Short answer
The cable cross section of a band-heater connection lead must not be derived from operating current alone. For pure nickel stranded wire in an ambient temperature up to 400 °C, permissible continuous current falls sharply. Conductor temperature, insulation material, routing, grouping, voltage drop and the temperature at the termination must also be checked.
The current stays the same. The permissible ampacity does not.
The temperature trap at the band heater
A connection that operates safely at 40 °C can be seriously undersized in the hot zone of an extruder or injection-moulding barrel. Nickel’s resistivity rises with temperature while the conductor can dissipate less heat into a hot environment.
One distinction is essential: ambient temperature is not conductor temperature. The tabulated factors refer to the temperature at the point of installation. Electrical losses normally raise the actual conductor temperature above it.
Ambient temperature
Temperature at the connection lead’s installation point. It determines the correction factor.
Conductor temperature
Temperature of the energised stranded wire including self-heating. Its limit must not be exceeded.
Termination
The connection to the heater and the terminal point are often more critical than the wire itself.
Insulation material
The insulation temperature class must cover the actual operating temperature.
What pure nickel stranded wire can carry at 400 °C
The following values are based on the manufacturer data documented in the works standard for pure nickel Ni 99.6. They assume a single conductor without grouping, with 40 °C as the reference ambient temperature; a factor of 0.34 is applied at 400 °C.
| Cross section | at 40 °C | at 400 °C |
|---|---|---|
| 0.25 mm² | 4.0 A | 1.4 A |
| 0.50 mm² | 7.7 A | 2.6 A |
| 0.75 mm² | 10.1 A | 3.4 A |
| 1.00 mm² | 12.1 A | 4.1 A |
| 1.50 mm² | 15.8 A | 5.4 A |
| 2.50 mm² | 22.1 A | 7.5 A |
| 4.00 mm² | 30.0 A | 10.2 A |
| 6.00 mm² | 39.1 A | 13.3 A |
| 10.00 mm² | 56.0 A | 19.0 A |
| 16.00 mm² | 75.0 A | 25.5 A |
Calculation example: 1.5 mm² at 400 °C
A 1.50 mm² pure nickel stranded wire has a base ampacity of 15.8 A at 40 °C. At 400 °C, the correction factor is 0.34:
If the same lead is to carry 10 A, 1.50 mm² is not sufficient. The recommendation rises to 4 mm². At 20 A, the requirement increases from 2.5 mm² at 40 °C to 16 mm² at 400 °C.
Five steps to a robust design
Determine operating current
Record rated power, voltage, circuit configuration and the actual operating mode of the band heater.
Define the hot zone
Determine ambient temperature along the complete lead, especially where it exits the heater.
Check material and insulation
Verify pure nickel grade, stranded-wire construction, insulation temperature class and rated voltage.
Apply correction factors
Consider temperature, grouping, routing and restricted heat dissipation together.
Assess termination and voltage drop
Check terminal, transition and cable run at operating temperature, especially for lengths over one metre.
Why the termination often limits before the wire
At higher currents, the theoretically required nickel cross section quickly becomes difficult to accommodate. At the same time, the exit, crimped joint and terminal are thermally and geometrically constrained. A different architecture is therefore often better than simply choosing a thicker wire:
Run pure nickel only through the hot zone. Make the properly engineered transition to copper in a cooler area.
The appropriate standards then apply to the longer copper section. The transition point must still be checked for permissible temperature, contact resistance, mechanical load and permanent connection quality.
Conclusion
Current alone is not enough to size a band-heater connection lead. At 400 °C ambient temperature, the required pure nickel cross section can increase by several standard sizes. Confusing ambient and conductor temperature, or ignoring the termination, means designing past the most critical point.
The better question is not only “How many amps flow?” but: Where do they flow, at what temperature and through which thermal path?
Frequently asked questions
What cross section is required for 10 A at 400 °C?
Under the documented conditions, at least 4 mm² of pure nickel stranded wire is recommended. Applicable minimum cross sections and the installation must also be checked.
Why does ampacity fall at high temperature?
Nickel’s electrical resistance rises while heat dissipation becomes less effective in a hot environment. The permissible continuous current must therefore be reduced.
Do the copper tables in DIN VDE 0298-4 apply?
Not to pure nickel stranded wire at 400 °C. This conductor material and temperature range are not covered by those tables.
Are ambient and conductor temperature identical?
No. Current causes additional self-heating, so the conductor temperature is normally above ambient.
Can copper be used outside the hot zone?
Yes, provided the transition is correctly designed in a sufficiently cool area and checked thermally, electrically and mechanically.
Have the connection lead sized safelySend current, voltage, ambient temperature, cable length, routing, grouping and a photo of the termination. We will assess the suitable construction for your band heater.
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