Why DC Cables Are Rated Separately From AC
The DC side of an array does not behave like the AC side, and the published ratings show the split: KUKA CABLE lists a rated voltage of DC 1.8/1.8 kV for its 1500 V solar cable, an ambient range of −40 °C to +90 °C, a sustained short-circuit allowance of +120 °C for 20,000 hours and a brief one of +250 °C for 5 seconds. An AC cable is rated against a waveform that crosses zero a hundred times a second; a solar dc cable lives under one polarity for decades.
Key takeaways
- Rated voltage on the published page is DC 1.8/1.8 kV, written as a direct-current figure rather than an AC line-to-neutral value.
- Ambient temperature is published as −40 °C to +90 °C. Fault events are rated twice: the insulation must hold +120 °C for 20,000 hours, or +250 °C for 5 seconds.
- Test figures are separate from the rating: a 10 kV spark test during production and a 6.5 kV AC test voltage for five minutes on the finished cable.
- Standards named are TUV 2PfG 1169/08.2007, EN 50618:2014 and IEC 62930 — PV-specific documents rather than general wiring rules.
Why direct current gets its own rating
Three physical differences drive the separate rating. First, a DC arc has no natural extinction point. On AC the current passes through zero twice per cycle, so a fault arc is re-struck or quenched by the waveform; on DC the same arc simply continues until the circuit is opened. Insulation and connection quality therefore matter more on the DC side than the same voltage would suggest on the AC side.
Second, the polarity never reverses. That constant field encourages electrochemical effects at any point where moisture, contamination and ionic movement meet — one reason PV cable insulation is specified as halogen-free and moisture-resistant rather than simply as thick.
Third, DC runs are long. Strings are wired in series to build voltage along a roof or a field, so the cable accumulates voltage stress over distances that a mains circuit would break into separate feeds. The published 1.8/1.8 kV figure is written for that duty.
The published figures for a 6 mm² DC cable
The manufacturer's 1500 V page describes a 6 mm² cable with a tinned copper conductor of class 5 flexibility to IEC 60228, XLPO insulation and an XLPO sheath, both electron-beam cross-linked and LSHF, in black or red. Of the figures published on that page, the electrical ones carry the DC rating.
During production the cable sees a spark test at 10 kV. On the finished drum it sees a test voltage of 6.5 kV AC at 50 Hz applied for five minutes. Neither figure is a working voltage. They are factory checks that prove the insulation has no pinhole and no thin spot, and they sit well above the rated DC 1.8/1.8 kV for exactly that reason.
Mechanical figures come from the same page: a minimum bending radius of 4 × the outer diameter, tensile strength before ageing of at least 8 N/mm², and elongation before ageing of at least 125%. Those limits are what keep the cable intact while it is pulled into a tray and clipped down, before any current ever flows.
The temperature limits, read apart
Read the bars as durations, not as a temperature scale. The 90 °C bar is the ambient ceiling the cable is expected to live inside. The 120 °C bar covers a fault current that lasts long enough to heat the conductor, and its 20,000-hour allowance makes it a design case rather than an incident. The 250 °C bar is five seconds long, which is the time a protection device has to clear the fault.
Twenty thousand hours is 833 days, or about 2.3 years of continuous operation — my own conversion, and it assumes the allowance is expressed as a cumulative period at that temperature, which the page does not state. Whichever way the figure is read, the three bars belong to different questions, and a datasheet that merges them into a single temperature rating is hiding the useful part.
Insulation chemistry on the DC side
Cross-linking is the reason the jacket survives the temperature story. Both the insulation and the sheath are described as XLPO that has been electron-beam cross-linked, so the polymer forms a three-dimensional network instead of softening the way a thermoplastic does as it warms. The same construction is listed as LSHF, low smoke and halogen-free, which changes what comes off the cable if it is ever heated to destruction.
The published test list shows what the manufacturer measures to back this up: UV accelerated weathering, water resistance, cable abrasion, cold bending, cold elongation, tensile testing after ageing, solderability, static flexibility and construction and dimensional testing. A 2,000-hour xenon lamp UV test is described as arranged on the solar cables and equated with about 360 weathering cycles.
