So what is voltage drop? The voltage drop of a circuit is in direct proportion to the resistance of the conductor and the magnitude of the current. If you increase the length of a conductor, you increase its resistance — and thus increase its voltage drop. If you increase the current, you increase the conductor voltage drop. Thus, long runs often produce voltage drops that exceed NEC recommendations. We try to make sure most if not all circuits are 5% or less in voltage drop from the utility. We achieve this by either reducing the load on a circuit or increasing the wire size.
NEC recommends, not requires, that engineers adjust for voltage drop when sizing conductors. NEC 210.19(A), 215.2(A), and few other sections. The NEC recommends that the maximum combined voltage drop for both the feeder and branch circuit shouldn’t exceed 5%, and the maximum on the feeder or branch circuit shouldn’t exceed 3%. This is a recommendation and not a requirement because it is a performance issue, not a safety issue.
If this is not required, why does PEI follow this recommendation? There are a few reasons.
- System efficiency. If a circuit supports much of a load, a larger conductor will pay for itself many times over in energy savings alone.
- System performance. Lighting loads perform best when voltage drop is minimal. You get the light of a higher-wattage system simply by running larger wires.
- Troubleshooting. If you follow the NEC voltage drop recommendations, you don’t have to guess whether your field measurements indicate a problem or if the voltage is low due to not accommodating voltage drop in the design.
- Load protection. Undervoltage for inductive loads (motors) can cause overheating, inefficiency, and shorter life span of equipment.
For every project, if it requires new service equipment, we do short circuit analysis and voltage drop calculations. Typically this level of calculation is not done by most MEP consultants unless the facility is industrial or government in nature. We feel, however, that our clients deserve this level of service as well. We have purchased and utilize software such as Easy Power and ETAP which cost us over $20,000, and we use these software to calculate at minimum short circuit and voltage drop, but also capable or running coordination studies, arc flash hazard calculation and etc.
If we didn’t do these calculations then we would not even know about some of the circuits being over 5% voltage drop, but we do, and therefore, we try to be under 5% max.