Construct-AI-primary Electrical Cable Sizing
Overview
Perform cable sizing calculations per IEC 60364-5-52 or NEC Article 310: determine design current, select cable type, apply derating factors (ambient temperature, grouping, burial depth), verify ampacity meets design current, calculate voltage drop, verify short-circuit withstand, and document the final cable specification. This ensures cables are correctly sized for safe, efficient, and code-compliant operation.
Announce at start: "I'm using the electrical-cable-sizing skill to calculate cable sizes with derating, voltage drop, and short-circuit verification."
When to Use This Skill
Trigger Conditions:
- New electrical circuit design requires cable size selection
- Existing cables need verification for increased load
- Cable route conditions change (temperature, grouping, burial)
- Voltage drop complaints require cable upgrade assessment
- Short-circuit withstand verification needed for new protection settings
- Compliance documentation requires cable sizing calculations
Prerequisites:
- Load data available (design current in amps, voltage, power factor)
- Cable route information (length, routing method, installation conditions)
- Installation method identified (air, buried, conduit, tray, ladder, duct)
- Ambient conditions known (ambient temperature, soil thermal resistivity for buried)
- Grouping information (number of circuits installed together, spacing)
- EE-CABLE-001 agent or equivalent cable sizing capability available
- Short-circuit current at cable location available
Step-by-Step Procedure
Step 1: Determine Design Current
Calculate the design current for the circuit:
Three-phase motor loads:
I_design (A) = P_rated (W) / (sqrt(3) x V_line x PF x efficiency)
Three-phase general loads:
I_design (A) = P_demand (W) / (sqrt(3) x V_line x PF)
Single-phase loads:
I_design (A) = P_demand (W) / (V_phase x PF)
Design current checklist:
- Rated/demand power confirmed in watts
- Operating voltage confirmed (line-line or phase)
- Power factor documented
- Motor efficiency included where applicable
- Design current for continuous load includes 125% factor (NEC) or per IEC installation method
Continuous vs. non-continuous loads:
| Load Type | Factor | Notes |
|---|---|---|
| Continuous (>3 hours) | 1.25x (NEC 210.20) | Required safety margin |
| Non-continuous | 1.0x | No additional factor |
| Motor circuits | Per NEC 430.22 / IEC 60364 | 125% FLC for motor |
Step 2: Select Cable Type
Choose cable construction based on application:
Cable type selection guide:
| Application | Recommended Cable Type | Standard |
|---|---|---|
| Indoor fixed wiring (dry) | PVC/PVC, XLPE/PVC | IEC 60502-1 |
| Indoor fixed wiring (fire-rated) | MICC, FP200 | BS 7629 |
| Underground direct burial | XLPE/PVC/SWA/PVC, XLPE/PE | IEC 60502-1 |
| Cable tray/ladder | XLPE/PVC, EPR/PVC (unarmored) | IEC 60502-1 |
| Conduit/duct | PVC/PVC, XLPE/PVC | IEC 60502-1 |
| Hazardous area | Armored, screened per zone | IEC 60079-14 |
| Marine/offshore | XLPE/LSF/SWA/LSF | IEC 60092-350 |
Cable construction components to specify:
- Conductor material: Copper (standard) or Aluminium (large feeders)
- Conductor class: Class 2 (stranded) for fixed, Class 5/6 (flexible) for connections
- Insulation: PVC (standard), XLPE (higher temperature, better current rating), EPR (flexible)
- Armor: SWA (steel wire), AWA (aluminum wire), STA (steel tape), or unarmored
- Sheath: PVC (standard), LSF (low smoke), PE (buried), LSZH (public areas)
Standard conductor sizes: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm²
Step 3: Apply Derating Factors
Determine the cable rating under actual installation conditions:
Derating factor categories:
Ambient Temperature (Ca): For XLPE insulated cables (90 deg C rated):
| Ambient Temp (deg C) | In Air | Buried (soil) |
|---|---|---|
| 25 | 1.00 | 1.00 |
| 30 | 0.96 | 0.93 |
| 35 | 0.91 | 0.87 |
| 40 | 0.87 | 0.79 |
| 45 | 0.82 | 0.71 |
| 50 | 0.76 | 0.61 |
Grouping Factor (Cg): For circuits installed together in air (touching):
| Number of Circuits | 1 | 2 | 3 | 4 | 6 | 8 | 10 | 12 | 16 |
|---|---|---|---|---|---|---|---|---|---|
| Grouping Factor | 1.00 | 0.80 | 0.70 | 0.65 | 0.57 | 0.52 | 0.50 | 0.48 | 0.44 |
Thermal Insulation Factor (Ci):
| Condition | Factor |
|---|---|
| Cable in thermal insulation (fully enclosed) | 0.50 |
| Cable in thermal insulation (partial contact) | 0.89 |
| Cable in conduit in thermally insulated wall | 0.78 |
Soil Thermal Resistivity (CC) for buried cables:
| Soil Type | Resistivity (K.m/W) | Factor |
|---|---|---|
| Wet soil | 0.5-1.0 | 1.00 |
| Damp soil | 1.0-2.0 | 0.91 |
| Dry soil | 2.0-3.0 | 0.84 |
Combined derating:
I_rating_adjusted = I_tabulated x Ca x Cg x Ci x CC x other
Where:
- I_tabulated = base current rating from manufacturer or standard tables
- All applicable derating factors are multiplied together
Step 4: Verify Ampacity
Check that adjusted cable ampacity meets design current:
Ampacity verification:
I_rating_adjusted >= I_design
Selection procedure:
- Start with a trial cable size
