Load Flow & Voltage Drop Analysis

For general branch circuits and feeders, the 3% and 5% figures come from informational notes — they are recommendations, not enforceable limits. A few specific articles do set firm limits for particular systems. We evaluate against the recommendations and against what your equipment actually tolerates.

Usually not on its own, since the general requirements sit in informational notes. It becomes a compliance issue when a specific article applies to the system in question, or when the drop is severe enough that equipment no longer operates as intended. It is always an operating and reliability problem.

An induction motor at reduced voltage draws more current to make the same torque, which raises winding temperature and shortens insulation life. Starting torque falls off sharply. Variable frequency drives ride through some sag, then trip on undervoltage. Contactors and control relays drop out before anything else does.

Yes, and it is one of the most common findings. Load flow shows actual loading on each transformer and feeder under expected operating conditions, including the ones that grew load by accretion over fifteen years. Anything running past its rating is flagged with the margin it is over by.

Load flow solves the whole system for power flow and bus voltages under normal loading. Voltage drop analysis is the conductor-level question: how much voltage is lost between a source and a load. Load flow gives you the system picture and the voltage drop answer falls out of it.

Metered or trended data gives the best results, and we will use whatever your power monitoring or utility bills provide. Where there is none, we build load from connected equipment with demand factors, then state the assumption in the report so the numbers can be revisited later.

It is most valuable during design and when adding significant load. Sizing conductors and transformers correctly on paper costs a few hours of engineering. Discovering the feeder is undersized after the concrete is poured and the gear is set costs considerably more.

Voltage drop follows from conductor impedance, length, and load current. Rules of thumb work for a single run, but system-level answers need a load flow model, because upstream drops stack on downstream ones. The NEC’s informational notes suggest keeping branch plus feeder drop to about five percent.