A 3.29% drop exceeds the commonly-cited NEC guideline of 3% for a branch circuit — consider a thicker wire gauge or a shorter run.
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How This Calculator Works
Every wire has some resistance, so some voltage is always lost between the source and whatever
it’s powering — voltage drop is how much. Enter the wire’s material and gauge, the current it
carries, its one-way length, and the source voltage, and this calculator shows exactly how many
volts (and what percentage) are lost over that run.
This matters most on longer wire runs — a garage or shed sub-panel, an extension circuit, or a
solar array’s home run back to an inverter — where too much voltage drop can make lights dim,
motors run hotter and less efficiently, and other equipment misbehave even though it’s still
technically receiving power.
The Formula
Voltage Drop=Circular Mils2×K×Current×One-Way Length
K is a resistivity constant specific to the wire material (copper or aluminum) — a long-
standing, stable physical property, not something that changes over time.
Circular Mils is the wire gauge’s cross-sectional area, looked up from the standard AWG
(American Wire Gauge) reference table.
The factor of 2 accounts for the round trip the current actually makes — out to the load
through one conductor and back through the other — even though you only enter the one-way
distance.
Worked Example
A 12 AWG copper wire carrying 20 amps over a 50-foot one-way run on a 120-volt
circuit:
12 AWG copper has a cross-sectional area of 6,530 circular mils, and copper’s K constant is
12.9.
Voltage drop: 6,5302×12.9×20×50≈3.95 volts.
As a percentage of the source: 3.95÷120×100≈3.29% — just over the
commonly-cited 3% guideline for a branch circuit, meaning a thicker gauge or a shorter run
would be worth considering.
Cómo funciona esta calculadora
Todo cable tiene algo de resistencia, así que siempre se pierde algo de voltaje entre la fuente
y lo que sea que esté alimentando — la caída de voltaje es cuánto. Ingresa el material y el
calibre del cable, la corriente que transporta, su longitud en un solo sentido y el voltaje de la
fuente, y esta calculadora muestra exactamente cuántos voltios (y qué porcentaje) se pierden en
ese tramo.
Esto importa más en tramos de cable largos — un subpanel de garaje o cobertizo, un circuito de
extensión, o el tramo de retorno de un arreglo solar hacia un inversor — donde una caída de
voltaje excesiva puede hacer que las luces se atenúen, que los motores funcionen más calientes y
de forma menos eficiente, y que otros equipos se comporten mal aunque técnicamente sigan
recibiendo energía.
La fórmula
Caıˊda de voltaje=Circular mils2×K×Corriente×Longitud en un sentido
K es una constante de resistividad específica del material del cable (cobre o aluminio) —
una propiedad física estable y bien establecida, no algo que cambie con el tiempo.
Circular Mils es el área de sección transversal del calibre del cable, obtenida de la tabla
de referencia estándar AWG (American Wire Gauge, o Calibre de Alambre Americano).
El factor de 2 contempla el recorrido de ida y vuelta que realmente hace la corriente —
hacia la carga por un conductor y de regreso por el otro — aunque solo ingreses la distancia en
un sentido.
Ejemplo resuelto
Un cable de cobre calibre 12 AWG que transporta 20 amperios a lo largo de un tramo de
50 pies en un solo sentido, en un circuito de 120 voltios:
El cobre calibre 12 AWG tiene un área de sección transversal de 6,530 circular mils, y la
constante K del cobre es 12.9.
Caída de voltaje: 6,5302×12.9×20×50≈3.95 voltios.
Como porcentaje de la fuente: 3.95÷120×100≈3.29% — justo por encima de
la guía comúnmente citada del 3% para un circuito derivado, lo que significa que valdría la
pena considerar un calibre más grueso o un tramo más corto.
The National Electrical Code (NEC) commonly-cited guideline is to keep voltage drop at or under about 3% for a branch circuit (5% combined with the feeder) — below that, equipment generally runs as expected; above it, motors and other equipment can run inefficiently or misbehave.
Why does the length I enter get doubled in the calculation?
Current has to travel the full round trip — out to the load through one conductor, then back through the return conductor — even though you only measure the one-way distance. The formula's factor of 2 accounts for that automatically, so just enter the one-way length.
Why does aluminum wire have more voltage drop than copper at the same gauge?
Aluminum has higher electrical resistance than copper, so a run of the same length and gauge loses more voltage in aluminum — which is why aluminum wiring is often sized a gauge or two thicker than copper would need for the same circuit.
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