Guide
Circuit Sizing Basics: Continuous Loads and Voltage Drop
Published July 19, 2026
A circuit rated for exactly the amperage a device draws is under-sized the moment that device runs long enough to count as a continuous load, and a wire correctly sized for that amperage can still deliver meaningfully less voltage than it should if the run is long enough — two separate limits, working independently, and both have to be satisfied rather than either one on its own. This is the conceptual companion to the EV charger circuit calculator and generator size calculator: the idea behind why those tools ask for run length as well as amperage.
This page explains the two rules involved. It does not replace a licensed electrician, a permit, or an inspection — actual circuit sizing depends on panel capacity, what else shares that panel, local code, and site-specific conditions that a general explanation can't account for, and getting it wrong carries real fire and shock risk.
Continuous load: why it's derated to 80 percent, not sized at 100
A continuous load is commonly defined as one expected to run for three hours or more at a stretch — an EV charger topping off overnight, an electric water heater, a large lighting load left on for an evening. Continuous loads generate sustained heat in a circuit's conductors and connections over that whole run time in a way a load that cycles on and off briefly doesn't, so electrical code requires continuous loads to be sized with a real safety margin rather than right up to a circuit's rated capacity.
The 125/80 pair, worked through
The requirement shows up as two numbers that describe the same relationship from opposite directions: a circuit's conductors and overcurrent protection are sized at 125% of a continuous load's rated current, which is mathematically identical to saying the continuous load itself may only draw up to 80% of the circuit's rated capacity. A 40 amp continuous load — a common EV charger draw — needs a breaker and wire rated for at least 50 amps (40 x 1.25), and that same 50 amp circuit is then only permitted to carry 40 amps of continuous load (50 x 0.8), which checks out as the same number arrived at from either direction.
Ampacity and voltage drop: two separate limits, not one
Ampacity is a wire's rated current-carrying capacity before it overheats — determined by gauge, insulation type, and how it's installed — and it's the limit most people think of as the whole story once the 125/80 sizing above is satisfied. Voltage drop is a separate, independent limit: a wire has real electrical resistance along its length, and that resistance causes a real, measurable voltage loss between the panel and whatever's plugged in at the far end, growing with run length regardless of whether the current flowing through it is well within the wire's rated ampacity.
A wire can pass an ampacity check with room to spare and still deliver noticeably reduced voltage at a distant outlet, which shows up as underperforming equipment, motors that draw more current to compensate for the lower voltage and run hotter as a result, or a charger or appliance that simply doesn't reach its rated output. Voltage drop is commonly kept within a recommended range — often cited as around 3% for a branch circuit alone and up to about 5% combined with any feeder ahead of it — as a performance and efficiency guideline rather than always a strict, universally enforced numeric limit; check how your local jurisdiction treats it, since enforcement varies.
Why the longer run usually loses to voltage drop first
For a circuit run entirely within a normal room-to-room distance, ampacity is typically the limit that actually decides wire gauge — a wire sized for the amperage is almost always short enough that voltage drop stays comfortably low. Stretch that same circuit out to a detached garage, a workshop, or an outbuilding, and the relationship can flip: voltage drop grows with distance in a way ampacity simply doesn't, so a wire gauge that would be entirely adequate for the amperage over a short run can fail the voltage-drop guideline over a long one, forcing a jump to a heavier gauge than the breaker size alone would suggest. This is the genuinely counter-intuitive part — 'the breaker size tells you the wire size' is a reasonable shortcut for a short run and an unreliable one for a long outbuilding circuit.
This explains the idea; a licensed electrician makes it real
Everything above is the reasoning behind why a run's distance matters as much as its amperage, not a set of numbers to design an actual circuit from. Real circuit sizing has to account for the specific panel's available capacity, everything else that panel already serves, the exact wire type and installation method, and local code and permitting requirements, all of which a licensed electrician is trained and required to verify. This page, and any calculator on this site touching circuit sizing, are for understanding the concept and estimating a starting point — not for skipping the permit, the inspection, or the electrician.
FAQ
Does the 125% rule apply to every circuit, or just continuous loads?
Just continuous loads specifically — a load that cycles on and off rather than running steadily for three hours or more doesn't generate the same sustained heating concern, so it's sized differently. Whether a specific device counts as continuous for code purposes is itself sometimes a judgment call worth confirming with an electrician.
If my wire passes the ampacity check, do I still need to check voltage drop separately?
Yes — the two checks are independent, and passing one says nothing about the other. A wire correctly sized for amperage over a long run can still fail a voltage-drop check, which is exactly the scenario covered above for a detached garage or outbuilding circuit.
Is voltage drop dangerous, or just a performance issue?
Primarily a performance and efficiency issue rather than an immediate safety hazard on its own, though a motor working harder to compensate for low voltage runs hotter and can shorten equipment life or trip its own protection more often — it's worth taking seriously even though it's a different kind of risk than an ampacity violation.
Can I just use a bigger wire than necessary to avoid worrying about voltage drop calculations?
Upsizing the wire gauge does reduce voltage drop for a given run, and it's a genuinely valid way to solve a long-run voltage-drop problem — but the correct gauge for a specific run length and load still needs to be calculated or confirmed, not guessed, since 'bigger is safer' isn't a substitute for actually checking the numbers on a circuit that matters.
Does this 125/80 relationship apply to a whole electrical panel, or just individual circuits?
The same underlying continuous-load derating principle applies at the panel and service level too, which is part of what a licensed electrician accounts for when evaluating whether an existing panel has enough real capacity to add a new continuous load like an EV charger without exceeding the panel's own rating.