Continuous loads confuse many field calculations because two different questions get mixed together. The overcurrent device and conductor ampacity may need to be sized at 125% of the continuous load, but voltage drop should be evaluated at the actual expected load current unless the project specification says otherwise.

That distinction matters on EV chargers, sign lighting, battery chargers, process heaters, data-room receptacle rows, and other loads expected to run for 3 hours or more. A 48A EVSE usually needs a 60A circuit by continuous-load rules, but the voltage-drop math for the conductors should still be checked at 48A so the result reflects the real circuit voltage at the equipment. NEC IEC

A Field Workflow That Keeps Ampacity and Voltage Drop Separate

Identify whether the load is continuous: NEC Article 100 defines it as maximum current expected for 3 hours or more.

Size the branch-circuit or feeder ampacity and overcurrent protection first. Common NEC checks are 210.19(A)(1), 210.20(A), 215.2(A)(1), and 215.3.

Run voltage drop at the actual design load current. For a 48A charger on a 60A circuit, enter 48A, not 60A, unless the equipment can really draw 60A.

If upsizing for voltage drop changes conductor size, re-check ampacity, terminals, conduit fill, box space, and equipment lug range before ordering material.

Code and Standard Checkpoints

NEC Article 100: a continuous load is one where maximum current is expected to continue for 3 hours or more.
NEC 210.19(A)(1) and 215.2(A)(1): branch-circuit and feeder conductors must cover noncontinuous load plus 125% of continuous load, subject to listed equipment rules.
NEC 210.20(A) and 215.3: overcurrent protection follows the same 125% continuous-load logic unless a listed 100% rated assembly is used.
NEC informational notes near 210.19(A)(1) and 215.2(A)(1) commonly guide designers toward about 3% branch or feeder voltage drop and 5% total. IEC 60364-5-52 uses installation method, grouping, and voltage-drop limits as part of cable selection.

Where the 125% Rule and Voltage Drop Point to Different Answers

Use these planning cases as a check on method. Final values still depend on insulation temperature, conductor material, terminal rating, ambient correction, grouping, raceway fill, and local approval.

Scenario	Ampacity check	Voltage-drop check	Practical result
48A Level 2 EV charger, 240V, 150 ft	48A x 125% = 60A minimum circuit; 6 AWG copper often satisfies ampacity at 75C	At 48A and 150 ft, 6 AWG copper is about 2.95%; 4 AWG is about 1.85%	6 AWG may pass a 3% design target, but 4 AWG gives margin for heat and future load
16A continuous sign circuit, 120V, 180 ft	16A x 125% = 20A branch-circuit ampacity	12 AWG copper is about 7.5%; 8 AWG is about 2.9%	The breaker size is not the hard part; voltage drop controls the conductor size
32A continuous IEC machine load, 400V three-phase, 70 m	IEC selection checks cable current capacity by installation method and grouping	10 mm2 copper can be near 2.2%; 16 mm2 can be near 1.4%	Choose the cross-section only after both thermal capacity and voltage drop are acceptable
24A battery charger, 208V, 220 ft	24A x 125% = 30A continuous-load circuit basis	10 AWG copper can exceed 5%; 6 AWG can land near 2.7%	A small continuous load can still need a large conductor on a long run
80A process heater feeder, 480V three-phase, 260 ft	80A x 125% = 100A feeder basis unless equipment listing changes it	3 AWG copper is near 2.0%; 1 AWG is near 1.25%	Ampacity may set the minimum, while voltage drop sets the engineered margin

Worked Examples with Specific Numbers

EV charger: 48A continuous load on a 60A branch circuit

A garage EVSE is 150 ft one-way from the panel on a 240V circuit. Continuous-load sizing gives 48A x 1.25 = 60A, so the branch circuit is built as 60A. For voltage drop, use the actual 48A charging current. With 6 AWG copper at roughly 0.491 ohm/kft, drop is about 7.07V or 2.95%. That is close to a 3% target; 4 AWG drops closer to 1.85%.

Outdoor sign: 16A continuous, 120V, 180 ft

A sign load runs all evening and qualifies as continuous. The circuit ampacity basis is 20A, but a 12 AWG copper run at 16A and 180 ft is roughly 9.1V, about 7.6% on a 120V circuit. Upsizing to 8 AWG gets close to 3%. The calculation explains why a normal 20A branch circuit can still be a poor design.

IEC machine feed: 32A for 3 hours or more, 70 m route

A production machine draws 32A for a full shift on a 400V three-phase supply. IEC 60364-5-52 requires cable selection by current-carrying capacity, installation method, grouping, and voltage drop. A 10 mm2 copper cable may be near 2.2% voltage drop, while 16 mm2 may be near 1.4%. If grouped with other loaded circuits, the thermal derating can make 16 mm2 the practical answer.

Checklist Before You Trust the Result

Do not enter the breaker rating as the voltage-drop current unless the load can actually draw that current.
For mixed loads, calculate noncontinuous load plus 125% of continuous load for ampacity, then use the realistic coincident load for voltage drop.
Check terminal temperature limits before assuming a 75C ampacity value is allowed on small equipment.
After upsizing, verify conduit fill, box fill, bending space, and the maximum conductor size accepted by the equipment lugs.
For critical equipment, use the stricter project specification if it requires 2% total voltage drop or a lower limit than NEC informational guidance.

Common Questions

Should I calculate voltage drop at 125% of a continuous load?

Usually no. Use 125% for ampacity and overcurrent sizing under NEC 210.19(A)(1), 210.20(A), 215.2(A)(1), or 215.3, then calculate voltage drop at the actual expected current such as 48A for a 48A EV charger.

Does a 60A EV charger circuit mean I enter 60A in the calculator?

Only if the EVSE can draw 60A. A 48A EVSE commonly uses a 60A circuit because 48A x 125% = 60A, but voltage drop should be checked at 48A.

What voltage-drop limit should I use for continuous loads?

Many NEC designs target about 3% for a branch or feeder segment and 5% total. Data rooms, medical equipment, or industrial specifications may call for 2% or less.

Can upsizing for voltage drop change code compliance?

Yes. Larger conductors can affect conduit fill under NEC Chapter 9, box sizing under NEC 314.16, and lug compatibility. Re-check the whole installation after upsizing.

How does IEC work differ from NEC for continuous operation?

IEC 60364-5-52 combines current-carrying capacity, installation method, grouping, ambient temperature, and voltage drop. The principle is similar: thermal suitability and voltage performance are separate checks.

Use the Calculator with the Right Current

Enter the actual load current for voltage drop, then use the wire-size, derating, and conduit-fill tools to confirm the installation can carry the continuous load safely.

Open Voltage Drop Calculator Related Derating Guide

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