Voltage-limited capacity
A long feeder may reach its permissible voltage drop before thermal current capacity is used.
Engineering
Cable Capacity Calculator
Convert derated conductor ampacity and planning reserve into usable line current, apparent power, input real power, and delivered load capacity.
DELIVERABLE FEEDER CAPACITY
Translate corrected ampacity into useful load without hiding power factor or reserve
This planning calculator converts a documented cable ampacity into an electrical capacity at a stated voltage, phase arrangement, power factor, and load efficiency. It first applies ambient and grouping corrections across parallel runs, then holds an explicit design reserve before reporting apparent, real-input, and delivered-load capacity. It does not create ampacity data or authorize future loading beyond the adopted wiring rules.
Delivered load capacity (kW)—
Usable current capacity (A)—
Derated installed ampacity (A)—
Apparent capacity (kVA)—
Input real capacity (kW)—
Held current reserve (A)—
DELIVERABLE FEEDER CAPACITY
Cable capacity conversion ledger
Use delivered capacity to compare a proposed feeder with a future load case, while retaining the intermediate current and kVA limits. A conductor may have thermal capacity but still be constrained by voltage drop, switchgear, transformer, protection, harmonics, fault duty, or operational policy.
| Capacity layer | Reference rating | Correction or conversion | Held reserve | Calculated outcome |
|---|
CURRENT CALCULATION PROCESS
Formula, substitution, intermediate values, and reconciliation
Iz = Itable × nruns × Camb × Cgroup; Iusable = Iz(1 − r); S = kphaseVIusable/1000; Pdelivered = S × PF × ηload
The calculation separates thermal capacity from service capacity. Corrected ampacity establishes the upper current boundary, the entered reserve withholds a portion, and voltage, phase, power factor, and efficiency translate usable current into a delivered kW estimate.
Intermediate values remain unrounded until display formatting.
HOW TO USE THIS MODEL
Convert ampacity to a defensible planning capacity
- Choose the governing conductor ampacity for the actual wiring method and termination temperature.
- Apply installation-specific ambient and grouping factors and count only matched parallel runs.
- State the voltage and phase basis on which connected equipment will operate.
- Enter expected power factor and load efficiency for the proposed load family, then declare the reserve policy.
- Review current, kVA, real electrical input, delivered kW, and the held reserve alongside voltage and protection constraints.
DELIVERABLE FEEDER CAPACITY FUNDAMENTALS
Capacity layers that must not be confused
- Thermal ampacity
- Allowable continuous conductor current under stated conditions.
- Usable current
- Corrected ampacity remaining after the entered planning reserve.
- Apparent capacity
- Voltage-current capacity before power factor is applied.
- Real input capacity
- Electrical kW available at the stated power factor.
- Delivered capacity
- Useful load output after downstream conversion efficiency.
- Planning reserve
- Deliberately unused capacity; it is not an emergency cable rating.
MODEL AND FORMULA
Move from thermal current to delivered service in distinct steps
Iz = Itable × nruns × Camb × Cgroup; Iusable = Iz(1 − r); S = kphaseVIusable/1000; Pdelivered = S × PF × ηload
The calculation separates thermal capacity from service capacity. Corrected ampacity establishes the upper current boundary, the entered reserve withholds a portion, and voltage, phase, power factor, and efficiency translate usable current into a delivered kW estimate.
SYMBOLS AND DEFAULT CASE
Variable definitions, units, and starting assumptions
| Symbol or input | Meaning | Unit or default |
|---|---|---|
| I_z | Installed corrected ampacity | A |
| I_table | Tabulated ampacity per run | A |
| r | Planning reserve fraction | dimensionless |
| I_usable | Corrected current remaining after reserve | A |
| S | Apparent capacity | kVA |
| P_delivered | Estimated useful load output | kW |
| phases | Circuit phases (1 or 3) | 3 |
| voltageV | Line voltage (V) | 400 |
| baseAmpacityA | Base ampacity per run (A) | 260 |
| parallelRuns | Parallel runs | 2 |
| ambientFactorPercent | Ambient factor (%) | 94 |
| groupingFactorPercent | Grouping factor (%) | 85 |
| powerFactorPercent | Expected power factor (%) | 90 |
| loadEfficiencyPercent | Load efficiency (%) | 95 |
| designReservePercent | Planning reserve (%) | 15 |
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
Capacity conflicts that require another calculation
Equipment-chain limits
Breaker, bus, transformer, contactor, generator, and terminal ratings can govern below conductor ampacity.
Load composition
Future nonlinear or motor loads may change power factor, harmonics, starting current, and neutral duty relative to the entered planning case.
WORKED DECISION CASES
Two capacity planning decisions
Factory expansion reserve
The feeder has thermal headroom, but the team holds 20% reserve and evaluates motor starting voltage before allocating new production equipment.
EV charging bank
Coincidence controls and charger power factor are included in the capacity case; demand management is treated separately from permanent cable ampacity.
TECHNICAL GLOSSARY
Cable capacity terms
- Installed ampacity
- Sum of corrected capacity across declared parallel paths.
- Usable capacity
- Planning result after a reserve is withheld.
- kVA
- Apparent power, the product of voltage and current with phase factor.
- Power factor
- Fraction of apparent power converted to real electrical power.
- Conversion efficiency
- Fraction of real input delivered as useful output.
- Reserve policy
- Declared fraction kept unavailable for routine planning.
EVIDENCE AND DATA LINEAGE
Keep the ampacity-to-service conversion auditable
Retain the ampacity source and edition, conductor material and size, wiring method, correction factors and installation evidence, parallel-run details, voltage basis, load-family power factor and efficiency, reserve authority, equipment-chain ratings, and unrounded current/kVA/kW results.
LIMITS AND EXCLUSIONS
Boundaries of this capacity conversion
- It does not derive ampacity or check thermal transients, voltage drop, faults, protection, harmonics, neutral current, or installation mechanics.
- It assumes balanced operation, equal parallel sharing, and constant power factor and load efficiency at the capacity point.
- The planning reserve has no code authority unless the governing design documents assign it one.
RELIABLE SOURCES
References for this method and its boundaries
FREQUENTLY ASKED QUESTIONS
Cable capacity questions
Why report both kVA and kW?
Cable current relates directly to kVA, while useful load kW depends on power factor and efficiency.
Can reserve replace code derating?
No. Code correction factors are applied first; reserve is a separate planning decision.
Can reserve be zero?
Use only a documented reserve policy; a zero holdback does not increase the underlying code ampacity.
Does parallel cable capacity add linearly?
The calculation assumes equal, compliant paths. Real sharing depends on conductor and installation symmetry.
Is delivered capacity a guaranteed equipment output?
No. It is a conversion estimate at the entered power factor and efficiency.
Does this include voltage drop?
No. Calculate voltage performance separately using route, impedance, load current, and starting or transient cases.
IMPORTANT ENGINEERING NOTE
Do not allocate load from cable kW alone
A qualified electrical professional must confirm the adopted ampacity method, voltage performance, protection and fault duty, switchgear and source limits, load characteristics, physical installation, and applicable code before approving additional connected load.