, a larger conductor, higher distribution voltage with local regulation, shorter path, or lower-resistance connectors may be more robust. Do not use a cable-length number to choose among these without checking thermal loss, protection, connector ratings, and serviceability.

Verification at the Actual Assembly

Build the intended cable and connector assembly. Measure source-terminal and load-pin voltages simultaneously at maximum relevant current after temperatures stabilize. Use Kelvin connections when characterizing low connector resistance, but preserve the installed contact and crimp path when checking end-to-end voltage. Repeat after mating cycles or environmental exposure when those are part of the product requirement. Compare observed load voltage with the minimum allowed value and reserve, then inspect cable, contact, fuse, and switch temperature. The Four-Wire Cable Resistance Guide gives a separate conductor-versus-contact measurement workflow.

• Using nominal supply voltage instead of minimum loaded source voltage.
• Giving cable the full voltage margin without subtracting contacts or switches.
• Counting one-way wire resistance but forgetting the return conductor.
• Using cold reference resistivity for a hot harness.
• Applying one current to every segment of a branched harness.
• Confusing load voltage at the regulator with voltage at the load pins.
• Assuming remote sensing removes cable heat or all sense-lead fault risk.
• Calling a voltage-drop length an ampacity or installation approval.

Engineering Sources

Analog Devices remote-load voltage note discusses cable drop, remote sensing, and regulator compensation boundaries. Analog Devices connector-and-wiring drop note shows why the complete remote path matters. These are application examples, not a current-rating or wiring-code approval.