The Title 24 5 Per Cent Voltage Drop Limit
Combined voltage drop across feeder and branch circuit conductors, measured to the farthest connected load or outlet, must not exceed 5 per cent. It appears in the Energy Code because voltage drop is wasted energy, not only lost performance.
Combined, and to the farthest point
Two details do the work. It is the combined drop across both the feeder conductors and the branch circuit conductors — not 5 per cent each. And it is measured to the farthest connected load or outlet, so the worst case governs, not an average or a typical run.
Treating them separately is the common error, and it produces a design that reads as compliant on each leg and fails on the sum.
Why an energy code cares
Voltage dropped in a conductor is energy converted to heat in the wall. It does no work; it is simply lost between the service and the equipment. On a long run in a large building that is a continuous, invisible loss for the life of the installation.
There is a compounding effect too. Equipment fed below its rated voltage tends to draw more current to deliver the same output, which increases the drop further. Capping the drop keeps that from becoming self-reinforcing.
What it means for conductor sizing
Voltage drop is a function of length, current and conductor size. Since the length is set by the building and the current by the load, the variable that remains is conductor size — so in practice this requirement upsizes cable on long runs.
That is a cost that lands on the electrical package and is decided by the distribution layout. Locating a panel closer to the load it serves is usually cheaper than upsizing the conductor to reach it from further away, which makes this a planning question as much as a specification one.
The exceptions are safety-driven
Voltage drop permitted by California Electrical Code Sections 647.4, 695.6 and 695.7 is excepted. Those cover sensitive electronic equipment and fire pump installations.
Fire pumps are the clearest case: a fire pump has to start and run under fault conditions, and its conductors are sized for that duty rather than for efficiency. Where life safety and energy conservation genuinely conflict, the Energy Code stands down.
Where it gets missed
Voltage drop is familiar to every electrical engineer as a performance calculation, which is exactly why its presence in the Energy Code gets overlooked — it is assumed to be somebody else's requirement, already handled. It is worth confirming the calculation exists and covers the combined path to the farthest outlet, rather than assuming the electrical design has it in hand.