I am reviewing a set of drawings where the EE has used an undersized neutral to feed an MCC. The phase conductors are 300MCM, the neutral is #4. The only loads connected to the neutral are control transformers 347-120V. The conductors are all to run in a single conduit along with a #3/0 EGC and are protected by a 300A CB.
The Canadian Electrical Code allows the neutral to be sized to carry only the unbalanced load, so I am not concerned about code compliance. I would like to know what you think about installing an undeersized neutral. Are there safety issues, have you had experiences with faults in such installations?
OK, first I'm not at all familiar with the CEC, as I'm stateside.
As to your inquiry, No, I have not seen any situations of damage or problems with undersized neutral conductors in systems as you described.
Occasionally I come accross a spec with a 'undersized neutral' by a PE/EE. Upon review of the plans and the loads, I may elect to install a full neutral and send a CO to that effect. Please note that the 'load' must be considered; if it's into a panel, I prefer a full neutral, primarialy based on future modifications. Note that this is my opinion, and NOT based on NEC requirements. We perform a lot of repeat work at the same complexes, and always try to 'think ahead', within reason.
Also, if you are bidding, you may end up 'high' if you do not follow plans and specs without offering explanations (if possible)
Normaly a MCC doesn't need a neutral. I like the idea of an undersized neutral that is used just for the control transformers. It can get overloaded though. I have seen many machines with lights, receptacles and other 120 volt devices connected to the control circuits. Is the line side of the control circuit tapped in the bucket with it's own fuse in place of the transformer? Not sure if it is a violation though.
I also don't know about the CEC, but I might have a couple of issues about this.
1) Is the MCC the service equipment? If so, is it listed as such? Also, in the NEC, the nuetral would have to be sized as a GEC, as per 250.24(B).
2) Is the nuetral sized large enough to clear a phase to nuetral fault? There is a change in the 2005 NEC that states that nuetral conductors must be sized not only to carry the unbalanced load, but also large enough to qualify as an EGC, in case there is a line to nuetral fault.
2. The nuetral conductor is not sized to carry a line to neutral fault. That is my issue with this design - what happens to the neutral if there is a line to neutral fault? It seems to me that the neutral conductor could potentially experience major damage during a fault and would have to be replaced. It is to be contained in the same conduit as the line conductors, so I suspect that in the evnet of a line to neutral fault all of the conductors in the conduit would be damaged.
Steve, if it is not sized to handle a fault, either the conductor itself will reach its short time withstand rating and begin to anneal or vaporize, or the conductor may have too much impedance to open the protective device quickly. Either way, I see this as a problem.
Yes, I understand that, strictly speaking, it isn't always necessary to have any neutral, let alone a full-size one, serving a motor control center. I also understand that it is not the purpose of the code to plan for future uses. But....
I would strongly encourage the installation of a full-size neutral anyway- even if all loads are three-phase motors. Or, perhaps I should say, especially if all loads are three-phase motors.
Why? Because we're not in the '60's anymore. Today's production equipment more and more uses variable speed controls and electronic components. The two don't mix well- that's why there is such a concern these days over "power quality" and "harmonics." In some cases, it is even necessary to double the size of the neutral.
Sure, the place may have only simple, old-fashioned three-phase motors today....but what about tomorrow? Or, will the PoCo hit him with 'power factor' penalties, and he'll want to add a bank of capacitors? Then he'll be glad you planned for the neutral.