Continuous-load circuits expose a sizing mistake that ordinary receptacle circuits sometimes hide: the current does not relax. An EV charger at 48 amps, a row of commercial LED drivers at 18 amps, a process heater at 36 amps, or a ventilation load that runs through an entire shift keeps the conductor warm and keeps voltage drop present for hours. If the design only checks breaker size or a short ampacity table lookup, the installation can be legal-looking but still leave equipment operating at low voltage every day.

The first field scenario is familiar from EV and shop projects. In a review of a 240V, 48A Level 2 charger circuit routed about 165 feet one way from a garage subpanel, the minimum 60A continuous-load circuit was not the whole answer. The conductor could be made thermally legal, but the voltage-drop screen showed that the branch alone could spend roughly 3% before the upstream feeder was counted. Once the feeder was added, the total path was too close to 5%, so the final design moved up one conductor size instead of pretending the 125% rule had solved performance.

This article is written for electricians, engineers, and careful DIYers who already know that continuous loads are treated differently but want a practical workflow for conductor sizing. The key distinction is simple: NEC 210.20(A), NEC 215.3, NEC 625, and similar rules tell you how to size conductors and overcurrent protection for sustained current and heat. Voltage drop then asks whether the load still receives enough voltage after the real route length, conductor material, and system voltage are considered. IEC 60364-5-52 uses different organization, but it reaches the same engineering sequence.

TL;DR

A continuous load is expected to run for 3 hours or more.
NEC sizing often uses 125%, but voltage drop still depends on actual sustained current.
Use 3% branch and 5% total voltage drop as planning targets unless the project sets tighter limits.
EVSE, lighting rows, heaters, and process loads need feeder budget coordination.

The design baseline in this article is anchored to the National Electrical Code , electric vehicle charging station , the International Electrotechnical Commission . Those references matter because code language, conductor physics, and equipment behavior usually fail in the same place: a circuit that was technically legal on paper but poorly optimized for the distance, load, or operating temperature in the field.

"The 125% continuous-load rule is a heat and protection rule. It does not erase the resistance of a 150-foot branch circuit, so EVSE and lighting circuits still need a separate voltage-drop calculation."
— Hommer Zhao, Technical Director

What Makes Continuous-Load Voltage Drop Different

A continuous load is commonly understood in NEC work as a load where the maximum current is expected to continue for 3 hours or more. NEC Article 100 defines the term, while NEC 210.20(A) and 215.3 apply the familiar 125% sizing rule to branch-circuit and feeder overcurrent devices where continuous loads are present. EV charging adds its own emphasis because NEC 625 treats EVSE load as continuous. That is why a 32A charger is normally paired with a 40A circuit and a 48A charger is normally paired with a 60A circuit.

Voltage drop does not use the 125% factor in the same mechanical way for every design decision. The load equipment experiences the voltage produced by actual current through actual conductor resistance. For an EVSE drawing 48A for several hours, 48A is the sustained current that creates the voltage loss during charging. For conductor and overcurrent sizing, the continuous-load rule can require a 60A circuit. A disciplined design records both numbers: the code sizing basis and the voltage-drop operating current.

Three entity definitions keep the workflow clear. A continuous load is an electrical load expected to operate at maximum current for 3 hours or longer. Voltage drop is the reduction in voltage caused by current flowing through conductor impedance. Ampacity is the maximum current a conductor can carry under stated conditions without exceeding its temperature rating. Mixing those definitions is how designers end up with a circuit that passes one check while failing another.

Size for sustained heat first. Use NEC 210.20(A), NEC 215.3, NEC 625, equipment rules, terminal ratings, ambient correction, grouping, and NEC Table 310.16 before treating a conductor as acceptable.
Calculate drop at the real sustained current. A 48A EVSE, 36A heater, or 18A lighting row holds that current long enough for voltage loss and conductor temperature to become normal operating conditions, not short events.
Coordinate feeder plus branch budgets. NEC informational guidance around 210.19(A)(1) and 215.2(A)(1) points designers toward about 3% on branches and 5% total. A long feeder can consume the margin before the branch is even pulled.
Do not let the breaker rating hide the load. A 60A breaker for a 48A continuous EV load does not mean voltage drop should be screened as if the charger normally draws 60A or as if distance is irrelevant.

