HVAC School - For Techs, By Techs
Understanding Voltage Drop - Short #251
· Summarized
Key takeaways
- Voltage drop is critical to understanding electrical performance and is affected by numerous factors.
- Always measure voltage drop under load, not during startup conditions.
- Follow National Electrical Code recommendations for maximum allowable voltage drop.
- Prioritize proper wire sizing and connection integrity to mitigate voltage drop issues.
- Recognize that longer conductors can cause reduced performance without necessarily overheating.
Summary
Understanding Voltage Drop
Voltage drop is the reduction in electrical potential energy (voltage) along a conductor due to resistance. The episode discusses how voltage drop occurs across conductors and loads, emphasizing the importance of understanding this concept for practical applications in the field. Bryan uses a water analogy where voltage is compared to pressure, current to flow, and resistance to friction, making it easier to visualize the phenomenon.
Key Factors Influencing Voltage Drop
Multiple factors influence conductor resistance and voltage drop. These include the length of the wire, gauge, material (like copper vs. aluminum), and temperature. Longer wires introduce more resistance, while smaller gauge wires also increase resistance. Bryan warns that as temperature rises, resistance increases, potentially leading to thermal runaway. Proper connections are vital, as poor connections can exacerbate resistance issues.
Voltage Drop Measurement Nuances
It's crucial to measure voltage drop under load, as opposed to startup conditions. Bryan explains how voltage drops are indicative of the load's current, which affects the entire system's performance. The podcast also touches on the misconception that lights dimming means there's a problem, clarifying that voltage drop should ideally be measured while the system is running.
Design Recommendations and Code Compliance
According to National Electrical Code recommendations, the total voltage drop should aim for no more than 5% from the service panel to the furthest outlet, with specifications for feeders and branch circuits. Bryan explains the difference between designed and undesigned voltage drop and emphasizes correct wire sizing based on minimum circuit ampacity rather than breaker size.
Common Causes of Voltage Drop Issues
Undersized conductors and improper connections are leading causes of voltage drop. Bryan highlights the importance of following design specifications to avoid significant voltage drops due to insufficient wire size and stressed connections, which can lead to overheating and safety hazards.
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Show notes
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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