A cable selection chart is your secret weapon when sizing cables for your project. Get it wrong, and you risk voltage drop, overheating, or a wasted budget. Cable installation costs can even hit 10% of a solar plant’s budget, so it pays to nail the numbers.
In this guide, you’ll learn how to read a cable selection chart, weigh the key selection factors, and customise your choice for real-world conditions. By the end, you’ll have a clear process to pick the right cable every time.
Explore Main Selection Factors
Before you dive into the chart, let’s be clear on what drives cable sizing. Four main criteria keep your system safe and efficient:
- Current carrying capacity
- Voltage rating
- Insulation temperature and short-circuit performance
- Voltage drop limits
You can explore each of these in more depth in our cable selection criteria.
Current Carrying Capacity
This tells you how much continuous current a cable can handle without overheating. It’s also called ampacity and depends on conductor material, installation method, and ambient temperature.
Voltage Rating
Expressed as U₀/U, the voltage rating reflects the insulation’s quality. In AC systems, U₀ and U must at least match the system voltage. In DC setups, the system voltage should not exceed 1.5 times the cable’s rated value.
Insulation and Short-Circuit Performance
Your insulation must tolerate normal operating voltages and short-circuit stresses. Materials like PVC handle up to 70 °C, while XLPE and EPR reach 90 °C. That extra heat resistance can matter in tight cable trays or high-load conditions.
Voltage Drop Limits
Too much drop means your end device sees lower voltage than intended. The IEEE rule B-23 caps voltage drop at 2.5% of supply voltage to keep motors and electronics happy. We’ll dig into how to calculate that soon.
Read the Cable Selection Chart
Now let’s crack open that chart. It lines up load currents, recommended cable cross-sections, and voltage drop per metre.
Charts vary by standard and region, but most share a similar layout:
| Current (A) | Cable size (mm²) | Voltage drop (V/m) |
|---|---|---|
| 10 | 1.5 | 3.3 |
| 16 | 2.5 | 2.1 |
| 25 | 4.0 | 1.4 |
| 32 | 6.0 | 1.0 |
| 40 | 10 | 0.7 |
Table 1. Example cable selection chart for common loads.
Want the full range? Check cable selection tables for detailed sizes up to 300 mm².
Cable Selection Chart Layout
- The current column shows the expected load in amps
- The cable size column lists the conductor’s cross-sectional area
- The voltage drop column gives the drop per metre at the rated current
Interpreting the Values
Match your load to the nearest current rating, note the suggested mm² size, then verify the voltage drop suits your run length. If it doesn’t, you’ll need a larger cable.
Factor in Voltage Drop
Voltage drop might feel like a minor detail, but over long runs it adds up. Ever measured how much voltage you lose on a 50 m cable? A 2% drop on a 230 V circuit at 50 m means 4.6 V lost before your device sees power.
Calculating Drop
Use this simple formula to estimate drop:
Voltage drop (V) = I × R × L
- I is the load current in amps
- R is cable resistance (Ω/m)
- L is one-way length in metres
Using Online Calculators
If maths isn’t your thing, free calculators let you plug in current, cable size, and length. They spit out the drop and even suggest a larger size if you overshoot the 2.5% limit.
Compare Cable Standards
Not all cable charts line up. Different standards tweak the numbers based on local conditions.
Key Standards
- IEC: widely used in Europe and many global markets
- NEC: National Electrical Code, key in the US
- Australian Standards: specific to harsh climate conditions
- Chinese Standards: often less restrictive on size
Regional Impact
A chart based on NEC might call for a larger mm² size than its IEC counterpart for the same current. Always pick the chart tied to your project’s regulatory region.
Adjust for Installation Conditions
Real-world installs rarely match lab conditions. You need to apply correction factors when things differ from the chart’s assumptions.
Ambient Temperature
Higher air temperatures reduce ampacity. Correction factors are laid out in SS638 rules in Singapore, which cap voltage drop at 4% for some runs.
Grouping Circuits
Bundled cables heat each other up. More circuits means lower ampacity per cable. Chart notes usually list grouping derating factors.
Installation Method
Whether your cable is in conduit, tray, or free air changes its cooling. Each method has its own set of multipliers.
Consult Cable Construction Types
Choices go beyond size and ampacity. The cable’s build impacts performance and cost.
Twin Core and Earth
Common in UK homes, this has two insulated cores and an earth conductor under PVC sheathing.
3-Core and Earth
Used for two-way switches, it adds a third phase or switch loop in the sheath.
Armoured Cables
Metal armour protects underground or overhead runs. They handle mechanical stress but cost more.
Communication and Fire-Resistant
Low-voltage data runs use thin insulation. Fire-resistant cables keep critical alarms live under extreme heat.
For more on construction and ratings, see the Electrical Engineering Portal’s deep dive.
Use Ampacity Tables Effectively
Ampacity tables let you cross-check your chart pick. They list current ratings per size under specified conditions.
- Always validate the recommended size against the table
- Adjust for derating factors like ambient heat and grouping
- Keep copies of relevant tables in your project folder
If you need a refresher on ampacity rules, our cable selection guide walks you through each step.
Implement your Cable Choice
You’ve got the data, now put it into action. Here’s a quick process you can follow:
- Calculate your maximum load current and diversity factor
- Locate that current on your cable selection chart
- Check voltage drop for your run length, apply correction factors
- Verify ampacity in a reference table
- Confirm insulation rating and construction type
- Order your cable and schedule installation
This simple loop keeps you on track and helps prevent costly mistakes.
Key Takeaways and Next Steps
- A cable selection chart aligns current, size, and voltage drop in one view
- Understand ampacity, voltage rating, insulation, and short-circuit criteria
- Factor in voltage drop carefully, keeping it below 2.5% of the supply
- Choose the right standard chart for your region to stay compliant
- Adjust for real-world conditions like temperature and circuit grouping
Ready to make cable selection a breeze? Check out Zable Cable’s handy tools and expert support. Get in touch with Zable Cable today and streamline your next project.
