How to Determine Voltage Drop for Long Branch Circuits

Voltage drop is the reduction in electrical potential that occurs when current flows through the inherent resistance of a conductor. It is not a defect in your wiring; it is an unavoidable consequence of physics. However, when you are running feeders to a detached garage, wiring a subpanel, or installing an EV charger at the end of a long driveway, failing to calculate this drop can result in dimming lights, overheating motors, and nuisance breaker trips. To properly determine voltage drop, you must look past the breaker's ampacity rating and calculate the actual resistance of the wire run based on its length, material, and temperature rating.

The Physics of the Drop (and What It Changes)

At its core, voltage drop is just Ohm's Law ($V = I \times R$) applied to the wire itself. Every conductor has resistance. When current (amps) pushes through that resistance, some of the electrical pressure (voltage) is consumed by the wire before it ever reaches the load.

What it changes in a real circuit: Voltage drop changes the available voltage at the terminal of your device. If you send 120V into a 150-foot run of 14 AWG wire, the receptacle at the end might only read 112V under load. For resistive loads like incandescent lights, this just means dimmer output. But for inductive loads like air compressors, well pumps, or HVAC blowers, it forces the motor to draw more current to produce the same mechanical power ($P = V \times I$). This excess current generates heat in the motor windings, degrades insulation, and can trip thermal overloads.

Think of it like water pressure in a long garden hose; the friction of the hose walls reduces the pressure at the nozzle, even if the spigot is fully open.

Reference Data: Copper Wire Resistance and Drop

To determine voltage drop accurately, you need the exact resistance of the wire. The table below uses the 75°C column for stranded copper wire, which is the standard baseline for most modern THHN in conduit and NM-B (Romex) terminations.

Note: The distances below represent a 100-foot one-way run. Because current must travel to the load and return to the panel, the actual wire length used in the calculation is 200 feet (round-trip).

Worked Example: How to Determine Voltage Drop on a 120V Branch

Let’s walk through a real-world scenario. You are wiring a dedicated 120V, 20A circuit from a subpanel to a heavy-duty air compressor in a detached workshop. The one-way distance is 120 feet. The compressor draws a continuous 16A while running.

Step 1: Test 10 AWG Copper
You might assume 10 AWG is plenty, since it is rated for 35A at 75°C. Let's do the math using the standard formula: $VD = I \times R_{total}$.

• Current (I): 16A
• Total Wire Length: 120 ft × 2 = 240 ft
• Resistance of 10 AWG: 1.24 Ω per 1000 ft
• Total Resistance: $1.24 \times (240 / 1000) = 0.2976 \Omega$
• Voltage Drop: $16A \times 0.2976 \Omega = 4.76V$
• Percentage Drop: $(4.76 / 120) \times 100 = \mathbf{3.96\%}$

A 3.96% drop exceeds the 3% NEC recommendation. We need to step up the wire size.

Step 2: Recalculate with 8 AWG Copper

• Resistance of 8 AWG: 0.778 Ω per 1000 ft
• Total Resistance: $0.778 \times (240 / 1000) = 0.1867 \Omega$
• Voltage Drop: $16A \times 0.1867 \Omega = 2.98V$
• Percentage Drop: $(2.98 / 120) \times 100 = \mathbf{2.48\%}$

At 2.48%, 8 AWG copper passes the 3% threshold. You must pull 8 AWG wire for this run.

Where You Meet This in Practice (and Common Confusions)

If you want to know how voltage drop manifests on the jobsite, you will encounter it most frequently in three scenarios:

1. Detached Structures: Feeder cables running underground in PVC conduit to garage subpanels often exceed 100 feet, requiring upsized aluminum (like 2-2-2-4 SER or individual THHN) to keep the 240V drop under 3%.
2. Level 2 EV Chargers: A 48A continuous EV charger load at the far end of a driveway will generate massive heat and severe voltage drop if run on standard 6 AWG wire over long distances. Tools like the Southwire Voltage Drop Calculator are essential here to justify the cost of 4 AWG or 3 AWG copper.
3. Low-Voltage Landscape Lighting: Because the base voltage is only 12V or 24V, even a 2V drop is a massive percentage. This is why landscape transformers use multi-tap outputs (12V, 13V, 14V, 15V) to intentionally over-supply voltage to compensate for the drop over long 12 AWG direct-burial runs.

Common Confusions to Avoid

Voltage Drop vs. Voltage Sag: Voltage drop is a steady-state reduction caused by the physical resistance of your wire. Voltage sag (or dip) is a temporary, utility-side event that happens when a massive load (like a neighbor's welder or your own central AC compressor) kicks on, momentarily pulling the transformer voltage down. Upsizing your wire will fix voltage drop, but it will not fix utility voltage sag; you need a UPS or power conditioner for that.

Voltage Drop vs. Power Loss: Many DIYers think the 'lost' voltage just vanishes. It doesn't. It dissipates as heat inside your walls. If you have a 5V drop on a circuit carrying 20A, you are generating 100 watts of heat ($P = V \times I$) continuously along the length of that cable. In tightly bundled conduit or insulated walls, this ambient heat can degrade wire insulation over time.