Accurate SY cable sizing is critical for maintaining electrical safety, meeting load requirements, and ensuring the cable performs reliably in its installed environment. Each application demands careful consideration of current-carrying capacity, voltage drop, mechanical protection, and compatibility with terminations and cable glands.
This guide provides a clear, practical approach to selecting SY cable, supported by sizing charts and technical recommendations aligned with industry best practice.
Table of Contents
- What is SY cable?
- Why selecting SY cable size matters
- Common SY cable sizes & sizing chart
- How to determine correct SY cable size
- Cable gland & termination considerations
- Practical installation tips
- Frequently Asked Questions
- Final Thoughts
What is SY cable?
An SY cable (steel-braided, PVC-sheathed control cable) is a multi-core flexible cable used in industrial process automation. The galvanised steel wire braid (S) provides mechanical protection, while the PVC (Y) outer sheath offers basic environmental resistance.
SY cables are typically rated at 300/500 V and are widely used for instrumentation, control, and motor connections.
Why selecting SY cable size matters
Choosing the right size of SY cable is key to a safe, long-lasting system. Here’s why:
- Current capacity: Prevents overheating and voltage drop.
- Correct fit: Matches cable glands, reducing mechanical stress.
- Compliance: Avoids falling short of wiring regulations and site specs.
- Efficiency: Minimises energy loss over long runs.
Common SY cable sizes & sizing chart
SY cables are available in a range of sizes and core configurations. Here’s a helpful chart to guide you:
No. of cores | Conductor size (mm²) | Approx. outer diameter |
2 | 0.75 – 6 | 9.0 – 17.9 mm |
3 | 0.75 – 50 | 8.3 – 37.4 mm |
4 | 0.5 – 95 | 9.1 – 56.3 mm |
5 | 0.75 – 25 | 10.5 – 29.8 mm |
SY cables larger than 25 mm² are less common but available for high-load applications.
How to determine correct SY cable size
1. Calculate your load
Estimate the current (A) based on voltage and wattage of your equipment. Add diversity where applicable.
2. Check cable’s ampacity
Use the manufacturer’s datasheet or BASEC-approved cables to verify current-carrying capacity. For instance, a 3-core 2.5 mm² SY cable may carry ~14 A depending on conditions.
3. Factor in voltage drop
Long runs = larger cable. Longer cable runs result in greater voltage drop, which can affect equipment performance. Selecting a larger cable size may be necessary to maintain voltage within acceptable limits.
4. Think environment and installation
- Indoor dry site? PVC sheath is fine.
- Outdoor or in conduit? You may need an alternative to SY.
- Flexing or vibration? Ensure high braid coverage.
5. Check outer diameter for gland fit
Always confirm that the chosen cable fits your gland. A 4-core 6 mm² cable may have an OD of 16–20 mm and make sure to check before cutting.
Cable gland & termination considerations
Choose the right gland
Use brass or stainless-steel glands for durability. Match the gland size to your cable’s outer diameter using a manufacturer’s chart.
Glanding SY cable
- Strip the PVC sheath.
- Fold braid back or twist into a pigtail.
- Clamp securely to the gland to maintain mechanical integrity and earth continuity.
- Connect cores to terminal blocks or connectors.
Tip: Always refer to our gland selection guide for matching your SY cable precisely.
Practical installation tips
- Record the use of SY cable in your certificates, as it isn’t covered under BS 7671.
- Don’t use it underground unless fully protected. The braid can corrode, and the PVC sheath isn’t UV-stable.
- Inspect regularly: The transparent sheath allows you to monitor the braid visually.
- Allow spare capacity if future upgrades or loads are likely.
- Choose branded, high-braid options from trusted suppliers like Batt Cables for best performance.
Frequently Asked Questions
1. What is the smallest SY cable available?
Typically, 2- or 3-core 0.75 mm² cable, with outer diameters starting from around 8.3 mm.
2. Can SY cable be used outdoors?
Not recommended unless protected from UV and moisture. Consider CY or SWA cables for such environments.
3. What’s the difference between SY, CY, and YY cable?
- SY: Galvanised steel wire braid – for mechanical protection.
- CY: Copper braid – for EMI protection.
- YY: No braid – used for general, light-duty internal applications.
4. Is SY cable compliant with BS 7671?
No. It’s not a harmonised cable under BS 7671. It may still be used with justification and risk assessment so always record it on the installation certificate.
Final thoughts
SY cable sizing must be accurate to avoid performance issues or potential safety risks. Always ensure the cable you choose aligns with the electrical load, installation environment, and mechanical protection requirements, including compatibility with glands and terminations.
Browse our full range of cables at Batt Cables for reliable solutions and fast delivery.
