Neutral and harmonic loading
Nonlinear loads can increase neutral and conductor heating even when phase currents appear balanced; the entered grouping factor is not a harmonic study.
Engineering
Cable Load Calculator
Translate real load demand into line current, current per parallel run, derated cable ampacity, utilization, and spare continuous-current capacity.
CIRCUIT LOADING
Keep load demand, power factor, conversion losses, and cable derating on one traceable current basis
This calculator is for electrical designers and facility engineers who need to test whether a proposed conductor set can carry a stated load. It converts delivered real power to apparent input power, resolves single- or three-phase line current, distributes that current across parallel runs, and applies entered ambient and grouping factors. It is a screening calculation: the entered tabulated ampacity and correction factors must come from the governing wiring method and jurisdiction.
Calculated line current (A)—
Current per run (A)—
Derated ampacity/run (A)—
Installed ampacity (A)—
Ampacity utilization—
Spare current (A)—
CIRCUIT LOADING
Circuit load and ampacity reconciliation
Use the utilization result to identify whether the proposed conductor set has enough continuous-current capacity under the entered installation factors. A pass on ampacity does not prove acceptable voltage drop, short-circuit withstand, protective-device coordination, neutral loading, or termination temperature.
| Load step | Power or installation basis | Electrical factor | Current basis | Calculated outcome |
|---|
CURRENT CALCULATION PROCESS
Formula, substitution, intermediate values, and reconciliation
S = Pout / (PF × ηload); I = 1000S / (kphase × V); Iz = Itable × Camb × Cgroup × nruns; utilization = I / Iz
The method first works backward from delivered power through load efficiency and power factor. It then applies the correct phase multiplier to find line current and separately derates the selected conductor ampacity before comparing current with capacity.
Intermediate values remain unrounded until display formatting.
HOW TO USE THIS MODEL
Establish the real current before judging the cable
- Confirm whether the supply is single-phase or balanced three-phase and enter the voltage on the matching basis.
- Use a coincident real load at the stated duty, then document operating power factor and downstream load efficiency.
- Select the actual wiring method and obtain the tabulated ampacity for the conductor, insulation, and termination limits.
- Apply ambient and grouping factors that match the physical installation; do not invent a convenient combined factor.
- Compare per-run current, derated per-run capacity, total utilization, and remaining current before checking voltage drop and protection.
CIRCUIT LOADING FUNDAMENTALS
What determines cable loading
- Real power
- The useful electrical input measured in kilowatts after the effects of phase angle are separated from apparent power.
- Apparent power
- Volt-amperes demanded from the source; it rises when power factor or downstream efficiency falls.
- Line current
- Current in each energized line conductor, calculated on the selected single- or three-phase voltage basis.
- Tabulated ampacity
- A code- or standard-based current value for a defined conductor, insulation, ambient, and installation reference condition.
- Correction factor
- A multiplier that adjusts reference ampacity for a stated environmental or grouping condition.
- Parallel conductors
- Matched conductors intended to share current; unequal routing, terminations, impedance, or temperature can prevent equal sharing.
MODEL AND FORMULA
Convert delivered demand to current, then derate the conductor independently
S = Pout / (PF × ηload); I = 1000S / (kphase × V); Iz = Itable × Camb × Cgroup × nruns; utilization = I / Iz
The method first works backward from delivered power through load efficiency and power factor. It then applies the correct phase multiplier to find line current and separately derates the selected conductor ampacity before comparing current with capacity.
SYMBOLS AND DEFAULT CASE
Variable definitions, units, and starting assumptions
| Symbol or input | Meaning | Unit or default |
|---|---|---|
| Pout | Delivered real load demand | kW |
| PF | Operating power factor | dimensionless |
| eta_load | Load conversion efficiency | dimensionless |
| k_phase | Phase multiplier: 1 or sqrt(3) | dimensionless |
| V | Circuit line voltage | V |
| I_z | Installed corrected ampacity | A |
| phases | Circuit phases (1 or 3) | 3 |
| voltageV | Line voltage (V) | 400 |
| realPowerKw | Delivered real load (kW) | 120 |
| powerFactorPercent | Power factor (%) | 88 |
| loadEfficiencyPercent | Load efficiency (%) | 94 |
| parallelRuns | Parallel cable runs | 2 |
| conductorAmpacityA | Tabulated ampacity per run (A) | 230 |
| ambientFactorPercent | Ambient correction factor (%) | 91 |
| groupingFactorPercent | Grouping correction factor (%) | 80 |
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
Design questions a utilization percentage cannot answer
Protection and fault duty
A conductor can carry normal current yet fail short-circuit thermal withstand or be poorly coordinated with the upstream protective device.
Voltage at the equipment
Ampacity and voltage drop are separate constraints. Long circuits may require a larger conductor even when utilization is modest.
WORKED DECISION CASES
Two load decisions with different evidence
Motor control center feeder
A plant uses coincident motor demand, measured power factor, motor efficiency, and the actual tray grouping arrangement. Two parallel runs meet ampacity, after which the engineer checks starting voltage and protection.
Commercial kitchen renovation
Connected nameplate totals overstate simultaneous demand, but a single optimistic diversity number would understate current. The designer records the adopted demand basis and verifies termination temperature ratings.
TECHNICAL GLOSSARY
Cable loading terms
- Demand load
- Expected simultaneous operating load used to size a circuit.
- Power factor
- Ratio of real power to apparent power.
- Ampacity
- Maximum current a conductor may carry continuously under specified conditions.
- Derating
- Reduction of a reference current rating through applicable correction factors.
- Coincidence
- Degree to which connected loads operate at the same time.
- Current sharing
- Distribution of total circuit current among conductors connected in parallel.
EVIDENCE AND DATA LINEAGE
Retain the load schedule and installation basis
Keep the load list revision, coincidence assumptions, measured or manufacturer power factor and efficiency, system voltage basis, conductor construction, wiring method, insulation and termination temperatures, ambient record, grouping geometry, applicable ampacity table, correction-factor references, parallel-run details, and unrounded calculation output.
LIMITS AND EXCLUSIONS
What this loading screen excludes
- It assumes balanced phase loading and equal current sharing between parallel runs.
- It does not calculate voltage drop, fault current, short-circuit withstand, protective-device coordination, harmonics, neutral current, or cable pulling limits.
- The result is only as authoritative as the entered ampacity and correction factors for the governing code and installation.
RELIABLE SOURCES
References for this method and its boundaries
FREQUENTLY ASKED QUESTIONS
Cable load questions
Why does lower power factor increase current?
The source must supply more apparent power for the same real power, so current increases at a fixed voltage.
Why include load efficiency?
If the entered kW is useful output, electrical input must cover the load conversion loss. Enter input kW directly only with 100% for this field.
Can I enter one combined derating factor?
You can if it is documented and mathematically equivalent, but separate factors make the installation basis easier to audit.
Does current divide perfectly between parallel cables?
The arithmetic assumes matched runs. Real designs require equal conductor size, material, length, routing, and sound terminations.
Is unused ampacity an overload allowance?
No. Spare current is a planning margin within the entered steady-state model, not permission to exceed code or equipment ratings.
Does this select a protective breaker?
No. Protective-device rating, trip curve, fault current, coordination, and conductor protection are separate checks.
IMPORTANT ENGINEERING NOTE
Do not energize a feeder from this utilization result alone
Final cable selection must be reviewed by a qualified electrical professional using the adopted code, verified system and load data, equipment terminal ratings, protection study, voltage-drop and fault-duty checks, installation constraints, and manufacturer instructions.