Voltage Drop

Voltage drop is the reduction in voltage between the source and the load caused by the resistance of the conductors carrying the current. Every wire has resistance, so some voltage is inevitably lost as heat along the run; the longer the cable and the thinner its gauge, the larger the drop and the less voltage actually reaches your equipment. For a mining setup, where a single machine can pull 3,000 watts continuously for years, conductor sizing is not a formality — it is the difference between wiring that quietly wastes money and wiring that delivers what the panel promised.

How much is acceptable

The U.S. National Electrical Code does not set a hard limit but recommends, in an informational note, keeping branch-circuit voltage drop at or below 3% and the combined feeder-plus-branch drop at or below 5%; Canadian practice under the CEC works to comparable figures. For a single-phase circuit the drop is approximately Vd = 2 x I x R x L / 1000, where I is the load in amps, R is the conductor's resistance in ohms per 1,000 feet, and L is the one-way run length in feet — the factor of 2 accounts for current traveling out and back. Three-phase circuits substitute 1.732 (the square root of 3) for the 2. Worked example: a 12.5 A, 240 V miner on 100 feet of 12 AWG copper (about 1.6 ohms per 1,000 ft) drops roughly 4 V — 1.7%, comfortably in spec. Stretch the same circuit to 250 feet and the drop approaches 10 V, past the guideline and into the territory where equipment starts noticing.

Why miners should care

ASIC power supplies tolerate an input window, but excessive drop costs you twice. First, the lost voltage is dissipated as heat in the walls — pure waste on a load that runs 24/7, and at mining duty cycles even a 2% loss compounds into real kilowatt-hours over a year. Second, a sagging supply can pull the delivered voltage low enough to hurt PSU efficiency or trigger under-voltage faults, especially on long runs from a distant panel to a shed or container, and especially at startup when inrush current momentarily multiplies the drop. Marginal voltage is a classic source of the maddening intermittent restart: fine at idle, faulting under full hash.

The same physics applies downstream of the wall. Inside the machine, low-voltage DC paths are even less forgiving: at 12–15 V, a few hundred millivolts lost across an undersized conductor or a corroded connection is several percent of the entire budget, which is why hashboards receive power over solid copper busbars rather than wire harnesses, and why a warm connector is always a finding rather than a curiosity. Whether at 240 V AC or 12 V DC, the rule is identical — resistance in series with a big load taxes it forever.

Designing it out

Two levers dominate. Running 240 V instead of 120 V halves the current for the same power, and since drop scales with current — and the wasted percentage scales again with the lower base voltage — the 240 V circuit loses roughly a quarter as much, which is a major reason serious mining circuits are wired at the higher voltage on a NEMA L6-30 or similar receptacle. The second lever is copper: when a far-flung site is involved, upsizing the conductor one gauge is cheap insurance bought once, against a resistive loss paid every hour the machine runs. Plan conductor size against your load factor and total apparent power, measure voltage at the receptacle under full load rather than trusting the math alone, and remember that the circuit breaker protects the wire from overcurrent — nothing but design protects you from voltage drop.