- Look up base ampacity from standard table for the installation method
- Apply all derating factors to get adjusted ampacity
- Compare adjusted ampacity to design current
- If adjusted ampacity < design current, move to next larger size and repeat
- Select the smallest cable size where adjusted ampacity >= design current
Ampacity verification checklist:
- Base ampacity from correct table for installation method
- All applicable derating factors applied and documented
- Adjusted ampacity >= design current (I_rating_adjusted >= I_design)
- Next smaller cable size does NOT satisfy the requirement (confirm minimum size)
- Neutral conductor sized appropriately (full size for harmonics, or per calculation)
- Earth/ground conductor sized per fault current and duration (IEC 60364-5-54)
Step 5: Calculate Voltage Drop
Verify voltage drop across the cable run is within limits:
Three-phase voltage drop:
VD (V) = sqrt(3) x I_design x L x (R x cos(phi) + X x sin(phi)) / 1000
Single-phase voltage drop:
VD (V) = 2 x I_design x L x (R x cos(phi) + X x sin(phi)) / 1000
Voltage drop (%):
VD (%) = VD (V) / V_nominal x 100
Where:
- I_design = design current (A)
- L = cable length (m)
- R = cable resistance at operating temperature (Ohm/km)
- X = cable reactance (Ohm/km)
- cos(phi) = power factor
- sin(phi) = sqrt(1 - cos(phi)^2)
Acceptable voltage drop limits:
| Circuit Type | Maximum VD | Standard Reference |
|---|---|---|
| LV main feeder from supply | 3% | IEC 60364-5-52 / NEC 210.19 |
| LV branch circuits | 3% | IEC 60364-5-52 / NEC 210.19 |
| Total installed (incoming to load) | 5% | IEC 60364-5-52 / NEC 210.19 |
| Motor starting | 10-15% (momentary) | IEC 60034 |
If voltage drop exceeds limits:
- Select next larger cable size and recalculate
- Reduce cable length if possible (reroute)
- Install additional cable in parallel (large feeders)
Step 6: Verify Short-Circuit Withstand
Verify the cable can withstand the short-circuit current without thermal damage:
Adiabatic equation (IEC 60364-5-54 / IEC 60949):
S_min = sqrt(I_sc^2 x t) / k
Where:
- S_min = minimum conductor cross-section (mm²)
- I_sc = short-circuit current (A) at cable location
- t = fault clearing time (seconds)
- k = material factor (copper XLPE/PVC: k = 143, copper PVC: k = 115)
Short-circuit check:
- Calculate S_min for the available short-circuit current and protective device clearing time
- Verify selected cable cross-section >= S_min
- If cable size is insufficient:
- Increase cable size, OR
- Reduce fault clearing time (faster protection), OR
- Install current-limiting device
Short-circuit verification checklist:
- Short-circuit current at cable location documented
- Protective device clearing time for the fault current identified
- Material factor k selected for cable construction
- S_min calculated per adiabatic equation
- Selected cable cross-section >= S_min
Step 7: Document Cable Specification
Produce complete cable specification documentation:
Cable specification format:
| Field | Value |
|---|---|
| Cable tag | — |
| From → To | — |
| Circuit reference | — |
| Design current (A) | — |
| Cable type | — |
| Conductor material/size | — |
| Number of cores | — |
| Insulation/Armor/Sheath | — |
| Installation method | — |
| Route length (m) | — |
| Ambient temperature (deg C) | — |
| Grouping factor | — |
| All derating factors applied | — |
| Base ampacity (A) | — |
| Adjusted ampacity (A) | — |
| Ampacity check | PASS/FAIL |
| Voltage drop (V, %) | — |
| Voltage drop check | PASS/FAIL |
| Short-circuit current (kA) | — |
| Protective device / clearing time | — |
| S_min (mm²) | — |
| SC withstand check | PASS/FAIL |
| Standard reference | IEC 60364-5-52 / NEC Article 310 |
Step 8: QA Review
Perform quality review of cable sizing results:
QA checklist:
- All derating factors correctly identified and applied
- Cable size is the minimum that satisfies all criteria (not oversized)
- Voltage drop within limits for the entire circuit
- Short-circuit withstand verified with actual protective device clearing time
- Earth conductor sized per fault thermal requirements
- Standard references documented for all calculations
- Results reviewed by second check (EE-QA-001 or peer)
Success Criteria
- Cable sized correctly per applicable standard (IEC 60364-5-52 or NEC Article 310)
- All applicable derating factors applied (ambient, grouping, burial, insulation)
- Adjusted ampacity >= design current (cable will not overheat under sustained load)
- Voltage drop within limits (<=3% branch, <=5% total from supply to load)
- Short-circuit withstand verified (S_min <= selected cross-section)
- Earth/ground conductor correctly sized for fault thermal duty
- Cable specification complete with all calculation data and standard references
- QA review completed with zero critical findings
Common Pitfalls
- Incorrect Derating — Applying wrong ambient temperature factor, missing grouping factor, or double-counting factors produces unsafe cable sizing. Always use the factor for the actual installation condition.
