Topic guide
Feeder sizing, steady-state loading, and voltage drop within NEC limits across your distribution system.
A load flow study solves your system for normal operating conditions rather than fault conditions. It calculates the real and reactive power moving through every transformer, feeder and bus, and the resulting voltage at each one. Voltage drop analysis is the conductor-level piece of the same question: how much voltage is lost between the source and the load, given the conductor size, length, material and the current it carries. Run the load flow and the voltage drop answers come with it.
The 3% and 5% figures everyone quotes come from informational notes in the NEC — roughly 3% on a branch circuit and 5% total including the feeder. Informational notes are not enforceable requirements, which surprises people. A few specific articles do impose firm voltage drop limits for particular systems, and those are real. For general feeders and branch circuits, the numbers are engineering guidance, and the practical limit is whatever the connected equipment tolerates.
That tolerance is where the consequences show up. An induction motor at reduced terminal voltage draws more current to produce the same torque, so windings run hotter and insulation life gets shorter; starting torque falls off faster still. Variable frequency drives ride through some sag and then trip on undervoltage, usually at the least convenient moment. Contactors and control relays drop out before anything else does, which is why an intermittent voltage problem often presents as a control fault nobody can reproduce. None of it announces itself as a voltage problem.
The same analysis catches the other slow failure mode: capacity. Facilities add load by accretion — a new compressor here, a production line there — and no one recalculates the feeder. Load flow shows the actual loading on each transformer and feeder against its rating, which is how a transformer running past nameplate gets found before it fails on the hottest afternoon of the year. It is most valuable during design and before adding significant load, since conductor sizing is cheap on paper and expensive in conduit. Our load flow and voltage drop service page covers scope and turnaround; the articles below get into the calculations and the sizing decisions.
Where the 3% and 5% voltage drop figures actually come from, how the calculation works, and why rules of thumb fail once feeder and branch drops stack.
More on Load Flow & Voltage Drop →What a load flow study calculates, how it finds overloaded transformers and marginal feeders before they fail, and when to run one ahead of adding load.
More on Load Flow & Voltage Drop →Ready for the service itself? Load Flow & Voltage Drop
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