Current concentration
Short high-current peaks can dominate heating even when average energy efficiency looks favorable.
Engineering
Cable Efficiency Calculator
Estimate real power delivered through a cable circuit, conductor I²R loss, efficiency, voltage drop, and annual loss energy from one measured operating point.
FEEDER ENERGY BALANCE
Reconcile source power with conductor heat before calling a circuit efficient
This page helps energy engineers and electrical designers quantify how much measured source power is lost as conductor heat at one steady operating condition. It calculates phase-correct real input power, temperature-basis conductor resistance entered by the user, I²R loss across loaded conductors, delivered real power, voltage drop, and annual loss energy. It does not replace an ampacity or thermal model; it makes the energy balance explicit.
Cable efficiency—
Source real power (kW)—
Delivered power (kW)—
Conductor loss (kW)—
Voltage drop—
Annual loss energy (kWh)—
FEEDER ENERGY BALANCE
Cable energy-balance ledger
Efficiency is meaningful only when all measurements describe the same steady state and resistance uses the same temperature and AC/DC basis as the circuit. Compare loss energy with the cost and feasibility of a larger conductor or shorter route, not with an arbitrary universal efficiency target.
| Energy-balance step | Electrical measurement | Path model | Time basis | Calculated outcome |
|---|
CURRENT CALCULATION PROCESS
Formula, substitution, intermediate values, and reconciliation
Psource = kphaseVI PF; Rpath = RacL / (1000nruns); Ploss = nconductorsI²Rpath; ηcable = (Psource − Ploss) / Psource
The calculator uses simultaneous voltage, current, and power factor to establish source real power. It treats the entered resistance as the effective resistance of one conductor at operating condition, divides it across parallel paths, and applies I²R to every loaded conductor.
Intermediate values remain unrounded until display formatting.
HOW TO USE THIS MODEL
Build an efficiency result from simultaneous measurements
- Record RMS voltage, line current, and true power factor at the same operating condition.
- Confirm phase basis and whether the resistance value is AC or DC and corrected to conductor operating temperature.
- Use actual one-way route length and the number of electrically matched parallel runs.
- Review source power, heat loss, voltage drop, and delivered power before annualizing the loss.
- Use a load profile rather than one operating point when current varies materially, because loss follows current squared.
FEEDER ENERGY BALANCE FUNDAMENTALS
Energy layers in a cable circuit
- Source real power
- Real electrical power entering the modeled feeder.
- Conductor resistance
- Opposition that converts part of electrical energy to heat; it changes with material and temperature.
- Copper or aluminum loss
- Thermal loss proportional to current squared and resistance.
- Delivered power
- Source real power remaining after modeled conductor loss.
- Voltage drop
- Reduction in RMS voltage along the resistive path at the stated load and power factor.
- Loss duration
- Operating hours used to convert an instantaneous kW loss to kWh; a variable load needs interval integration.
MODEL AND FORMULA
Close the circuit energy balance before annualizing it
Psource = kphaseVI PF; Rpath = RacL / (1000nruns); Ploss = nconductorsI²Rpath; ηcable = (Psource − Ploss) / Psource
The calculator uses simultaneous voltage, current, and power factor to establish source real power. It treats the entered resistance as the effective resistance of one conductor at operating condition, divides it across parallel paths, and applies I²R to every loaded conductor.
SYMBOLS AND DEFAULT CASE
Variable definitions, units, and starting assumptions
| Symbol or input | Meaning | Unit or default |
|---|---|---|
| P_source | Source real power | kW |
| k_phase | Phase multiplier: 1 or sqrt(3) | dimensionless |
| R_path | Equivalent resistance of one loaded path | ohm |
| R_ac | Entered conductor resistance | ohm/km |
| P_loss | Conductor heat loss | kW |
| eta_cable | Delivered power divided by source power | dimensionless |
| phases | Circuit phases (1 or 3) | 3 |
| voltageV | Source voltage (V) | 400 |
| currentA | Line current (A) | 185 |
| powerFactorPercent | Power factor (%) | 90 |
| lengthM | One-way route length (m) | 180 |
| resistanceOhmPerKm | Conductor resistance (ohm/km) | 0.153 |
| parallelRuns | Parallel runs | 2 |
| operatingHours | Annual operating hours | 4200 |
Percent inputs are converted to decimal factors once. The live calculation process above substitutes the current values in order, names intermediate quantities, reports the final result, and closes with a reverse or conservation check.
DEEP ENGINEERING ANALYSIS
Why a high cable efficiency can still hide a design problem
Temperature feedback
Higher conductor temperature raises resistance, which raises loss and may increase temperature again until the installation reaches equilibrium.
Power-quality effects
Skin effect, proximity effect, harmonics, and neutral current can make effective AC loss exceed a simple DC-resistance estimate.
WORKED DECISION CASES
Two efficiency assessments requiring different records
Process feeder retrofit
An energy team compares measured loss against a larger parallel feeder. The saving uses interval currents, actual cable temperature, and outage cost rather than extrapolating one noon reading.
Data-center distribution
The feeder appears efficient at rated load, but harmonic current and neutral heating require an AC-resistance and power-quality assessment beyond the simple balanced model.
TECHNICAL GLOSSARY
Cable efficiency terms
- I²R loss
- Power converted to heat in a conductor, proportional to the square of current.
- AC resistance
- Effective conductor resistance including frequency-dependent effects.
- RMS current
- Current magnitude that produces equivalent heating in a resistor.
- Delivered power
- Modeled source real power minus conductor loss.
- Load profile
- Time series of operating current or power used for energy integration.
- Energy balance
- Reconciliation of input, useful output, and modeled loss on the same boundary.
EVIDENCE AND DATA LINEAGE
Retain synchronized electrical and thermal evidence
Keep meter timestamps and accuracy, phase configuration, voltage and current RMS definitions, power-factor method, cable material and size, AC/DC resistance basis and temperature, measured or estimated conductor temperature, route length, parallel-run equality, operating-hour profile, and the unrounded energy balance.
LIMITS AND EXCLUSIONS
Boundaries of this steady resistive model
- It excludes dielectric, sheath, screen, circulating, harmonic, skin, proximity, and neutral losses unless included in the entered resistance.
- It assumes balanced phases, constant current, and equal parallel current sharing.
- It is not an ampacity, thermal, fault, protection, or voltage-regulation compliance calculation.
RELIABLE SOURCES
References for this method and its boundaries
FREQUENTLY ASKED QUESTIONS
Cable efficiency questions
Why does the model use loaded conductor count?
Each current-carrying path dissipates I²R heat; a balanced three-phase set has three loaded phase conductors.
Is voltage drop identical to efficiency loss?
No. Voltage drop is a voltage-quality metric; efficiency compares real power delivered with real power supplied.
Can I use resistance at 20°C?
Only as a preliminary screen. Operating-temperature AC resistance is a better basis for an energized cable.
Why is annual loss based on entered hours?
The tool assumes those hours occur near the entered current; a varying load should be integrated interval by interval.
Does parallel cabling halve loss?
At the same total current, two equal paths halve equivalent resistance and therefore modeled loss, provided current sharing is equal.
Can efficiency exceed 100%?
No. The page rejects a loss equal to or greater than source real power because that indicates inconsistent inputs or units.
IMPORTANT ENGINEERING NOTE
Treat the result as a measured-point energy screen
Before changing a cable or operating limit, a qualified electrical professional should verify the measurement boundary, conductor temperature, AC resistance, power quality, installation thermal conditions, applicable code, protective devices, fault withstand, and equipment voltage requirements.