What the DC page does not publish
Several figures a system designer might want are absent. The site publishes no current-carrying capacity table, no derating factors for bundled or conduit-routed cables, no DC voltage drop figures and no per-metre price or minimum order quantity for any cross-section. It also does not publish a warranty term.
That is a normal silence for a cable maker that sells into many markets, where ampacity tables are set by local installation codes rather than by the manufacturer. It does mean the buyer has to supply the missing half of the calculation. Take the published cross-section, diameter and temperature limits, apply the derating rules from the code that governs the site, and keep the resulting current as a project figure rather than as a manufacturer claim.
| Item | Published value | What it is used for | Where it appears |
|---|---|---|---|
| Conductor | Tinned copper, class 5 to IEC 60228 | Flexibility and corrosion behaviour at cut ends | Product page 1020 |
| Rated voltage | DC 1.8/1.8 kV | Continuous operating voltage on the string side | Product page 1020 |
| Spark test | 10 kV during production | In-line check for jacket faults | Product page 1020 |
| Test voltage | 6.5 kV AC, 50 Hz, 5 minutes | Type and routine proof on the finished cable | Product page 1020 |
| Ambient range | −40 °C to +90 °C | Sizing the routing and the support system | Product page 1020 |
| Short circuit | +120 °C (20,000 h), +250 °C (5 s) | Coordination with the protection device | Product page 1020 |
| Standards | TUV 2PfG 1169/08.2007, EN 50618:2014, IEC 62930 | Declaring the design basis on the datasheet | Product page 1020 |
Worked example
Compare the published figures with each other rather than with a memory. The spark test at 10 kV is 5.6 times the rated 1.8 kV, and the finished-cable test at 6.5 kV AC is 3.6 times it. Both ratios are my own division of figures from the same page, and they assume a straight comparison of kilovolt values, which ignores the difference between a surface spark test, an AC proof voltage and a continuous DC rating.
Now run the temperature comparison the same way. The 5-second limit of 250 °C is 2.8 times the ambient maximum of 90 °C, while the 20,000-hour limit of 120 °C is 1.3 times it. My arithmetic again, on published numbers. The pattern is the point: the shorter the event, the higher the temperature the insulation must survive. A specifier checking a solar dc cable against a project should confirm the conductor construction, the rated voltage, the two short-circuit limits and the standards name, and should treat any datasheet that lists one temperature figure as incomplete.
Frequently asked questions
Can a standard AC cable be used on the DC side of an array?
The published pages describe a PV-specific construction for the DC side, with XLPO insulation and sheath, a rated voltage of DC 1.8/1.8 kV and the standards TUV 2PfG 1169/08.2007, EN 50618:2014 and IEC 62930. The site gives no rating for a general-purpose AC cable on a DC circuit, so that substitution is not supported by anything published.
Is the 10 kV spark test the same as the cable rating?
No. The spark test at 10 kV runs during production, while the finished cable receives a test voltage of 6.5 kV AC, 50 Hz for five minutes. Both are factory checks on insulation integrity, and both sit above the continuous rating of DC 1.8/1.8 kV rather than describing an operating condition.
What to check on the DC datasheet
Four lines decide whether the sheet is complete. The conductor row should name tinned copper and the IEC 60228 class, because class 5 is what lets a 6 mm² cable bend to 24 mm of radius — four times the published 6.0 mm diameter, a figure I calculated from the published table. The voltage row should read DC 1.8/1.8 kV with the test voltage listed separately. The temperature row should carry the ambient range and both short-circuit limits. The standards row should name the PV documents rather than a generic cable standard.
Then ask for the paper behind the rows. The manufacturer publishes a spark test during production, a five-minute test voltage on the finished cable, a certificate list of more than 30 types including TUV, IEC, CE, SAA and UL, and a laboratory that has operated since December 2015. Every figure quoted above appears on the 1500 V solar cable page, with the range summarised in the solar cable section, the laboratory work in the test list and the certificates collected on the certificates page.
The DC and AC ratings compared in this article are the manufacturer's own website figures; both temperature limits were confirmed 24 Sept 2026[1].