Comparison Table: Continuous-Load Circuit Checks

These examples show how continuous-load sizing and voltage-drop review work together. Values are planning screens; final designs still need local code, terminals, conductor insulation, ambient temperature, and equipment instructions checked.

Load Type	Sustained Load	Typical NEC Circuit Basis	Route Example	Voltage-Drop Risk	Better Design Move
Level 2 EVSE	32A at 240V	40A circuit under 125% sizing	130 ft one way	8 AWG Cu may be close once feeder drop is included	Compare 6 AWG Cu before release
High-output EVSE	48A at 240V	60A circuit under NEC 625	165 ft one way	Branch can approach 3% by itself	Use larger conductor or reduce route
Commercial LED row	18A at 277V	125% continuous-load branch review	240 ft one way	Higher voltage helps, but long rows still drift	Split rows or increase conductor
Process heater	36A at 208V	45A minimum design current, often 50A circuit	110 ft one way	Voltage sag can reduce heat output	Check 6 AWG and source voltage
Ventilation fan bank	22A at 480V 3ph	Feeder sized for continuous motor load	420 ft one way	Distance can still control at 480V	Run three-phase voltage-drop screen
Sign or exterior lighting	12A at 120V	15A continuous-load sizing basis	95 ft one way	120V budget is small	Use 10 AWG or split circuit

"For a 48-amp EV charger, I want the 60-amp circuit sizing documented and the voltage drop checked at the sustained charging current. If the feeder already spends 2%, the branch cannot casually spend another 3.5%."
— Hommer Zhao, Technical Director

Example 1: 48A EV Charger, 240V, 165 Feet One Way

A 48A Level 2 EVSE is a continuous load, so the branch-circuit and overcurrent sizing discussion normally lands at 60A because 48A x 125% = 60A. That is the code sizing basis. Now check voltage drop at the sustained 48A charging current. With 6 AWG copper at roughly 0.395 ohms per 1000 feet, a 165-foot one-way route has 330 feet of loop conductor. The drop is about 48 x 0.395 x 0.330 = 6.3 volts, or 2.6% at 240V before upstream feeder drop is included.

That 2.6% branch result may be acceptable if the feeder is short and stiff. If the garage subpanel feeder already drops 1.8% under charging load, the total path is about 4.4%. That is inside a 5% planning target but leaves little margin for hot conductor resistance or low utility voltage. Moving to 4 AWG copper lowers the branch drop to roughly 1.6%, which can be a better long-term choice when the charger will operate for several hours at a time.

Example 2: 277V Lighting Circuit, 18A Continuous, 240 Feet One Way

A commercial lighting row at 277V and 18A can look comfortable because the system voltage is higher than 120V. A 3% branch-circuit target gives 8.31 volts of allowable drop. If the run uses 10 AWG copper at about 0.999 ohms per 1000 feet, the 480-foot loop produces roughly 18 x 0.999 x 0.480 = 8.6 volts, or 3.1%. That is close enough that warm conductor resistance, splices, or a feeder already spending voltage may push the installed result beyond the target.

There are two clean fixes. One is to upsize the conductor. The other is to split the lighting row so the farthest driver is not at the end of one long loaded path. For LED drivers, voltage tolerance varies by equipment, but the design habit should be consistent: do not spend the entire branch budget before the feeder and actual field temperature are considered.

Example 3: 208V Process Heater, 36A Continuous, 110 Feet One Way

A 208V heater drawing 36A for long production cycles is not a short-duration appliance load. The continuous-load sizing basis is 36A x 125% = 45A, so a 50A circuit may be the practical selection after standard ratings and equipment instructions are reviewed. At 36A operating current over 110 feet one way, 8 AWG copper produces about 36 x 0.628 x 0.220 = 5.0 volts, or 2.4% at 208V.

