Conductors and insulation
What you’ll learn
- Why copper and aluminium conduct, and why insulation doesn’t.
- The three things that set a conductor’s resistance: material, length and cross-section.
- How AWG sizes and metric mm² sizes work, and the “three sizes doubles the area” rule.
- What an insulation rating tells you — and why ampacity always comes from your code book.
A conductor is a material that lets current flow easily; an insulator is one that almost won’t let it flow at all. A wire is both: a conductor to carry the current, wrapped in insulation to keep it where it belongs. This lesson is about what makes a conductor good, what sets its resistance, how sizes are named, and what the insulation is rated for.
Why some materials conduct
In metals like copper and aluminium, the outermost electron of each atom is only loosely held. Those electrons drift from atom to atom easily, so a small voltage moves a lot of them — low resistance. In insulators like PVC, rubber, glass or air, the electrons are held tightly, and it takes a very large voltage to force any current through. Push hard enough and any insulator breaks down — that is why insulation has a voltage rating.
Copper is the everyday conductor: it conducts very well, bends without cracking and holds a screw terminal. Aluminium is lighter and cheaper but conducts less well for its size, so an aluminium conductor has to be larger to do the same job. It also forms a hard oxide skin and slowly creeps away from a clamping screw, which is why it needs terminals made and marked for it.
What sets a conductor’s resistance
Three things, and they are common sense once you picture current as traffic:
- Materialcopper beats aluminium
- Length ↑ → R ↑twice as long, twice the R
- Area ↑ → R ↓twice the area, half the R
Put together, that is R = ρ × L ÷ A: resistance equals the material’s resistivity (ρ, “rho”) times the length, divided by the area. You will rarely calculate ρ yourself — conductor resistances come from the tables in your code book or the maker’s data — but the ratios are worth owning, because they let you reason about any run. Temperature matters too: a copper conductor’s resistance rises as it warms up.
Sizes: AWG and mm²
North American building wire is sized in AWG (American Wire Gauge) up to 4/0, then in kcmil (thousands of circular mils) above that. AWG runs backwards — the bigger the number, the smaller the wire — because it grew out of how many times wire was drawn through a die. Most of the world, and much equipment data, uses mm² of cross-section instead.
The AWG series is geometric. Three gauge steps double the area (and halve the resistance); six steps double the diameter. So a #10 has about twice the copper of a #13 and about four times the copper of a #16.
Worked: reasoning about a run without a table
- A conductor in a run measures 0.5 Ω end to end (a practice number).
- Same wire, run twice as far: resistance doubles → 1.0 Ω.
- Back to the original length but three AWG sizes larger: about twice the area → about 0.25 Ω.
- Twice as far and three sizes larger: the two changes cancel → about 0.5 Ω again. That is the idea behind upsizing a long run for voltage drop.
What the insulation is rated for
The letters printed on a conductor or cable name its insulation type, and the markings tell you what it is rated for. The ratings that matter first:
- Voltage — the most it is built to insulate against. 300 V and 600 V (1000 V in some metric-rated products) are common markings.
- Temperature — how hot the insulation may run. This is what limits current: current through resistance makes heat, and the insulation must not cook.
- Location — dry, damp or wet; sunlight resistance; direct burial. The marking says where it may go.
Ampacity — the current a conductor may carry continuously — comes from the tables in the CEC (CSA C22.1) or the NEC, whichever you work under. It depends on the size, the material, the insulation’s temperature rating, the surrounding temperature and how many conductors are grouped together. This site never quotes those tables: look the value up in your own current book, every time.
Try one: which has the lower resistance?
A #12 copper conductor 30 m long, or a #12 copper conductor 60 m long? The 30 m one — half the length, half the resistance. Now a #9 copper 60 m long versus the #12 at 30 m: #9 is three sizes up (twice the area) and twice as long, so they come out about equal.
En la obra
Tips from the job
- Read the jacket. The printed marking tells you the conductor size, material, insulation type, voltage and temperature ratings — that is what the inspector reads too.
- Before you pick a size, find the ampacity in the CEC (CSA C22.1) or the NEC, whichever you work under, for that insulation and that installation. The same gauge can carry different currents in different conditions.
- Check every terminal for its marking (CU, AL/CU, CO/ALR). If it doesn’t say aluminium, don’t land aluminium on it.
- Keep a stripper that matches the gauge. A nicked conductor has less copper at the nick — less area, more resistance, a hot spot.
Common mistakes
- Reading AWG the wrong way round — thinking #14 is bigger than #12.
- Guessing a conductor’s ampacity from memory or from a chart on a forum instead of the current code book.
- Nicking or ringing the copper with a knife when stripping.
- Assuming a conductor rated for dry locations is fine in a wet one. The insulation marking decides that.
Check yourself
8 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.
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Why is copper used for most building wire?
Answer and why
B. Copper’s outer electrons move easily from atom to atom, so it has low resistance, and it bends, strips and terminates well. Every real conductor still has some resistance.
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Double the length of a conductor, same size and material. Its resistance…
Answer and why
D. Resistance is proportional to length: twice the distance for the current to travel, twice the resistance.
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Double the cross-sectional area of a conductor, same length. Its resistance…
Answer and why
A. Resistance goes down as area goes up: twice the area is twice the room for current, so half the resistance.
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In the AWG system, which is the larger conductor?
Answer and why
C. AWG counts backwards: the bigger the number, the smaller the wire. #10 is larger than #14, which is larger than #18.
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Going up about three AWG sizes (for example #14 to #11) roughly does what to the area?
Answer and why
B. The AWG series is geometric: every three gauge steps is almost exactly double the area (every six steps doubles the diameter).
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Where does the current a conductor is allowed to carry (its ampacity) come from?
Answer and why
C. Ampacity is a code-book value. It depends on the conductor, its insulation’s temperature rating and the installation conditions — look it up every time; do not guess from the gauge.
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What limits how much current an insulated conductor can safely carry?
Answer and why
A. Current through resistance makes heat (P = I² × R). The limit is set by how hot the insulation can get without breaking down — which is why insulation has a temperature rating.
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Why does aluminium conductor need terminals marked for it?
Answer and why
D. Aluminium grows a hard oxide skin and slowly flows away from a clamping screw. Terminals marked for aluminium (for example AL/CU, or CO/ALR on devices) are designed to hold it. Follow the marking and the maker’s instructions.
Preguntas frecuentes
Is stranded wire better than solid?
Neither is “better”; they suit different jobs. Stranded is flexible — easier to pull through a long conduit run and better where there is movement or vibration. Solid holds its shape, is easy to loop around a device screw, and is common in smaller residential sizes. The code book and the terminal’s marking say what is allowed where.
Why does the same wire have different ampacities?
Because ampacity is about heat. A conductor in free air sheds heat easily; the same conductor bundled with others in a hot attic doesn’t. The insulation’s temperature rating, the surrounding temperature and the number of current-carrying conductors together all change the number, and the tables and correction factors in your code book account for them.
Ver también
- Ampacity — what the word means and where the numbers live.
- Combination circuits — the lesson before this one.
- Reading prints and symbols — the next lesson.
- Hot, neutral, ground — what each conductor in a cable does.
- All first year lessons
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