5 Common Voltage Drop Myths Debunked
Separating fact from fiction: addressing the most common misconceptions about voltage drop calculations and requirements.
Misconceptions about voltage drop are surprisingly common, even among experienced electrical professionals. Let's examine and debunk five persistent myths that can lead to improper designs or unnecessary costs.
Myth #1: Voltage Drop is Optional Because It's Just an Informational Note
The Myth
"Since NEC voltage drop limits are in informational notes rather than code text, I don't need to worry about them."
The Reality
While technically not mandatory code requirements, voltage drop limits represent industry best practice and are increasingly enforced by AHJs, required in specifications, and expected by clients. Ignoring voltage drop leads to equipment problems, callbacks, and liability issues. Many jurisdictions adopt stricter requirements than the NEC informational notes.
Myth #2: If Ampacity is Adequate, Voltage Drop is Fine
The Myth
"I sized the conductor for ampacity from NEC Table 310.16, so voltage drop will automatically be within limits."
The Reality
Ampacity and voltage drop are independent calculations. A conductor sized for ampacity might have excessive voltage drop on long runs. For circuits over 50-75 feet, voltage drop often becomes the controlling factor and may require larger conductors than ampacity alone would indicate. Always check both.
Myth #3: 5% is Always Acceptable
The Myth
"The NEC says 5% total is fine, so I always design to that limit."
The Reality
The 5% total limit is the maximum for combined feeder AND branch circuit drops. Many applications require tighter limits: data centers often specify 2% total, sensitive electronic equipment may need 1.5%, and motor circuits benefit from even lower drops. The 5% figure assumes normal residential/commercial loads—not a one-size-fits-all target.
Myth #4: DC and AC Voltage Drop Are Calculated the Same Way
The Myth
"I can use the same formula for both AC and DC circuits."
The Reality
DC calculations use pure resistance, but AC calculations should account for impedance, which includes reactance as well as resistance. For small conductors and short runs, the difference is minimal. But for large conductors (especially in steel conduit) and three-phase systems, using DC resistance values can significantly underestimate actual voltage drop.
Myth #5: Temperature Doesn't Affect Voltage Drop
The Myth
"The resistance values in NEC tables are constants that don't change."
The Reality
NEC Table 8 values are for 75°C. Conductor resistance increases approximately 3.9% for every 10°C increase in temperature. A heavily loaded circuit operating at 90°C will have noticeably higher voltage drop than predicted using standard table values. In hot environments, this effect is even more pronounced.
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Now that we've cleared up these common myths, use our voltage drop calculator to get accurate results based on sound engineering principles.
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