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Topic Title: DC Cable Grouping Factors for PV
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Created On: 25 February 2014 08:33 PM
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 25 February 2014 08:33 PM
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peggler1976

Posts: 2
Joined: 25 February 2014

Hi, can someone explain if the same regs apply for DC cables as AC cables when grouping? The reason I ask is there seems to be little info available for this situation and most calculation software assumes too many parameters are AC.

Anyone got any calculation examples?

I have a 20kW SMA inverter with 4 circuits of 23 modules on each. I've done basic calculations in the sma sunny design software but it takes no account of multiple cables in a system.

This particular system will have approx. 10 of these 20kW inverters and a common cable trench or cable tray system. I want to be sure that the cables will not be derated by the thermal effects of each other, or is it a bit different when dealing with DC cables due to lower current bit higher voltage?. My plan is to use multiple 4 core swa cables and then split into solar cables at each end for the connections. We have a long cable run > 300m, so its important to get the calcs right.

A bit off the norm so could just to with a bit more direction if any of you can help?
 25 February 2014 10:03 PM
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broadgage

Posts: 1264
Joined: 07 August 2007

Direct current has exactly the same heating effect as AC current, presuming that the AC current is RMS, as may be assumed to be the case.

Therefore derating of cables for grouping should be calculated in exactly the same way for AC or DC.

The nominal circuit VOLTAGE is not relevant as regards derating for grouping, only the current.
Voltage drop should be calculated, and there may be an economic case for use of larger cables than required by current carying capacity.
PV modules are expensive, and reducing cable losses from say 4% to 2% may be worthwhile.
 25 February 2014 10:15 PM
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peggler1976

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Thanks for your response. I thought as much that the thermal effects were the same, but would I treat using a 4 core swa cable with 2 DC circuits within it as two cables touching when doing grouping?
 26 February 2014 06:34 AM
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broadgage

Posts: 1264
Joined: 07 August 2007

Considering a single 4 core cable as being a pair of 2 core cables that are touching would be one approach, but may be unduly pesimistic and give rise to a larger cable size than is needed.

One the other hand, selecting the accepted rating of a 4 core cable from the regs is probably unduly optimistic.
AFAIK, the publised rating of 4 core cables assumes a 3 phase and neutral circuit with only 3 cores loaded, not all 4 cores.

The rigourous approach would be too calculate the heat produced with 3 cores fully loaded, and then to calculate the reduced current that if present in all 4 cores would produce the same heating effect.

This may be OTT for small and relatively cheap cables though.
I would be inclined to calculate on the first basis of considering the 4 core cable as being a pair of touching 2 core cables.
Although this gives a pessimisticly large cable size, the difference in cost should be small and perhaps justified on voltage drop grounds.

Remember that peak PV output will, by definition occur at times of bright sun, and cables may need de rating for high ambients on a sunny day.
Also worth remembering that PV technology is still improving and that failed modules might be replaced with ones of similar size but slightly greater output.

Watch out for the voltage rating, remembering that most standard SWA cables are limited to 1,000 volts between cores.
If the inverter trips on a cold but bright day, then the DC cables will be subject to the full open circuit voltage of the series connected modules.
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