Why is the last few feet (or metre) to a motor usually flexible or liquidtight flexible conduit rather than rigid?

B. It takes up vibration, lets the motor be adjusted on its base (belt tension), and makes removal easier

C. Rigid conduit is not allowed in industrial buildings

D. Flex carries more conductors

Where is rigid PVC conduit most at home?

B. Underground, in slabs, and in wet or corrosive locations where steel would rust

A. Exposed in a hot mechanical room ceiling

C. As the only bonding path for a circuit

D. Anywhere exposed to physical damage

What is a main advantage of cable tray in an industrial plant?

B. Large numbers of cables can be installed, added and changed without pulling through a raceway

C. Any cable can go in it without restriction

D. It is always the cheapest method for a single circuit

Teck (in Canada) and MC cable (in the US) are both…

B. Armoured cables with their own conductors inside, used where a raceway would be pulled otherwise

D. Only for low-voltage control

What is busway (bus duct) typically used for?

B. Carrying large currents in a compact run — service entrances, risers, and plug-in feeds along a factory floor

A. Low-current lighting branch circuits

Before choosing a wiring method for a run, which question is NOT one you should be asking?

D. Which method was used on the last job, so I can do the same without checking?

A. Is the location wet, corrosive, hazardous, or exposed to damage?

B. Is this method permitted for this occupancy and use in my code book?

C. What does the spec and drawing call for?

Tercer año Lesson 11 of 13 · about 12 min

Commercial wiring methods

What you’ll learn

The main raceways and cables on a commercial or industrial job, and the job each is good at.
How the location — wet, corrosive, damage, heat, vibration — steers the choice.
The questions to ask the code book and the specification before a run goes in.
A little pulling physics: why bends multiply tension.

Before this: Conduit bending basics · Second year

A wiring method is the way conductors get from one place to another and stay protected: pulled into a raceway, built into a cable assembly, laid in tray, or carried as bars in a busway. On a house it is mostly one kind of cable; on a commercial or industrial job you will use half a dozen methods in one building, each where it suits. What is permitted where is your code book’s call, and what is required on this job is the specification’s. This lesson is about why each one exists.

One building, several methods, each doing the job it is best at. Which is permitted in each place is set by your code book and chosen on the drawings.

Raceways: the pipe first, the wire later

Method	Good at	Watch for
EMT (thin-wall steel)	Fast, light, bends by hand; the default indoor commercial raceway	Physical damage, wet or corrosive places; set-screw vs compression fittings
Rigid steel (RMC) / IMC	Physical protection, outdoors, some hazardous locations; threaded joints	Heavy, slow, needs threading; corrosion in some soils
Rigid PVC	Underground, in slabs, wet and corrosive areas; no rust	Big thermal expansion; not a bonding path; sunlight and damage ratings
Flexible metal conduit	Short runs to equipment that moves or vibrates; awkward spaces	Bonding through it is limited; dry locations
Liquidtight flex	Motors and equipment in wet, oily or washdown areas	Fittings must match; bonding rules for its length

Cables, tray and busway

Armoured cable — AC90 and Teck90 in Canada, AC and MC cable in the US — puts the conductors, usually a bonding conductor, and a metal armour into one assembly. It goes in far faster than pipe and wire, handles vibration, and Teck’s jacketed armour suits industrial and tray work. Cable tray is an open ladder or trough that supports many cables at once; cables are laid in, not pulled, so adding a circuit later is easy — but only tray-rated cables go in it. Busway is copper or aluminum bars in a housing for large currents: a service riser, or a plug-in run down a factory floor that machines tap off anywhere.

The questions to ask before a run goes in

Where is it? Dry, damp, wet, corrosive, hot, cold, exposed to damage, hazardous, a return-air plenum, underground, in concrete?
What does the specification say? The owner may require more than the minimum — steel instead of cable, particular fittings.
What does the code book permit there? For this method in this location and occupancy — and the conditions attached to it.
How is it bonded? Is the raceway or armour the bonding path, or does a bonding conductor go with it?
How will it be supported, pulled and terminated? Supports, bends, pull points, fittings — the numbers are in the book and the maker’s instructions.

Worked: why bends multiply pulling tension (practice numbers)

Three conductors weighing 0.5 lb/ft each (a practice weight — the real one is on the cable data sheet): 1.5 lb/ft in total.
A 200 ft straight horizontal run with a friction coefficient of 0.5: tension = weight × length × friction = 1.5 × 200 × 0.5 = 150 lb (about 667 N).
Through a 90° bend, tension multiplies by e^μθ, θ in radians: 90° = 1.571 rad, so e^{0.5 × 1.571} = e^0.785 = 2.19.
Tension leaving the bend = 150 × 2.19 = about 329 lb (about 1,460 N). A second 90° would multiply it by 2.19 again.
So feed from the end nearest the bends, where the tension is still small, and compare against the cable maker’s maximum pulling tension.

