CEC Level 5 Design Constraints
Simon, After reconsidering your question, I believe there should be no restriction on cable length for Level 5 power supplies.

Many power supplies have remote sensing and this DC cable length line loss can be compensated.  There are practical limitations on the noise and step response to remote sensing, however,this is a control issue and not an efficiency issue per se. Since DC cable conductor size is supplier design specific and not in the Level 5 Spec, you could choose a 50m cable extension if you wanted and choose to connect remote sensing lines to the load. Not every supply has this option, but the same supplier I showed above does have these in open frame Level 5 with sense +/- pins. http://www.slpower.com/data/collateral/GNT200_Enclosures_AN.pdf shows one example.

The following table shows the efficiency for various power levels of DC PSUs with Level 5 ratings.


I had to add this for your information. Answer as an alternative to "Standard Practise" commercial cabled PSU's. But you can custom choose PSU's with conectors and sense lines so you can create any length cable and design that you supply, for your needs.


Hope that helps with another perspective.