Polyvinyl
Chloride Conduit
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PVC conduit is approved by the NEC for many types of applications.
But there are some situations where it cannot replace metallic conduit. It's not approved for hazardous locations or in return
air plenums. Check with the inspection authority for other restrictions. The standard length is 10 feet and sizes range from
1/2" to 6". Schedule 40 PVC is the standard weight. Schedule 80 has a heavier wall. PVC can be installed directly
underground, concrete encased underground, exposed, in concrete walls, and in unit masonry.
One coupling is furnished with each length of conduit
and is usually attached to the conduit. PVC must be bent with a special hot box which heats the conduit until it becomes pliable.
Once heated to the right temperature, the tube is bent and then allowed to cool. PVC fittings fit both Schedule 40 and 80
conduit. Couplings, terminal adapters, female adapters, expansion fittings, end bells, caps, conduit bodies, pull boxes, outlet
boxes and elbows require a special cement. The glue is air-drying and comes in half-pints, pints, quarts, and gallon containers.
The smaller containers have a brush attached to the cap for applying the cement to the conduit or fittings. PVC conduit can
join other types of conduit if you use the right fittings to tie the two types together.
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Polyvinyl Chloride Conduit
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PVC is nonconductive. That
makes a bonding conductor necessary to ensure electrical continuity from the device to the service panel. You probably won't
need a bonding conductor when PVC is used as communications conduit or in some application that doesn't include electrical
wiring. When installed exposed, PVC requires extra support to keep it from sagging.
Some job specs restrict use of PVC to specific locations. One common
restriction is to limit PVC to underground installations encased in a concrete envelope. Many specifications restrict its
use to certain applications.
PVC conduit can be cut with a hand hacksaw, a wood crosscut saw, or with a power cutting machine. The inside
cut edge should be reamed to remove the cutting burr. Use a pocket knife or a file.
Power and communications duct is usually called P&C duct. It's
made of PVC in 25-foot lengths and in diameters from 1" to 6". There are two types of P&C duct. One is called
EB for encased burial. The other is DB for direct burial. Fittings for P&C duct include couplings (one is furnished with
each length), end bells, caps and plugs, terminal adapters, female adapters, elbows, and expansion fittings. The elbows are
available in various shapes and with either long or short radii. Fittings can be used either on type EB or DB. Use a special
cement to weld the fittings to the conduit.
Bend P&C duct with a hot box. It can be cut with the same tools as PVC conduit.
The inside cut edge must be reamed to remove the cutting burr.
P&C duct is used for underground systems only, never above ground.
ABS underground duct is
used and installed the same as PVC P&C duct. It requires a special ABS cement to weld the fittings to the conduit. The
job specifications or the utility company may require either P&C, ABS, or PVC duct, depending on the specific use.
Galvanized Rigid Conduit
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Galvanized Rigid Conduit
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GRS or RSC (for rigid steel
conduit) is made with nonferrous steel and has a corrosion-resistant coating on the inside. The outer coating is either hot-dipped
galvanizing or electroplate. It comes in diameters from 1/2" to 6" and in 10-foot lengths with a thread on each
end. A coupling is furnished on one end of each length. GRS can be cut with a hand hacksaw, a pipe cutter, or with a cutting
machine. The inner cut edge must be reamed to remove the burr. Use a pipe reamer or a file.
After the pipe has been cut and reamed, it can
be threaded. Use a hand die for threading on a small job. Where there's more cutting and threading to be done, use a threading
machine. Several types are available. Small portable electric threading tools cut sizes up to 2". Larger threading machines
can cut, ream and thread conduit diameters up to 6". Another good choice for GRS up to 6" is a threading set that
uses a tripod vise stand and a threading head that clamps to the pipe in the vise stand. The threading head is turned with
a universal joint connected to a power vise. Another set uses a tripod vise stand to hold the conduit. The threading head
clamped on the conduit is turned with a reduction gear assembly powered by an electric drill. This rig works well on diameters
over 2".
Use enough cutting oil to keep the die cool and lubricated during thread cutting. Cutting oil comes in clear or dark
and in small cans, gallons and barrels. Use an oil can to keep a film of oil ahead of the dies. Commercial oiling units hold
about a gallon of cutting oil and recirculate oil back to the cutting teeth as oil drips into the catch basin. Most threading
machines have automatic oilers that filter the oil as it's reused.
Elbows are available for all sizes of GRS. Long radius bends are available for the
larger sizes. Some specifications require concentric bends for all exposed conduit installed parallel on a common hanging
assembly or trapeze.
GRS fittings include couplings, locknuts, bushings, one-hole straps, two-hole straps, heavy duty two-hole
straps, expansion fittings, threadless compression couplings, threadless set-screw couplings, threadless compression connectors,
threadless set-screw connectors, three-piece union-type couplings, strut clamps, beam clamps, hanger clamps, condulets, split
couplings, caps, and plugs.
Galvanized rigid conduit is bent about the same way as EMT except that the bender is made for bending rigid
conduit. Hand benders are used on conduit up to 1". There are hand benders for 1-1/4" and 1-1/2" rigid steel
conduit, but it takes a lot of effort to make the bend. Power benders can be used on all sizes of conduit, even the 1/2".
There are three common
types of rigid steel benders: one-shot benders create a single standard radius arc. Segment benders must be moved along the
conduit as each few degrees of bend are made. The electric sidewinder bender has up to three bending shoes in place ready
to bend any of three sizes of conduit. The sidewinder saves labor on larger rigid conduit jobs.
Supports for rigid conduit must be
no more than 10 feet apart from support to support and within 18 inches of junction boxes or cabinets.
Trapeze hangers are often used to carry
multiple runs of GRS conduit. Trapeze hangers can be made from strut, angle iron, or channel iron. The trapeze is supported
from the structural frame of the building
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with threaded rod - usually
either 3/8" or 1/2" diameter. The upper part of the rod is attached to beam clamps or concrete anchors. The lower
portion of the rod is run through the trapeze and is secured with double nuts and flat washers.
Like other hangers, trapezes have to
be placed within 10 feet of each other and should be sized to support the total weight of the conduit and all cable. Trapeze
hangers can be stacked one over the other with conduit clamped on each one.
IMC Conduit
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IMC Conduit
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Intermediate metal conduit
(IMC) has a thinner wall than GRS. It comes in the same sizes and uses the same fittings as GRS. The same tools can be used
for cutting, threading, and bending. It's made about the same way as GRS, comes in 10-foot lengths and is galvanized for corrosion
resistance. The difference is that IMC is lighter and easier to install than GRS. Some specifications restrict its use to
specific applications.
PVC Coated Conduit
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PVC Coated Conduit
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Both GRS and IMC conduit
come with a PVC coating for use in highly corrosive locations. Aluminum tubing also comes with a PVC coating, but applications
are restricted to specific uses. The PVC coating is either 10, 20 or 40 mils thick, and is bonded directly to the conduit
wall. Most fittings made for use with GRS are available with a PVC coating.
To thread PVC coated conduit, the PVC coating must be cut back away from the end to
be threaded. When PVC coated conduit is put in a vise, be sure the coating is protected from the vise jaws. Also be careful
when you're bending PVC coated conduit not to damage the coating. If the coating is damaged, patching material is available
to restore the surface. The material comes in a spray can. Apply several thin layers to repair worn spots.
Conduit Take-Off