- Missing Grouping Factors — Cables installed close together must be derated for mutual heating. A circuit alone in a tray has no grouping, but 4 circuits in a conduit have significant derating. Always count adjacent loaded circuits.
- Insufficient Short-Circuit Rating — A cable may handle the load current but fail catastrophically if its cross-section is below S_min for the available fault current. Always verify SC withstand with the actual protective device clearing time.
- Using Room Temperature Tables for Hot Environments — Standard ampacity tables assume 30 deg C ambient. In 45-50 deg C environments (near equipment, tropical), derating can reduce capacity to 60-70% of tabulated value.
- Ignoring Harmonics — Non-linear loads (VFDs, IT equipment) produce neutral current that can exceed phase current. In these cases, neutral must be sized equal to or larger than phase conductors.
- Wrong Installation Method Table — Cable installed in conduit underground has different ampacity than the same cable in free air. Always use the table that matches the actual installation method.
Cross-References
Related Skills
electrical-load-analysis— Design current and demand data feed cable sizingelectrical-protection-coordination— Protective device clearing time required for SC withstandelectrical-sld-processing— SLD provides circuit topology and cable routing informationelectrical-arc-flash-analysis— Cable SC rating relates to incident energy calculation
Related Agents
EE-CABLE-001(OpenClaw) — Primary cable sizing agentEE-LOAD-001(OpenClaw) — Load analysis input/outputEE-SC-001(deep-agents) — Short-circuit current dataEE-QA-001(OpenClaw) — Quality validation of cable sizing results
Standards & References
- IEC 60364-5-52 — Electrical installations of buildings — Selection and erection of wiring systems
- IEC 60364-5-54 — Selection and erection of earthing arrangements
- IEC 60949 — Calculation of thermally permissible short-circuit currents
- NEC Article 310 — Conductors for General Wiring
- NEC Article 250 — Grounding and Bonding
- BS 7671 (IEE Wiring Regulations) — Requirements for Electrical Installations
Example Usage
Scenario: Size cable for a 75kW motor feeder, 400V 3-phase, DOL start, 45m run in cable tray with 3 other circuits, ambient 40 deg C
- Design Current: 75kW / (sqrt(3) x 400V x 0.88 PF x 0.94 eff) = 130.2A. Continuous duty per NEC requires 125%: I_design = 162.8A
- Cable Type Selected: XLPE/PVC/SWA/PVC, 4-core, copper, Class 2 stranded (industrial motor feeder)
- Derating Factors: Ca (40 deg C, XLPE) = 0.87, Cg (4 circuits in air, trefoil) = 0.65, Ci (none) = 1.00. Combined derating = 0.87 x 0.65 = 0.566
- Ampacity Verification: Trial 70mm²: base ampacity (free air, XLPE) = 248A. Adjusted = 248 x 0.566 = 140.4A < 162.8A. FAIL. Trial 95mm²: base = 298A. Adjusted = 298 x 0.566 = 168.7A >= 162.8A. PASS.
- Voltage Drop: 95mm² Cu: R = 0.242 Ohm/km (at 90 deg C), X = 0.080 Ohm/km. VD = sqrt(3) x 130.2 x 45 x (0.242 x 0.88 + 0.080 x 0.47) / 1000 = 3.5V = 0.88%. Well within 3% limit.
- Short-Circuit: Available I_sc = 15kA, MCCB clearing time = 0.2s, k = 143 (Cu XLPE). S_min = sqrt(15000^2 x 0.2) / 143 = 46.8mm². 95mm² >> 46.8mm². PASS.
- Final Cable Spec: 95mm² 4C Cu XLPE/SWA/PVC, I_design = 162.8A, I_rating_adj = 168.7A, VD = 0.88%, SC withstand OK, earth 35mm² per Table 54.7.
Performance Metrics
Target Performance:
- Ampacity check: 100% of cables pass I_rating_adjusted >= I_design
- Voltage drop: 100% of circuits within 5% total, 3% branch
- Short-circuit withstand: 100% of cables have S_min <= selected cross-section
- Standard compliance: All calculations reference specific IEC/NEC clauses
- QA review: Zero critical findings in final cable specification
- Processing time: Individual circuit sizing completed within 5 minutes