Cable Derating: 12 Cables in a Tray at 40°C — NEC's 90°C Advantage

Cable derating is where the standards appear to agree — similar grouping factors, similar temperature corrections — until you look at the starting point. NEC's approach of using the 90°C conductor rating as the base ampacity, then derating to the termination temperature, can result in a cable one or two sizes smaller than BS 7671 or IEC 60364 would require. This article examines whether that's clever engineering or a hidden risk.

The Scenario

A cable tray in a plant room:

• 12 single-core cables (6 circuits, 2 conductors each) on a perforated cable tray
• Each circuit: 32A three-phase
• Cable type: Copper, PVC insulated (or THHN for NEC)
• Installation: Single layer on perforated tray, cables touching
• Ambient temperature: 40°C
• Required: minimum cable size for each circuit

Side-by-Side Results

The NEC 90°C Advantage Explained

How NEC Uses Two Temperature Ratings

NEC 310.15(B) permits an approach that other standards do not allow in the same way:

1. Start with the 90°C ampacity from Table 310.16 (highest current rating)
2. Apply derating factors (grouping + temperature) to this higher starting value
3. The final derated value must not exceed the termination temperature rating (usually 60°C or 75°C)

This means: even though the termination can only handle the 75°C rating, the cable itself can run hotter in the middle of the run (up to 90°C), giving more thermal headroom for derating.

Why Other Standards Don't Allow This

BS 7671, IEC 60364, and AS/NZS 3008 take the position that the cable temperature should be referenced to the conductor operating temperature, not the insulation temperature limit. If the cable is PVC rated at 70°C, you use the 70°C ampacity as your starting point — period.

The reasoning: conductor temperature affects resistance, which affects voltage drop and losses. A cable running at 90°C in the middle of a grouped installation has higher resistance (and therefore higher voltage drop) than one running at 70°C. The IEC/BS approach is more conservative but also more thermally consistent.

The Real-World Impact: Cable Tray Fill

For our 12-cable scenario, the cable sizes are:

The NEC installation uses 65% less tray space than BS 7671. For a large industrial plant with hundreds of cable runs, this means smaller trays, lighter supports, and significantly lower installation costs.

Is NEC's Approach Safe?

This is the fundamental question. The engineering community is split:

Arguments For (NEC Position)

1. Cable insulation is rated for 90°C — it can safely operate at that temperature
2. The termination temperature limit protects the weakest point (the connection)
3. Decades of US installations prove the method works in practice
4. Manufacturers design THHN cable specifically for this application

Arguments Against (IEC/BS Position)

1. Conductor at 90°C in a grouped tray transfers heat to adjacent cables
2. The 70°C/75°C tables already account for the thermal environment
3. Higher conductor temperature accelerates insulation ageing
4. Voltage drop and I²R losses increase — the cable is less efficient

Derating Factor Sources

Grouping Factors

All four standards agree on approximately 0.50 for 12 cables in a single layer on a tray. The values come from heat-transfer calculations validated by IEC TC 20 testing. Small differences exist in exact boundaries (e.g., "7-9 cables" vs "7-12 cables" in different tables), but the physics is the same.

Temperature Correction

The temperature correction factor is derived from:

Where T_max is the conductor temperature rating, T_reference is 30°C (standard ambient), and T_ambient is the actual ambient temperature.

For 90°C cable at 40°C: Ct = √((90-40)/(90-30)) = √(50/60) = 0.913 For 70°C cable at 40°C: Ct = √((70-40)/(70-30)) = √(30/40) = 0.866

The 90°C cable retains more of its capacity at elevated ambient temperatures — another advantage of the NEC approach.

Practical Guidance

1. Always use the derating method required by your applicable standard — mixing approaches is a compliance violation
2. For international projects, the IEC/BS method is the safe default
3. NEC's 90°C method is only valid for 90°C-rated conductors (THHN, THWN-2) — not for all cable types
4. Verify voltage drop independently — a cable sized by NEC 90°C derating may need upsizing for voltage drop
5. Cable tray fill calculations differ between standards — size the tray for the cable sizes you actually use

Key Takeaways

1. NEC allows 10mm² where IEC/BS requires 25mm² — a 2.5× difference in copper for the same circuit
2. The NEC 90°C base rating starts from a higher ampacity, giving more derating headroom
3. All four standards use similar grouping factors (~0.50 for 12 cables) — the divergence is in the starting point
4. Temperature correction is more favourable for 90°C cable (0.91 vs 0.87 at 40°C)
5. Neither approach is wrong — they represent different engineering philosophies about thermal margins

Related Resources

• Grenfell Tower: Cable Tray Fire Spread — Overloaded cable trays and the thermal cascade that follows
• The Grouping Factor Trap — Why grouping derating catches experienced engineers
• Operating Temperature vs Maximum Temperature Rating — The temperature assumption behind every derating factor
• Cable Sizing: The 50m Office Feeder — Base case cable sizing before derating complicates matters
• The Complete Cable Sizing Comparison — Every derating factor, all four standards, compared
• View all worked examples →