Understanding Voltage Drop – Short #251

In this short podcast episode, Bryan drops some knowledge to help with understanding voltage drop, a few different causes of it, and NEC recommendations.

Voltage is electrical potential or “pressure,” and voltage drop is the reduction in electrical potential energy. We often think of it happening across conductors (which add resistance), but it also happens across loads (like contactor coils). Voltage drop across loads is usually designed, but voltage drop across conductors is usually undesigned and undesirable.

Several factors contribute to conductor resistance. Length is a major one; long runs of wire introduce more resistance to the circuit than shorter runs. Size/gauge also matters; smaller-gauge wire has more resistance than larger-gauge wire. Copper is the most common material for wiring, but we use other materials (including steel or aluminum), and those have different resistance values. Temperature also affects resistance, as they both increase and decrease as the other one does.

When we measure voltage drop, we want to make sure we’re doing it under load, NOT on startup. Voltage drops that happen on startup can be mitigated with more suitable infrastructure (including larger wires) or soft starts.

Undersized conductors don’t have sufficient cross-sectional area for the applied load. To avoid voltage drop due to undersized conductors, we should size conductors based on minimum circuit ampacity (MCA), not breaker size (MOCP). Poor connections can also cause resistance to jump, which reduces voltage; we need to pay attention to the connection design (including torque specs and proper lugging) to avoid making mistakes. Long wires don’t cause overheating by themselves, but they still contribute to voltage drop and cause reduced performance (including drawing higher current on startup).

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