That looks reasonable for the branch alone. But if the heater is fed from a long 208Y/120V panel feeder already carrying other continuous process loads, the branch calculation is not the end of the review. The feeder and branch must be checked together so the heater does not lose output or cycle longer than expected because the system voltage sags during normal production.

Common Continuous-Load Voltage-Drop Mistakes

Treating 125% as a voltage-drop shortcut

The 125% rule helps size the circuit for sustained current and heat. It does not prove the delivered voltage will be acceptable at the far end of a long route.

Ignoring feeder voltage already spent upstream

A branch circuit at 2.8% may look acceptable until a 2.2% feeder is included. Continuous loads often run long enough for the full path loss to matter.

Using nameplate maximum without understanding operating current

Some loads have programmable limits, duty settings, or staged elements. Record the actual continuous current used for voltage-drop screening and keep the code sizing basis visible beside it.

Forgetting temperature rise during long operation

Conductor resistance rises as conductors warm. A circuit that barely passes using cool resistance values may not hold the same drop after hours of operation.

A Practical Workflow for Continuous-Load Circuits

Use this sequence before buying conductor for EVSE, lighting, heaters, process equipment, ventilation, or other loads expected to run for 3 hours or longer.

1. Identify whether the load is continuous. Use the NEC Article 100 definition, the equipment article such as NEC 625 for EVSE, and the actual operating profile. Three hours or more at maximum current changes the design conversation.
2. Establish the code sizing basis. Apply 125% where required by NEC 210.20(A), 215.3, 625, or equipment-specific rules, then verify conductor ampacity, terminals, ambient correction, and grouping.
3. Calculate branch voltage drop at sustained current. Use the real one-way route, conductor material, and operating current. Compare copper and aluminum sizes when the route is long or the load is expensive to operate.
4. Add feeder drop before approving the design. A continuous branch circuit should not be approved in isolation when the feeder is long, heavily loaded, or already near 2% drop under the same operating condition.
5. Document the decision. Record voltage, phase, sustained amps, continuous-load sizing current, conductor size, route length, branch drop, feeder drop, and total drop so the upsizing decision is transparent.

FAQ

Do I calculate voltage drop at 100% or 125% for a continuous load?

Use the design current that the circuit is expected to carry. For many NEC continuous-load branch circuits, that means 125% sizing under NEC 210.20(A) and 215.3, so a 32A continuous EV load is commonly checked as a 40A circuit while voltage drop is evaluated against the real sustained current and feeder budget.

Is an EV charger a continuous load under the NEC?

Yes. NEC 625 treats EV charging loads as continuous for branch-circuit and feeder sizing. A 32A Level 2 EVSE normally needs a 40A circuit, and a 48A EVSE normally needs a 60A circuit before voltage drop is checked.

What voltage-drop target should I use for continuous lighting?

A common design target is 3% on the branch circuit and 5% total feeder plus branch, matching NEC 210.19(A)(1) and NEC 215.2(A)(1) informational guidance. Long LED rows often perform better when the branch stays closer to 2% to 3%.

Does 125% continuous-load sizing automatically solve voltage drop?

No. The 125% factor helps conductor and overcurrent sizing for sustained heat, but distance still creates resistance. A 60A EV circuit at 48A over 180 ft can still need larger copper or aluminum conductors for performance.

How does IEC 60364 handle continuous-load voltage drop?

IEC 60364-5-52 coordinates design current, installation method, grouping, ambient correction, conductor size, and allowable voltage drop. The terminology differs from NEC, but the practical sequence is the same: thermal legality first, voltage performance second.

When should I upsize wire beyond the continuous-load minimum?

Upsize when the route is long, source voltage is weak, the equipment runs for 3 hours or more, or the feeder has already used 2% or more of the total voltage-drop budget. Compare at least two conductor sizes before releasing material.

Need to Check a Continuous Load Before You Pull Wire?

Use the voltage drop and wire size calculators to compare the code-minimum continuous-load conductor against the next size up. If the feeder budget is tight or the equipment will run for hours, send the voltage, amps, route length, and conductor material through the contact page for a second review.

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Continuous Load Voltage Drop: 125% Current, Wire Size, and NEC/IEC Design