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Calculate conductor voltage drop per NEC 210.19 recommendations (≤3%).
Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. The NEC recommends keeping voltage drop to 3% or less for branch circuits and 5% total for feeders and branch circuits combined.
Increase wire gauge (lower AWG number), shorten wire runs, use copper instead of aluminum, or increase system voltage. For long runs exceeding 100 ft, always recalculate to confirm NEC compliance.
The NEC (National Electrical Code) recommends a maximum voltage drop of 3% for branch circuits (NEC 210.19) and 2% for feeders, with a combined total not exceeding 5%. While these are recommendations rather than requirements, most inspectors and engineers follow the 3% rule.
Copper has significantly lower resistance than aluminum, resulting in less voltage drop for the same wire size. Copper's resistivity constant K is approximately 12.9, while aluminum's is 21.2. For equivalent performance, aluminum wire must be sized up by two AWG sizes.
For three-phase circuits, the formula uses √3 (approximately 1.732) instead of 2 as the multiplier for distance. Our calculator handles this automatically when you select "Three Phase" from the Phase dropdown.
Find the minimum wire size needed to keep voltage drop within acceptable limits.
American Wire Gauge (AWG) is a standardized wire sizing system. Lower AWG numbers indicate thicker wire with more current capacity. Common residential sizes are 14 AWG (15A), 12 AWG (20A), and 10 AWG (30A).
Wire must be sized for both ampacity (current capacity) and voltage drop. For long runs, voltage drop often dictates a larger wire than ampacity alone requires. Always check both criteria before finalizing your selection.
For a 30A, 240V single-phase circuit with copper wire at 100 feet, you'll typically need 10 AWG wire. At 150 feet or more, consider upgrading to 8 AWG to keep voltage drop under 3%. Use our calculator above to get exact results for your specific run.
Aluminum wire is acceptable for service entrances and feeders, and can save money on long runs. However, you must upsize by two AWG compared to copper, use anti-oxidant compound on connections, and use rated terminations. It's generally not recommended for branch circuits.
Calculate conduit fill percentage per NEC Table 1, Chapter 9.
NEC Chapter 9, Table 1 limits conduit fill to: 53% for one conductor, 31% for two conductors, and 40% for three or more conductors. Our calculator uses the 40% rule for 3+ wires automatically.
EMT is lightweight and common for indoor exposed wiring. PVC Schedule 40 is used for underground and wet locations. RMC (Rigid) offers the most mechanical protection and is used in harsh environments and service entrances.
NEC Chapter 9, Table 1 specifies: 53% for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors. Exceeding these limits makes wire pulling difficult and can cause heat buildup, both of which are code violations.
Yes. All conductors including the equipment grounding conductor (EGC) must be counted when calculating conduit fill. The grounding conductor takes up physical space in the conduit regardless of its function.