Do I Need to De-rate a Neutral Cable Buried Underground?

http://cr4.globalspec.com/search/sitesearch?do=show&order=asc&sort=textmatchrank&srch=cable%20sizing

Which would be preferable:

• Sizing based upon a selection of educated guesses by well-meaning anonymous strangers on a global internet engineering forum that cannot see the installation with no assurance of correctness of each guess?
• Sizing based upon the application of the sizing criteria contained in the applicable national electrical code/standard by a local Electrical Engineer with all the commercial come-back that this approach confers?

Tough call, isn't it?

If you are using an armoured multicore cable including the neutral conductor core, you have only 2 options, either 50% or 100% rating of phase capacity. The selection of 50 or 100% rating for neutral will be determined by the kind of connected load (balanced or unbalanced) and the estimated neutral return current. Selection of earth conductor is an altogether different subject and is determined not by the load current, but by the prospective fault current.

You need to know and understand the power distribution standards for this location and application! Depending on your circumstance and integrity you now have a few choices.

If you are actually employed at a company installing power distribution cabling then the safest thing for everyone is this. Identify and research the likely proper codes for this installation. Draw a conclusion and tell your boss this conclusion but inform your boss that you have no authority or experience to support your conclusion. Ask your boss if they have somebody with authority who could verify your work.

I hope that you are instead a student with a homework assignment. CR4 does not do homework.

The neutral should be rated to carry at least the maximum unbalanced load. If the load is all 3 phase, then there is no neutral amperage. However, I would consider at least 1 400 mm2. The grounding should be covered by the local code. There is no load except in the case of a fault. The NEC requires a grounding conductor based on the circular mil area of the phase conductors. For your conductor the grounding conductor would be a min of 85 mm2 (3/0 cu).

Neutral carries same amount of current that live carries so neutral is not all that neutral electrically. You will know the I*I*R losses to decide if cable is good enough.

In = √((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

But that doesn't take in to account harmonics in the neutral. Harmonics can drive the neutral current to above phase current.

I if was in you're shoes, I wouldn't down rate.

Shyam quote "Neutral carries same amount of current that live carries so neutral is not all that neutral electrically. You will know the I*I*R losses to decide if cable is good enough."

The poster is using a 3 phase cable. There may be little or no current in the neutral.

You all should know how much I love to follow regional standards. Have you ever wondered why three phase circuit protection devices typically do not interrupt neutral?

Judging by the answers you have received, you might want to call on someone more qualified to do so. That person or company will get the information needed and the proper laws applicable for you.

Yes, Yes & Yes! Whether it is phase conductor or neutral conductor, all de-rating factors are applicable for buried cables.