Voltage Drop Settings Conflict With a Long Feeder to a Remote Panel
A warehouse project has a 480V service with a 400A main distribution panel and a 100A panel LP-1 located 410 feet away, feeding 18 high-bay luminaires at 277V plus four 3-phase motor loads. The electrical settings specify a 2% voltage drop for branch circuits and 3% for feeders, with copper conductors and 75-degree terminations. The engineer's calculation shows LP-1's feeder at 2.9% drop using 1/0 copper, which passes the feeder limit but the combined branch-plus-feeder is 4.6%, exceeding the 5% total target only marginally. The contractor proposes reducing to #2 copper to save cost, arguing the panel schedule shows adequate ampacity.
1. Evaluate the contractor's proposal using the configured voltage drop settings and explain what information the Revit panel schedule does and does not provide.
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Revit's Electrical Settings define voltage drop percentages used by the wire sizing and voltage drop calculation tools, but the panel schedule itself does not perform or display voltage drop by default. The schedule shows circuit loads, breaker sizes, and connected/general load totals; it will show #2 copper as adequate for the 100A ampacity, which is why the contractor's argument appears to hold.
The configured limits are 2% branch and 3% feeder. With 1/0 copper the feeder is 2.9%, already near the 3% ceiling, and the combined total is 4.6%. Dropping to #2 increases resistance substantially, pushing the feeder well past 3% and the total past the 5% combined target. The proposal fails the project's own settings even though ampacity passes.
The correct response is to run the voltage drop calculation against the actual one-way circuit length (410 ft is the run; the calculation uses the one-way distance, so confirm the tool's convention), verify the load in kVA rather than assuming the breaker size, and document the result. If the contractor wants cost savings, evaluate parallel runs or a closer panel location rather than undersizing the feeder.
2. The 5% combined target is described as marginal. Explain how you would decide whether to hold the 1/0 design or accept a smaller conductor, and what you would document.
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First, confirm the actual load rather than the breaker rating. A 100A panel rarely draws 100A; compute demand load from the connected luminaire and motor loads using the applicable demand factors. Voltage drop should be evaluated at design load, not at breaker size, though many specifications require evaluation at full breaker rating for future capacity.
Second, confirm the calculation inputs: conductor material, termination temperature, power factor, and whether the tool uses one-way or round-trip length. A mismatch here can swing the result by more than a percentage point.
Third, apply the governing criteria. If the project specification requires 5% total at design load, and the design load produces 4.6%, the design complies; the margin is thin but acceptable. If the specification requires 5% at full rated current, the design may not comply.
Document the load basis, calculation method, conductor size, resulting percentages, and the decision rationale in the calculation package and on the drawing general notes. Record the contractor's proposal and the rejection reason, so the decision is defensible in review.