Wires, Conductors & Sizing
Pick the wrong wire and you start a fire. This lesson covers AWG sizes, ampacity tables, voltage drop, and the conduit/cable types every electrician picks from daily.
The lesson
AWG — backwards numbering
American Wire Gauge: SMALLER number = BIGGER wire. 14AWG (skinny) handles 15A. 12AWG (medium) handles 20A. 10AWG handles 30A. 6AWG feeds electric ranges. The numbers go the wrong way — get used to it.
Ampacity — current carrying capacity
NEC Table 310.16 lists ampacity. Derate for bundled wires, high ambient temp, and continuous loads (>3 hrs continuous = 80% of breaker rating). A 20A breaker on a continuous load = only 16A usable.
Voltage drop
Long wire runs drop voltage. NEC recommends ≤3% drop. Formula: VD = (2 × L × I × K) / CM, but easier: use a voltage drop calculator. For long runs (>50 ft) of high current, upsize by one AWG.
Cable types you'll see
NM-B (Romex) — most residential interior. UF — direct burial outdoor. THHN — single conductors in conduit. MC — metal-clad for commercial. SE — service entrance. THWN — wet locations.
Conduit basics
EMT (thin-wall) — interior commercial. Rigid (RMC) — underground and outdoor. PVC — wet, corrosive, underground. Maximum bend = 360° total between pulls (4 × 90s, no more), or you can't pull the wire.
Tool list
- Wire strippers (matching gauges)
- Ampacity table (NEC 310.16 printable card)
- Voltage drop calculator (app or slide rule)
- Conduit bender (1/2" and 3/4" EMT)
- Fish tape
- Reaming tool (deburrs cut conduit)
Safety — Read or get hurt
- !!Aluminum wiring requires antioxidant paste and proper torque — improper terminations cause house fires.
- !!Energized work — gloves rated for the voltage class, face shield, long sleeves.
- !!Conduit cuts shred skin — always ream and deburr.
Take the mini quiz
6 questions · pass at 80%