First year Lesson 10 of 14 About 15 min

Boxes, devices and terminations

What you’ll learn

• How to strip a conductor cleanly and why a nick matters.
• The screw-terminal loop, the back-wire clamp and the push-in hole — and which to trust.
• Wire connectors and pigtails, and why the package instructions are part of the job.
• Why a loose connection gets hot, with the arithmetic.
• What box fill is and where the numbers for it come from.

Most electrical failures you will ever be called to are not in the middle of a wire — they are at the ends, where a conductor meets a terminal or another conductor. A good termination has full metal contact, holds its grip for decades and doesn’t damage the conductor getting there. This lesson is the workmanship that makes that happen.

Stripping

Strip with a stripper sized to the gauge — the hole cuts the insulation and stops at the copper. Take off only what the terminal needs: devices print a strip gauge on the back, and connectors give a strip length on the package. Check the copper afterwards. A nick or ring cut means less metal at that point, a weak spot that cracks when it is bent, and a small hot spot under load. Cut it off and strip again.

The screw terminal

Bend a hook about three-quarters of the way round the screw, clockwise, with the insulation stopping just short of the screw head. Close the hook with your pliers so it hugs the screw, then tighten. One conductor per screw unless the terminal is marked for more — if you need to feed through, splice and bring a short pigtail to the device.

Many devices also offer a back-wire clamp: a straight-stripped conductor goes in a hole and the terminal screw clamps a plate onto it. That is a strong connection and often allows two conductors. The push-in (back-stab) hole, with no screw, is a spring contact — it works, but it grips a much smaller area and is a common source of loose connections.

Splices and connectors

Twist-on connectors, push-in connectors, lever connectors, crimp sleeves and split-bolts all do the same job: hold conductors in solid metal contact. Each one is rated for a range of conductor sizes and combinations, and some for copper only. The rating and the installation steps are on the package — follow them, because the listing only covers the connector installed the way the maker tested it. Finish every splice with a tug test on each conductor.

Why a loose connection gets hot

A tight joint has almost no resistance. A loose one has a little — and the load current flows through it, making heat by P = I² × R. That heat oxidises the metal, which raises the resistance, which makes more heat. It is a runaway, and it ends in a melted device or a fire.

Worked: the same terminal, tight and loose

1. A receptacle feeds 15 A of load downstream through its terminals.
2. A good terminal, 0.002 Ω (a practice number): P = 15² × 0.002 = 225 × 0.002 = 0.45 W. Nothing you’d feel.
3. Worked loose to 0.2 Ω: P = 225 × 0.2 = 45 W — like a small soldering iron, sealed in a plastic box in a wall.
4. The voltage lost there: V = I × R = 15 × 0.2 = 3 V. The lights downstream barely notice, so nobody knows until it scorches.

Torque

“Tight” is not a number. Terminals on breakers, panels, lugs and many devices carry a specified torque on the equipment or in the instructions, and both codes expect it to be met. Too loose and you get the heat above; too tight and you can crush strands, crack the terminal or cold-flow aluminium. Use a torque screwdriver or wrench and the value on the label.

Boxes and box fill

Every box has a volume (stamped inside or listed by the maker) and every conductor, device, clamp and fitting inside takes an allowance of that volume. Box fill is the sum that proves they fit — and there has to be room left to fold everything in without damaging it. The allowances per conductor size and the counting rules are in the CEC (CSA C22.1) or the NEC, whichever you work under; they differ between the two, so use your own book. Do the sum when you pick the box, not when the device won’t go in.

Try one: 20 A through a 0.1 Ω joint

P = 20² × 0.1 = 400 × 0.1 = 40 W. Double the current and the heat goes up four times: at 10 A the same joint makes 10 W.

Sur le chantier

Check yourself

7 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.

Not started yet.

1. Which way should a conductor wrap around a terminal screw?

   A Clockwise — the same way the screw tightens B Counter-clockwise C It doesn’t matter D Straight under the screw, no hook
   Answer and why

   A. Clockwise, so tightening the screw pulls the loop in under the head. Counter-clockwise and the screw pushes the loop out as it tightens.

2. Where do you find how tight a terminal should be?

   A Tight as you can get it B On the device, the equipment label or the maker’s instructions C Half a turn past snug, always D In Ohm’s law
   Answer and why

   B. Terminals have a specified torque, and it is on the device, the equipment or the instructions. A torque screwdriver or wrench is how you know you hit it.

3. How do you work out whether a box has room for its conductors and devices?

   A If the cover goes on, it fits B Count to eight C Do the box fill calculation using the volumes and allowances in your code book D Ask the inspector afterwards
   Answer and why

   C. Box fill is a calculation: the box’s volume against the allowances the CEC or NEC gives for each conductor, device, clamp and fitting. The numbers are in the book — do the sum before you rough in.

4. A stripped conductor has a nick in the copper. What is the problem?

   A None, the insulation covers it B It changes the voltage rating C Less copper at the nick: a weak spot that can break and a small hot spot D It makes the breaker trip
   Answer and why

   C. The nick reduces the area — more resistance right there, and a place where the conductor will crack when it is bent. Re-strip it.

5. A loose terminal has 0.2 Ω of resistance and carries 15 A. How much heat is made at the terminal?

   A 3 W B 45 W C 0,75 W D 300 W
   Answer and why

   B. P = I² × R = 15 × 15 × 0.2 = 225 × 0.2 = 45 W, in a space the size of a screw head.

6. Can you put two conductors under one screw terminal?

   A Always B Never C Only if the terminal is marked or listed for it — otherwise use a pigtail D Only on the neutral
   Answer and why

   C. Most device screw terminals are for one conductor. Some are made for two (a back-wire clamp often is) and say so. When in doubt, splice and pigtail.

7. What does a twist-on or push-in wire connector’s packaging tell you?

   A Nothing useful B The conductor sizes and combinations it is rated for, and how to install it C Only the colour D The breaker size to use
   Answer and why

   B. Every connector is rated for a range of conductor sizes and combinations and has install instructions (strip length, whether to pre-twist). Follow them — that is what the listing depends on.

Start the quiz over

Questions fréquentes

Voir aussi

• Duplex receptacle wiring — the terminals on a receptacle and what each is for.
• Common wiring devices — what goes in the box.
• Fittings index — connectors, clamps and fittings, named.
• Reading prints and symbols — the lesson before this one.
• Grounding and bonding — the next lesson.
• All first year lessons