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Wire & Cable Fundamentals — Module 03 of 06

Electrical Performance Terms

Every cable datasheet lists electrical parameters — but knowing the numbers and understanding what they mean in the field are two different things. This is the most technically detailed module in the series, covering the five performance specifications that govern whether a cable run will be safe, code-compliant, and functional for its intended application. Each term gets a definition, a formula, an explanation of consequences, and a real-world scenario drawn from industrial and commercial electrical practice.

5–6 min Ampacity · Voltage Drop · Impedance · Capacitance · Inductance Free — No Account Required

Watch on YouTube Module 03 — Electrical Performance Terms

Ampacity — the thermal current ceiling — is governed by NEC Table 310.15(B) and must be derated for conduit fill, high ambient temperature, and bundled cables. Voltage drop — calculated as VD = I × R (DC) or I × Z (AC) — must not exceed 3% on branch circuits per NEC guidance, or equipment suffers. Impedance — the frequency-dependent total opposition to AC current — becomes critical in VFD cable runs and data cabling where impedance mismatch causes destructive reflected voltage waves. Capacitance (measured in pF/ft) acts as a low-pass filter in long instrumentation cable runs, degrading 4–20mA loop response and RS-485 communication at long distances. And inductance (measured in µH/ft) combined with cable capacitance creates LC resonant circuits in VFD motor leads — sometimes doubling peak voltage at the motor terminals and destroying winding insulation.

WHAT YOU'LL LEARN

Ampacity: definition, NEC Table 310.15(B), and derating rules
Voltage drop: VD = I×R formula and the 3%/5% NEC guideline
Impedance: Z = √(R² + (XL−XC)²) and frequency dependence
Capacitance: pF/ft, low-pass filter effect on 4–20mA and RS-485
Inductance: µH/ft, LC resonance, and VFD motor lead voltage spikes
Real-world scenarios: boiler room feeder, CNC machine run, VFD cable
How to find all five values on a manufacturer's cable datasheet
Quick-reference comparison table: all 5 terms, units, and gotchas

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Who This Is For

Built for the people who actually spec and install wire

These modules were designed with one goal: give you the technical background to make correct wire and cable decisions in the field, in the office, and at the order desk — without having to dig through manufacturer documentation every time. The content is grounded in NEC code references, ASTM standards, and real manufacturer datasheets from Belden and Alpha Wire.

Electricians and apprentices — learn the code basis for conductor and jacket selection, and understand what's on the label before you pull it through the conduit
Controls and automation engineers — understand impedance, capacitance, and inductance as they apply to VFD cable runs, motor leads, and instrumentation loops
AV and low-voltage integrators — apply impedance matching, signal integrity principles, and NEC plenum/riser ratings to data and AV cable runs
Instrumentation and process technicians — get the cable parameters that matter for 4–20mA loops, thermocouple extension cables, and RS-485/PROFIBUS networks
Procurement and supply chain professionals — read a spec sheet accurately, decode part number suffix codes, and catch substitution errors before they ship

Common Questions

Frequently Asked Questions

What is ampacity and why does it matter?

Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. It is governed by NEC Table 310.15(B) and must be derated when conductors are installed in conduit with multiple cables, in high ambient temperatures, or when bundled together. Exceeding ampacity causes insulation to degrade silently over time — and eventually fail. Always calculate ampacity with appropriate derating factors for your installation conditions, not just the free-air table value.

How much voltage drop is acceptable on a branch circuit?

The NEC recommends no more than 3% voltage drop on branch circuits and no more than 5% total across the feeder and branch circuit combined. Voltage drop above these thresholds causes AC motors to draw higher current to maintain torque — which accelerates winding insulation breakdown — and causes controls, sensors, and lighting to operate below their design voltage. On long runs, upsize the conductor gauge to bring voltage drop within limits.

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Wire & Cable Fundamentals - Electrical Performance Terms

Electrical Performance Terms