Advanced bending: segmented, parallel and PVC bends

Hand benders handle branch circuit conduit up to about 1¼ inches. Feeders, service conduits and large raceway banks need larger conduit, bent with mechanical, hydraulic or electric benders, and they often run in groups whose bends must nest neatly together. PVC, widely used underground and in corrosive areas, is not bent with a shoe at all but with heat. This lesson covers the equipment for large conduit, segmented (large-radius) bends, concentric and parallel bending of conduit banks, and the heat bending of PVC, with the calculations that make each one come out right.

Benders for large conduit

Several types of machine extend bending to larger sizes:

• Mechanical (ratchet or lever) benders: hand-operated, with a ratchet or long lever multiplying force, for EMT and rigid up to about 1¼ inches.
• Hydraulic benders: a hand or electric hydraulic pump drives a ram that pushes the conduit into a shoe, for rigid and EMT from 1¼ up to 4 inches. They may bend in one shot or in segments.
• Electric benders: motor-driven shoes that make one-shot bends quickly and repeatably, with a protractor or digital readout, for EMT, IMC and rigid up to 2 inches or more. They are standard on large jobs and in prefabrication shops.

Each bender and shoe set has its own take-up, gain and minimum stub length, given in tables supplied by the manufacturer. Large benders also have a minimum distance from the end of the conduit for a bend (because the conduit must be gripped), which affects how short a stub can be. Always use the shoe that matches the conduit type and size: a rigid shoe on EMT, or an EMT shoe on rigid, will flatten or kink it.

Segmented bends

Some bends need a larger radius than any shoe provides, for example sweeps in large feeders that must accept cables with large minimum bending radii, or a gentle curve following a curved wall. A segmented bend builds a large-radius curve from many small bends ("shots") spaced evenly along the arc. The method:

• Choose the radius R and the total angle (usually 90°).
• Choose the number of shots n; more shots make a smoother curve. Each shot is the total angle divided by n.
• The arc length of the bend is 2πR × (angle/360°). The spacing between shots is the arc length divided by n.
• Mark the shots along the conduit and bend each one to its angle, keeping all bends in the same plane.

A handy shortcut uses the fact that 2π/360 ≈ 0.01745: the arc length of a 90° bend is about 1.57 × R. For bends where the radius is set by the bender, the manufacturer's gain table replaces this calculation.

Concentric bends

When several conduits turn a corner together, their 90° bends must nest without crossing or changing spacing. In a concentric bend, each conduit is bent around the same centre point with a radius that increases by the centre-to-centre spacing of the conduits. The inside conduit has the smallest radius (often the bender's standard radius) and each outer conduit has a radius larger by the spacing. Because the radii differ, so do their arc lengths and their marks: for a 90° bend, each step outward adds about 1.57 times the spacing to the arc. Concentric bends look professional and keep conduits evenly spaced through the turn, but each conduit needs its own bending setup, so they are usually made with segmented methods or with sets of shoes made for the purpose.

The alternative for parallel 90° bends is to give every conduit the same radius and move the bend points: each outer conduit starts its bend further along by the spacing, so the stubs remain evenly spaced but the curves are not concentric. This is simpler with a hand or electric bender and is common for branch circuit conduits in groups.

Parallel offsets

When a group of parallel conduits makes an offset together, each conduit's marks must be shifted so the offsets line up and the spacing between conduits stays constant through the bends. If all conduits were marked at the same distance from a common start line, the offsets would cross over each other, because each conduit farther from the inside of the first bend must start its bends later. The correction for each successive conduit is the centre-to-centre spacing multiplied by tan(θ/2), the same factor as shrink per inch:

Bending PVC

Rigid PVC conduit cannot be bent cold; it is softened with heat and shaped by hand or with forms. Methods include:

• Hot boxes and heaters: an electrically heated box or tube into which the conduit section is placed until it becomes pliable.
• Heat blankets: electric blankets wrapped around the conduit, convenient in the field for larger sizes.
• Hot air or infrared heaters for small adjustments.

Heat the section evenly by rotating it, and avoid scorching, which weakens the conduit. Larger sizes (over about 2 inches) are prone to flattening and are often bent with plugs in the ends (to keep air pressure inside) or a flexible insert. Once shaped, the bend is cooled with a wet cloth or water to set it before it is moved. PVC remembers its original shape somewhat when reheated, so it can be corrected, but repeated heating degrades it. Many installers prefer factory-made elbows for standard 90° and 45° bends in large sizes, especially for underground duct banks, where factory sweeps with large radii are specified for cable pulling.

Because PVC expands with heat far more than steel, long exposed runs need expansion fittings (Lesson 2), and bends made on a hot day in direct sun will shift as the conduit cools.

Bending rigid conduit

Rigid and IMC are bent with the same principles as EMT, but they are stiffer and require more force and appropriately sized shoes. Hand-bending ½- and ¾-inch rigid uses the next larger EMT bender (whose shoe fits rigid of the smaller size), with its own take-up values. Bends near threaded ends can damage the threads, so plan stubs so that threads are cut after bending where possible, or protect them with a coupling during the bend. Galvanizing cracked during bending should be touched up with zinc-rich paint to restore corrosion protection.

Safety with power benders

Hydraulic and electric benders exert large forces. Keep hands and clothing clear of the shoe and the pinch points as the conduit bends; support long conduit lengths so they do not swing as they bend; never stand in the arc of a conduit being bent; check hydraulic hoses and fittings for leaks and damage before use; and follow the manufacturer's procedures for shoe changes and maximum sizes. Electric benders should be on a ground-fault protected supply and their foot or hand controls should stop motion immediately when released.

Using bending tables and apps

Manufacturers publish tables for every shoe: take-up, gain, minimum stub, offset multipliers and segment spacing for common radii. Many electricians now use bending apps or calculators that take the bender model, conduit size and desired dimensions and return the marks, and electric benders with digital controllers store programs for repeated bends. These tools speed the work and reduce arithmetic errors, but they are only as good as the inputs: the right bender model, accurate measurements and a clear understanding of which reference (arrow, star, centreline or outside of the bend) each output refers to. Check the first bend from any new tool or program against a tape measure before trusting it for a whole rack.

Quality checks on large bends

Large conduits are expensive and heavy, so mistakes are costly. Before bending, confirm the conduit type and size match the shoe, check that the conduit is free of dents, and mark the bend plane with a straight line along the conduit so that each shot or bend stays in plane. After each bend, check the angle and the plane; after the final bend, measure the overall dimensions against the layout. Look for flattening on the outside of the bend and wrinkling on the inside; a bend whose internal diameter is reduced must not be installed, because it makes pulling harder and may violate the code's requirement that bends not damage the raceway. Racks of large conduits should be dry-fitted in place before couplings are made up.

Prefabrication shops apply these methods at scale. A shop with an electric bender, a threading machine and a well-organized cutting station can produce racks of identical offsets and concentric bends far more consistently than field crews, and deliver them bundled and labelled by location. The field crew's job then becomes accurate measurement before fabrication and careful installation afterwards. Whichever approach is used, the geometry is the same, and an electrician who understands it can check prefabricated work, adapt it when field conditions differ from the model, and make the occasional bend that no table covers.

Large conduit work also requires attention to handling. Lengths of 3-inch or 4-inch rigid conduit weigh well over 50 kg each after bending, so racks of them are moved with lifts or carts, and installation at height uses mechanical lifting aids and more than one person.