Pull Box Sizing and Voltage Drop Planning. Coordinate NEC 314.28 pull-box dimensions with long feeder voltage-drop checks so upsized conductors have enough straight-pull, angle-pull, splice, and bending space.

Use this localized engineering guide with the same calculation sequence: calculate voltage drop from real load current and one-way route length, choose the final conductor and raceway, then verify NEC 314.28 pull-box dimensions or local IEC enclosure-space requirements before installation.

סיכום

Run voltage drop first, then size pull boxes around the final conductor and raceway arrangement.
NEC 314.28 straight pulls commonly require 8 times the largest raceway trade size.
Angle pulls and U-pulls need distance equal to 6 times the largest raceway plus the sum of other same-wall raceways.
IEC-style projects still need bending radius, pulling tension, terminals, IP rating, and enclosure heat checks.

Key Definitions for Pull-Box Work

A pull box is an enclosure installed in a raceway system to provide pulling access, directional changes, splicing space, or maintenance access for conductors.

A straight pull is a conductor pull where raceways enter and leave opposite walls without an angle turn inside the box.

An angle pull is a conductor pull where the conductors change direction inside the box, so NEC 314.28 requires extra distance from raceway entry to the opposite wall.

A splice enclosure is a pull box or junction box that also contains conductor splices, taps, or terminations and therefore needs listed connectors plus additional working space.

Voltage drop is the voltage lost in conductors because resistance and impedance increase with length, current, temperature, and conductor material.

Field Workflow: Voltage Drop Before Pull-Box Sizing

Step 1

Map the one-way route from panel to the farthest load, including vertical drops, offset bends, wall runs, and device-to-device routing. Use that route length in the voltage drop calculator.

Step 2

Select conductor ampacity first from NEC 310.16 or the applicable IEC cable table. A 100A feeder may start with #3 Cu or #1 Al before voltage drop pushes the design larger.

Step 3

Run the voltage-drop check at realistic load current and power factor. For a 100A feeder carrying 80A continuous over 260 ft, check 80A and review 100A if the owner wants full-load margin.

Step 4

If voltage drop forces a larger raceway, recalculate every pull box using NEC 314.28. Straight pulls, angle pulls, U-pulls, and splice boxes use different dimensions.

Step 5

Confirm practical pulling space. A box can pass the minimum dimension and still be hard to use if 4/0 Al conductors, compression connectors, grounding bushings, or multiple conduits crowd one wall.

NEC and IEC Checkpoints

These checks connect voltage-drop performance with enclosure dimensions, conductor ampacity, and practical pulling workmanship. The page uses public references for the NEC and the IEC while keeping the enforceable decision tied to the adopted local code.

NEC 314.28(A)(1) requires straight-pull boxes for conductors #4 AWG and larger to be at least 8 times the trade size of the largest raceway.
NEC 314.28(A)(2) covers angle and U-pulls; the distance from each raceway entry to the opposite wall must be at least 6 times the largest raceway plus the sum of the other raceways on the same wall.
NEC 314.28(A)(2) also requires raceway entries containing the same conductor to be separated by at least 6 times the trade size of the largest raceway involved.
NEC 210.19(A)(1) informational note and NEC 215.2(A)(1) informational note are commonly used as 3% branch-circuit and 5% total voltage-drop design targets.
NEC 310.16 ampacity remains a separate pass; upsizing for voltage drop can improve performance but does not remove terminal temperature, insulation, or overcurrent-protection checks.
IEC 60364 projects do not use NEC 314.28 dimensions, but enclosure selection still has to respect bending radius, pulling tension, heat, IP rating, and manufacturer limits.

Pull-Box and Voltage-Drop Examples

Use these examples as planning checks. Always enter the real box volume, conductor count, conductor size, route length, voltage, material, and load current for the final design.

