Why No DC-3 to DC-13 Conversion Coefficient Exists
A common request is to express a main-contact rating such as 20 A @ DC-3, 110 VDC as an equivalent DC-13 rating, often assuming a fixed factor such as 0.2. No such coefficient exists. According to IEC 60947-4-1, utilization category DC-13 is defined with switching conditions that differ from those of DC-1, DC-3, and DC-5. Because the test conditions are not the same, a rating established under DC-3 cannot be mathematically scaled into a DC-13 rating, regardless of contact material.
Some manufacturers (for example, Hyundai) publish contactors carrying multiple DC utilization categories (DC-1, DC-3, DC-5, DC-13) for the same device. That does not imply a conversion formula; each published value comes from testing under the specific category conditions. Siemens takes a different approach: rather than listing all categories on one product, it assigns categories per product family.
Siemens Product Assignment: 3RT vs. 3RH
On Siemens devices, the category split is explicit:
| Product family | Tested utilization categories |
|---|---|
| 3RT contactors | DC-1, DC-3, DC-5 |
| 3RH type | DC-13 |
The practical consequence: if your application requires a verified DC-13 rating, select a 3RH-type device that is tested and rated for that category instead of attempting to derate a 3RT main contactor's DC-3 value.
Getting a DC-13 Rating for a 3RT Main Contactor
If the application genuinely requires a DC-13 rating on a main contactor, the supported path is a formal verification request rather than a field calculation:
- Open a service request with the Siemens hotline at [email protected].
- Include a full description of your application (voltage, load type, switching duty).
- The request is forwarded internally to the laboratory, which can evaluate the device under the relevant conditions.
FAQ
Can I convert a DC-3 contactor rating to a DC-13 rating?
No. DC-13 has different switching conditions than DC-1, DC-3, or DC-5 under IEC 60947-4-1, so no conversion coefficient exists regardless of contact material.
Which Siemens contactors are rated for DC-13?
The 3RH type is tested according to DC-13. The 3RT contactors are tested according to DC-1, DC-3, and DC-5 only.
How do I get a DC-13 rating for a Siemens 3RT main contactor?
Submit a service request to the Siemens hotline at [email protected] with a description of your application; the inquiry is forwarded to the laboratory for evaluation.