Downsizing Neutrals in the NEC

Does the NEC also specify a maximum imbalance for the feeders?

You realize that sometimes there are special cases such as when it is impossible to imbalance the load except by a major fault. The thinking is this: if a load goes single phase, then the feeder and return line (a.k.a. neutral) had better be the same size or it will melt.

Now ask yourself this. Do you want to be responsible for burning down a building and possibly killing people because you were trying to mis-read the code? Do it right or don't do it.

This could cause burning of three or four air-conditioning units.

Please don't do any design for me. I'm scared.

No. De-rating/Down-sizing of the circuit neutral is totally dependent on what the downstream loading configuration of the feeder circuit is going to be.

The amount of current that the neutral is going to be carrying determines the AWG size of the neutral conductor.

Example: A 4 wire 277V 3 Phase lighting circuit that utilizes a phase and a neutral for each circuit presents a significantly higher load on a feeder circuit neutral than a 3 phase transformer that is Delta or Wye connected.

Example: A 4 wire 277V 3 Phase lighting circuit that utilizes a phase and a neutral for each circuit presents a significantly higher load on a feeder circuit neutral than a 3 phase transformer that is Delta or Wye connected.

Which is true, but of course there is actually no load carrying neutral going to that transformer so connected!! You need only the 3 phase connections........maybe a ground cable?

What am I missing here?????

A 3 phase transformer can be connected WYE primary with neutral and DELTA secondary without a neutral. The neutral my or may not be grounded depending on the location and application.

Or;

A 3 phase transformer can be connected as Delta primary with no neutral and WYE secondary with a neutral.

Often a Delta primary has one corner grounded for the purpose of metering and protection. Some refer to this ground as a neutral.

The transformer connection may be as an "open Delta".

All this and the many other types of equipment connections that a feeder may service is why the feeder service application should always dictate the neutral size.

That is FAR more than you said before.

Your original post was simply careless and only 50% correct....and could be called missleading!!

......the very small 0mm² neutral conductor.

Im just quoting the NEC. If its not sensible, then the NEC should be amended.

I will propose an amendment to it. Such as "....protected by Overcurrent Protection Device so that the load carried by the ungrounded conductor can not exceed its ampacity" ......

PS - no, i never have and probably never have used this in any design, altho in the BS7671 (IEC) countries like India or the middle East, this is standard practice. I just saw a job (apartments and commercial, high rise building) where the feeder was [3c x 300mm2 + 1c x 185mm2]. but that it an easily balanced load.

Yes, the devil is in the details.

The NEC is no different than any other code book as it has multiple disclaimers, and references that require intense awareness and thorough scrutiny.

Although the NEC is a very good guide often local codes are much more stringent and override it.

Knowing where you stand, what applies, and how to apply it is difficult at times but catamount to survival.

Friends,

Yes, in many cases the NEC does allow the neutral to be smaller than the phase conductors because it is supposed to only be carrying the imbalanced portion of the load. However, the increasing prevalence of electronic loads, such as electronic lighting ballasts, makes downsizing problematic. In single-phase (2 hot wires and a neutral, with the voltage on the two hot wires 180-degrwees apart), there is no problem with neutrals and electronic loads. However in three-phase (3 hot wires 120-degrees apart plus a neutral), the neutral current theoretically can be 150% of the phase current!

The reason is that the the non-linear load of electronics can impose significant amounts of harmonics onto the conductors. The "triplen" harmonics do NOT cancel out when you add the phases together, so neutral current can and often is larger than each of the phase currents. Codes and listing agencies have begun addressing this with rules that limit the percent of harmonics that can be imposed on the incoming circuits (one reason is the harmonics increase heating in the transformers so they have to be oversized or be designed with a "k-factor").

The NEC rules and exceptions come primarily from experience and analysis of failures. Therefore, the NEC does allow downsized neutrals in many cases, but it will require full-sized or even upsized ones in some cases.

--JMM

You might be interested in knowing that a recent (within a year or two) UL study has shown that concerns around harmonic generated neutral currents may not be as much of a big deal as the copper sellers would want us all to swallow. I will see if I can dig up a link on the issue.

North of 60,

I'd be happy to see that, if you find the link. --JMM

In the conductor sizes where a smaller neutral is permissable and very often part of the exfactory composite bundle I use aluminium. Saves a heap. Those copper barons....