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What is Voltage Drop?

Voltage drop is the reduction in electrical potential (voltage) along a conductor as current flows through it. Every conductor has some resistance, and according to Ohm's Law (V = I × R), this resistance causes a portion of the source voltage to be "dropped" across the conductor rather than being delivered to the load.

Why Does Voltage Drop Matter?

Understanding and calculating voltage drop is critical for several reasons:

• Equipment Performance: Motors, lighting, and electronic equipment are designed to operate at specific voltages. Insufficient voltage can cause motors to overheat, lights to dim, and electronics to malfunction or fail prematurely.
• Energy Efficiency: Voltage dropped across conductors is converted to heat—wasted energy that increases operating costs and can contribute to conductor heating.
• Code Compliance: The National Electrical Code (NEC) recommends voltage drop limits of 3% for branch circuits and 5% total for feeders plus branch circuits combined.
• Safety: Excessive voltage drop can cause equipment to draw more current, potentially overheating conductors and creating fire hazards.

The NEC Recommendations

While not mandatory requirements, NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2 provide important guidance:

• Branch Circuits: 3% maximum voltage drop recommended
• Feeders: 3% maximum voltage drop recommended
• Combined (Feeder + Branch): 5% maximum total voltage drop recommended

These recommendations ensure "reasonable efficiency of operation" for electrical equipment.