Cables & Conductors: Picking the Right Size and Standard

Cables & Conductors: Picking the Right Size and Standard

Cable selection isn’t only about “what’s on the drawing.” The right choice protects people and equipment, avoids nuisance trips, and keeps inspections smooth. Below is a practical framework—standards, sizing, derating, voltage drop, short-circuit, and fire performance—to help you specify with confidence.

1) Start with Standards & Conductor Class

Confirm that the cable complies with the specified standard and installation authority requirements. For conductors, match class to application:
• IEC 60228 conductor classes: Class 1 (solid), Class 2 (stranded), Class 5 (flexible)
• Common builds you’ll see on BOQs: IEC/BS power cables (e.g., BS 5467 XLPE/SWA/PVC, BS 6724 XLPE/SWA/LSZH), instrument/control, data, and fire-performance variants

2) Choose Insulation & Sheath for the Environment

Select materials that survive the installation method and exposure:
• XLPE for higher operating temperature & current capacity; PVC for general purpose
• LSZH/LS0H where low smoke & halogen-free is required (tunnels, public areas, indoors)
• UV/Outdoor/Armoured for external runs, direct burial, or mechanical protection

3) Size for Current — then Apply Derating

Base current capacity on installation method (tray, conduit, in ground, grouped) and ambient:
• Check derating factors for ambient temperature, grouping, soil thermal resistivity (buried), and installation depth
• Ensure the protective device rating and the cable’s continuous current rating align after derating

4) Verify Voltage Drop (VD)

Long runs can under-supply equipment. Calculate VD using route length and operating current:
• Many specs target ≤3–5% VD depending on system and load type
• Consider motor starting and sensitive ELV/controls—oversize if needed to keep VD in limits

5) Check Short-Circuit Withstand

The cable must withstand the prospective fault level for the device’s clearing time:
• Verify the adiabatic calculation / manufacturer’s short-circuit rating data
• Ensure the circuit protective conductor (CPC) and terminations are sized accordingly

6) Fire Performance & Safety

Where specified, insist on proper test regimes:
• Flame retardance & limited smoke (e.g., IEC 60332, 60754, 61034 families)
• Circuit integrity (fire-resistant cables) for life-safety systems as required by the design

7) Installation Reality: Routing & Termination

Plan the route and accessories early:
• Respect minimum bend radius and tray/conduit fill limits
• Match glands (brass/nylon/EMC) to armoured vs non-armoured builds and IP rating
• Specify lugs/ferrules to conductor CSA and termination hardware

Quick Spec Checklist

• Correct standard/build (e.g., XLPE/PVC or XLPE/LSZH, armoured where required)
• Conductor class matches installation (fixed vs flexible)
• Current capacity & all derating factors applied
• Voltage drop within project limits at operating current
• Short-circuit withstand verified for device clearing time
• Fire performance (LSZH/fire-resistant) where specified
• Glanding/termination & accessories defined (IP/EMC)
• Datasheets, CoC/DoC, and authority approvals on file

Why Getting It Right Pays Off

Correct sizing and construction cut heat losses, avoid nuisance trips, protect motors and power electronics, and reduce rework. Most “mystery” performance issues trace back to overlooked derating, VD, or termination details.

Need a Cable Schedule Review?

Share your single-line drawings, cable schedule, lengths, and installation method. We’ll propose OEM or approved equivalents with datasheets, VD and derating checks, and delivery timelines aligned to site progress.