De-Rating Factor for Cable

This is a question that appears in various contexts. To name two:

1. Cable will have a certain ampacity rating at a certain ambient temperature. At higher temperatures, it must be derated, because heat dissipation from the cable will be less.

2. The number of conductors in a conduit will influence their rating. More conductors and more current in less remaining space means that the cables will heat more; hence their ampacity must be derated.

For these and like reasons, there are several different formulas in electrical codes, each and all of which must be considered in any given application. There is no single or simple answer to this question.

thanks alot!

Once again, British Standard 7671.

Any reputed cable manufacturers' data sheets would give the continuous current ratings under standard conditions for various sizes, types & conductors of cables. Also, would be given are the de-rating factors for Ambient Temperature, Soil Temperature, Depth of Laying, Soil Thermal Conductivity, Grouping Factors (both Horizontal Grouping & Vertical Grouping), etc. Apply as many de-rating factors as applicable in your case and arrive at the final permissible current rating of the cable.

This is a GA, because it details several other situations which affect the ampacity of conductors. However, please let me give caution about including email addresses in open postings. Search bots may find these and generate lots of spam to the address. One work-around is to express the address in an off format, such as username[at]ip-com. Observant humans can reconstruct the correct address, but search bots will most likely miss fake formats such as this. (At least for a while.)

Please perform a search of CR4 as I and others have answered this question numerous times.

The short answer is the numerous different cable de-rating factors are stated in the electrical standards and helpfully on all major cable manufacturer websites and printed catalogues.

Cadet,

In the USA, use the National Electrical Code, NFPA-70, article 310 for the factors. In that section you will find derating for conduit fill and for ambient temperatures (in relationship to the cable insulation's temperature rating). Other nations will have similar codes and standards. In the NEC you will also find the derating factors for various typical arrangements of conduit in trench or duct bank. Study!!!

--JMM

Cadet Engineer it sounds like you are in training for a maritime career. The de-rating factor will vary depending on whose standards the vessel is being built to. I'm a marine electrical designer and constantly run into the same question. Lloyd's of London has tougher standards than ABS for instance but Germanishcher Lloyd is almost the same. Class of service will also affect the de-rating factor. I find recreational boats for US service to be more lenient that a passenger carrying vessel used in the same waters because the passenger vessels must meet CFR 45 not ABYC standards. Commercial vessels are often out-flagged and nationality of owner and builder is not always a good guide in determining what standard has to be met.

In land applications it is whatever power utility company that supplies power that sets the code and thus the de-rating factor to be used. To my surprise I found even same temperature rated cable had different de-rating factors depending on jurisdiction.

Longer Cable? Higher Temp? First of all, where are you. What kind of cable? What are you doing

I plan on running power to an outbuilding using 20 amp of current (or less). This only needs 12 AWG cable, but I will derate to use 10 AWG due to the distance.

The derating factor has nothing to do here... only where you are located...