Circuit condition	Voltage-drop result	Pull-box impact	Practical decision
100A, 240V feeder, 260 ft one-way	#3 Cu may exceed 4%	Upsizing to #1 Cu can require larger raceway and larger pull boxes	Check 8x straight-pull length before routing through tight framing
200A, 208V three-phase feeder, 180 ft	3/0 Cu may be near 3%	Parallel or larger conductors can change conduit count on each wall	Recalculate angle-pull dimensions before ordering enclosures
225A aluminum feeder, 320 ft	4/0 Al may not meet a 3% target	250 kcmil or parallel Al can make pulling tension and box size control	Compare a nearer distribution panel against oversized boxes
480V motor feeder, 125A, 420 ft	Voltage drop may pass at 480V while starting drop is marginal	Long runs often need intermediate pull points for tension, not only voltage	Check motor starting and pull-box spacing together
400V IEC pump feeder, 95 m	35 mm2 may exceed a 3% target depending on PF	Moving to 50 mm2 changes bending radius and gland/enclosure choice	Use manufacturer bend radius and gland spacing with local code
Feeder splice box with compression lugs	Larger conductor reduces drop but increases connector size	Splices need more usable space than a no-splice pull point	Size for listed connectors and maintenance access, not only minimum dimensions

Worked Calculator Examples

100A, 240V feeder, 260 ft one-way

Enter 240V, single-phase AC, copper, 100A, and 260 ft. #3 copper can land above 4% voltage drop, while #1 copper can move the feeder closer to 3%. If that change increases the raceway to 1-1/2 in, a straight-pull box should be checked at 8 times the raceway trade size, or about 12 in minimum before practical space is added.

200A, 208V three-phase feeder with angle pull

A 200A feeder at 208V over 180 ft is sensitive to voltage drop because the system voltage is low for the current involved. If the design moves from 3/0 copper to 250 kcmil or a parallel run, the box must be checked under NEC 314.28(A)(2). For a 2 in largest raceway plus another 2 in raceway on the same wall, the opposite-wall distance starts at 6 x 2 in + 2 in, or 14 in before workmanship margin.

400V IEC pump feeder, 95 m route

For an 18A pump at 400V and 95 m, the voltage-drop result may move a design from 35 mm2 aluminum to 50 mm2 aluminum depending on power factor and temperature. NEC 314.28 may not apply on an IEC project, but the same field issue remains: bend radius, gland spacing, heat, IP rating, and safe pulling tension have to match the final cable.

Pull-Box Review Checklist

Record the final voltage-drop conductor size and raceway trade size before sizing pull boxes.
Identify straight pulls, angle pulls, U-pulls, and splice boxes separately; NEC 314.28 dimensions are not interchangeable.
For straight pulls, check at least 8 times the largest raceway trade size.
For angle and U-pulls, check 6 times the largest raceway plus the other same-wall raceways, and verify same-conductor raceway separation.
Add practical space for compression lugs, grounding bushings, pulling grips, firestopping, and future maintenance.
For IEC projects, confirm enclosure manufacturer bending radius, gland spacing, IP rating, heat rise, and local inspection practice.

Common Questions

Does upsizing wire for voltage drop change pull-box size?

Often, yes. Larger conductors can require larger raceways, and NEC 314.28 dimensions are based on raceway trade size for #4 AWG and larger conductors.

Can I ignore voltage drop if the pull box gets too large?

No. Pull-box size and voltage drop solve different problems. A code-sized pull box does not guarantee good voltage at the load, and a low-drop conductor does not prove the pull box is large enough.

When does NEC 314.28 apply?

NEC 314.28 applies to boxes and conduit bodies containing conductors #4 AWG and larger under 600V rules. Straight pulls and angle or U-pulls use different minimum dimensions.

Should I use breaker size or load current for feeder voltage drop?

Use the actual design current where known, then check worst-case loading if the feeder can be used near its rating. A 100A feeder carrying 80A continuous should be checked at 80A and often reviewed at 100A.

Is the NEC 314.28 minimum always enough for pulling?

Not always. NEC 314.28 gives minimum dimensions, but pulling tension, sidewall pressure, connector size, conductor stiffness, and future maintenance can require larger boxes.

How do IEC jobs handle pull-box sizing?

IEC 60364 does not use NEC 314.28 raceway multipliers, but the same engineering concern remains: cable bend radius, terminal capacity, enclosure heat, IP rating, and maintainable wiring space must all be checked.

Check the Wire Size Before You Order Pull Boxes

Run the voltage-drop calculation, choose the final conductor and raceway, then verify every pull point while the route can still change.

פתח מחשבון מדריך מילוי מובילים ודרייטינג

Pull Box Sizing and Voltage Drop Planning