En la obra

Tips

Read the wiring-methods section of the specification before the first stick goes up.
Flag every change of location type on a run — indoor to outdoor, dry to wet — because the method or fittings usually change there too.
Pull PVC runs with expansion in mind: leave room and use expansion fittings on long exposed runs.
Plan pull points so no section collects too many bends.
Keep armour and raceway joints tight — they are often part of the fault path.

Common mistakes

Using EMT set-screw fittings where the location calls for raintight or compression.
Running a cable type in tray that is not rated for tray.
Forgetting the bonding conductor in PVC.
Rigid conduit straight onto a vibrating motor.
Choosing the method by habit instead of by location, spec and code book.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.Why is the last few feet (or metre) to a motor usually flexible or liquidtight flexible conduit rather than rigid?
2.Where is rigid PVC conduit most at home?
3.You pull cable through a raceway with a straight-run tension of 150 lb (667 N) arriving at a 90° bend. With a friction coefficient of 0.5, roughly what is the tension leaving the bend?
4.What is a main advantage of cable tray in an industrial plant?
5.Teck (in Canada) and MC cable (in the US) are both…
6.What is busway (bus duct) typically used for?
7.Before choosing a wiring method for a run, which question is NOT one you should be asking?

Answers

B. It takes up vibration, lets the motor be adjusted on its base (belt tension), and makes removal easier — Motors vibrate and get moved to tension belts or align couplings. A rigid connection would fatigue and crack. Liquidtight versions add protection where there is water, oil or washdown. Whether flex can serve as the bonding path, and how long it may be, are questions for your code book.
B. Underground, in slabs, and in wet or corrosive locations where steel would rust — PVC does not corrode, so it shines underground, in concrete and in corrosive areas. It is not conductive, so a separate bonding conductor is always pulled with it, and it expands and contracts a lot with temperature — long exposed runs need expansion fittings.
C. About 330 lb — Tension multiplies through a bend by e^(μθ), with θ in radians. For 90° (1.571 rad) and μ = 0.5, e^0.785 = 2.19, so 150 × 2.19 ≈ 329 lb (about 1,460 N). Bends multiply — which is why you pull from the end nearest the bends when you can.
B. Large numbers of cables can be installed, added and changed without pulling through a raceway — Tray is an open support system: cables are laid in, not pulled, and adding a circuit later is easy. Only cable types rated for tray may be used, and fill, spacing and bonding of the tray follow the code book.
B. Armoured cables with their own conductors inside, used where a raceway would be pulled otherwise — Both are armoured cable assemblies: the conductors, often a bonding conductor, and a metal armour (Teck adds an outer jacket). They go in fast and handle tray and industrial use. Which uses they are allowed, and whether the armour counts as a bonding path, depend on the product and your code book.
B. Carrying large currents in a compact run — service entrances, risers, and plug-in feeds along a factory floor — Busway is rigid copper or aluminum bars in a metal housing, rated for large currents. Plug-in busway lets machines be tapped off anywhere along its length, which suits factories that rearrange equipment.
D. Which method was used on the last job, so I can do the same without checking? — Every run starts with the location, the occupancy, what the code permits there, and what the specification requires. "Same as last time" is how the wrong method ends up in a return-air plenum or a corrosive room.

Preguntas frecuentes

EMT or rigid — how do I know which one?

Start with the spec and drawings; the engineer usually decides. Behind that decision is the location: EMT is light, fast and fine for most dry indoor work; rigid (RMC) and intermediate (IMC) steel are for physical damage, outdoors, some hazardous locations and where the wall thickness matters. Where each is permitted is in your code book.

Is AC90 the same as MC?

They are close cousins. AC90 (Canada) and AC / MC cable (US) are all armoured cable assemblies with the conductors inside, but the armour construction and whether the armour counts as a bonding path differ between products. Read the cable marking and check your own code book for its uses.

Why does the spec book matter if the code book already sets the rules?

The code sets the minimum that is allowed. The specification tells you what this owner is paying for, which is often more — EMT instead of cable, steel instead of PVC, particular fittings or colours. You install to whichever is stricter, and the spec is part of your contract.

Ver también

Lo básico del doblado de conduit
Fittings index — the pieces each method uses
Reading commercial and industrial prints
Grounding and bonding as a system — when the raceway is the bonding path
Hazardous locations: the ideas

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