size of neutral cable 4

In a four wire system ( red,yellow, blue, neutral) can we use a neutral cable
of half size the phase cable. If this is allowed, what are the conditions to be met.
V = 415V, I = 400 Amps, Length = 350 metre

 

[electricpete]

I don't have a direct answer. In a three-phase system with neutral (I'm assuming that's what you have), the neutral wire must carry any imbalance current due to unequal loading of the three phases. Also the neutral wire must carry third harmonics, whose contributions from each of the three phases add together in the neutral. This becomes important when your single-phase loads include electronics with nonlinear power supplies (computers etc). I think I've heard that the neutral is sometimes selected at the same size as the phase conductors for this reason... but I'm no code expert.

 

[rhatcher]

I am no code expert. However, the guideline I follow (I believe) exceeds code and is in my mind somewhat justified as I like to consider worst case scenarios when considering a design. For 3-phase 4-wire systems powering single phase loads you must consider that in the extreme it is possible to have load on only a single phase conductor. Take for example a 480/277Y lighting panel with 15 single phase breakers for 277V plant lighting. I have seen many of these where each phase feeds a certain row (or rows) of lights or certain areas of the facility due to the economics of design. Imagine if for some reason it is decided that only a certain part of the plant is to be lit (during a shutdown?) and only the 5 breakers of one phase are energized. Certainly in this case the neutral feeding the panel would have to be the same size as the phase conductors.
Of course, that is just an example and perhaps an extreme one at that. However, I can think of many more where this should be considered that may not seem extreme. Take a 208/120Y panel feeding receptacles. Do you really know that the receptacles will be evenly loaded (??) or do you agree that the more real case may be a few space heaters on one phase and some clocks and computers on another.

Again, this is not based on minimum code requirements but (my idea) of a common sense approach. I hope this helps.

 

[peterb]

I seem to remember a rule of thumb that says for a normally well balanced system, the neutral should be rated at >=40% of the phase conductors. Like the above posts, that is subject to a reading of the code.

 

[jbartos]

Suggestion: The current trend is to have the neutral rated even higher than the phase conductor because of harmonics. However, if you have the dedicated power supply to the dedicated load with relatively balanced, no harmonics, primarily resistive load or high power factor load, then the small neutral, e.g. 50% of phase conductor may suffice. However, this special situation is good to have analyzed beforehand.

 

[farhadkhaefi]

I think fult current and disconnection device time is very important for selection of neutral cable,
i think IEC60364-5-54 can help you
be happy

 

[mc5w]

One of the newer U.S. rules is that the neutral cannot be smaller than the equipment grounding conductor partly because of the popularity of autotransformers to run say a 240 volt electric heater off of 277 volts.

For your distance of 350 meters you need to size the hot wires, neutral, and equipment ground on the basis of voltage drop. For good voltage regulation and efficiency I would not be surprised if you need 600 amp or even 800 amp conductors for the phases and 400 amps for the neutral and equipment ground.

 

[Walden]

I believe that you are dealing with a supply in the UK and if so, then The IEE Regulations will apply. The half-sizing of the neutral conductor comes from early editions of the Regulations. The latest Regulation state that the neutral may be reduced in cross-sectional area if, in normal use, the current likely to be carried by the neutral conductor is significantly less than the current in the phase conductors.

As with most modern rules, the onus of responsibility is with the engineer to ascertain if the installation complies.

If you have a balanced three phase load and one of the phase protective devices fails, then the neutral current will rise to the phase value. I have seen neutral conductor current detectors fitted which switch off the phase conductors if an over-current occurs. But for small installations, the cost saving of a reduced neutral is out weighed by the current protection device and usually a full sized neutral is installed.

 

[RRaghunath]

How about full rating neutral up to 16Sqmm aluminium conductor size (i.e. 4cx16Sqmm supply cable)and 1/2 rated beyond that (such as 31/2 x 35Sqmm).

The above is what we follow widely in India. If we agree that the unbalance in phase currents and consequently the magnitude of neutral current is highest at lighting panel level and comes down at the uptream, the above appears reasonable.

 

[Walden]

What “rraganuth” writes is perfectly reasonable and appears to be a good compromise between safety and cost. The problem comes if the three phase supply is protected with single gang devices as opposed to a three gang device. If one of the single gang devices switches off on a fault, you, as the engineer, must decide how long the neutral cable can carry a current which may be well in excess of its rated value before the fault is rectified. As ever, the answer is influenced by the installation cost: the likelihood and frequency of a fault; the speed with which the fault will be detected and corrected and the cost of replacing the damaged cable.