Cable selection 2

[wonderchi]

Hi,

I am wondering if anyone can give me advice on when to use double netural cables for building services: eg...data centres (with harmonics) is the only one I am thinking about.

To take it to the next step...what is the deciding factor of using double netural cable and how you can calculate it?

Thanks

 

[EEJaime]

200% Neutrals are used when the loads are heavily laden with switching power supplies, electronics, such as lighting ballasts and other loads which may introduce heavy harmonics loads on the system. While it is possible to calculate if you have VERY specific load information and VERY specific electrical system configuration information, oftentimes it is a decision that is precautionary in nature. Some faciliies use high K factor rated transformers along with 200% rated neutral conductors in the distribution systems as a way of not having to worry too much about the exact neutral loading condition. We specify in these types of facilities simple added precautions such as not sharing any neutral conductors when 120V loads are fed by what would normally be a three circuit-four wire feeder. We simply take individual neutrals for each circuit.

Depending on the type of facility it may or may not make sense to use 200% rated neutrals, it really depends on the types and quantities of loads you have.

 

[waross]

You can get a feel for a situation by looking at the loads that DON"T generate harmonics on the neutral. This would be phase to phase loads and resistive loads such as incandescent lighting.
Then look at the combined neutral current of the loads that may generate harmonics.
Multiply the harmonic suseptible current by 2. If the total current is over the rating of the phase conductors then go to 200% neutrals.
This is a conservative estimate.
respectfully

 

[wonderchi]

Thanks for that..as with naming convention..what is the best/recommended naming of double neutral...

e.g 5 *1C 185mm^2 + E (XLPE/PVC) CU cable or
e.g 3 *1C + 2N 185mm^2 ...etc

 

[jghrist]

You can overestimate the harmonics in modern data control centers. Most major data center loads such as servers, routers, hubs, and storage systems are designed for low harmonics. See Harmonic Currents in the Data Center: A Case Study at

http://www.apcmedia.com/salestools/SADE-5TNRJN_R0_EN.pdf