Topics discussed in our "Code Forum" column involve complicated issues requiring extensive analysis. However, not every Code question warrants such in-depth treatment. Here are the latest short answers to questions posed on our website. Coverage includes topics in: Sec. 300-15, 373-5(c), 384-36, 430-1, 430-53, 430-97, 430-102(b), and 430-109(f).
Q. I was involved in an investigation of an electrical accident a while back. However, I noticed something in the Code I may have missed before. Table 430-97 of the 1999 edition specifies spacing requirements in motor control centers.
After doing some research, it appears this topic first appeared in the 1993 Code. Am I correct? What is the intent of this table? Does it apply strictly to safety so you have some room to move components? I was investigating an accident, where a bus bar cover accidentally contacted one of the phases in a motor control center (MCC) as an electrician tried to remove it. He didn't have much room, and I thought perhaps that's why this table is in the NEC. Am I right? The MCC was an older unit, predating the 1993 NEC.
I was also wondering if new MCC units must have an interlock or some type of guard over the lug connections? I thought I saw some newer ones advertised that way. Is this required, or are some manufacturers just putting them in?
A. The answer is yes and no. You are correct about the inception date of this table, but not its motivation. The 1993 NEC Correlating Committee imposed a scope on Art. 430 (Sec. 430-1) for the first time. The scope resulted from a turf battle between panels extending over several previous cycles. Art. 384 [specifically then Sec. 384-4, now relocated in the 1999 edition as Sec. 110-26(f)] covered external installation requirements, such as any rules about dedicated space above the MCC. Art. 430 got the inside of the units. To this end, the 1993 NEC extracted material from Art. 384 to develop the new part on motor control centers.
The table you reference came straight out of Sec. 384-36, where similar material remains to this day; where it has been for generations. This material addresses creepage distances and dielectric strengths. It has nothing to do with your problem. Locking mechanisms would be a design feature, not a Code requirement, except if a lockout is used for compliance with a rule, as in Sec. 430-102(b) Ex. Note, however, such rules typically apply to the individual circuit; you cannot use locking restrictions to an entire control center to cover a lock rule for an individual circuit. The Code generally doesn't require locked access to bus connections.
Q. I know this question is basic, but I need to find out if you can install two or more 3-phase motors out of one motor starter and overload relay set. I looked through the NEC, but didn't find anything that directly applies.
A. The answer is yes, but usually only in theory. The reason is the short-circuit and ground-fault protection for both motors must be within the limits given by Table 430-152, as per Sec. 430-53. For a single set of overloads, the protection must be within Code limits for each motor as well. As a practical matter, you aren't likely to achieve that and still get the motors to run; at least at the same time. If you go to multiple overload blocks, there are situations where this might work, especially for multiple small motors at relatively high voltage, where the smallest standard size circuit breaker is 15A.
Q. The local inspector just told us you can't use a connector bigger than 1 in. with Type NM cable because anything bigger isn't listed for this wiring method. He also told us you can only put the amount of Type NM cables in the connector it is listed for. How much of this is true, and is there any way around this? I think they are overly concerned about the product listing. We've also had some arguments lately about the number of conductors you could terminate on a neutral or equipment grounding bus in a panel. I've heard it's three wires to a hole on some panels, and just one on others. Is this true?
A. The practice of securing multiple Type NM cables to a panel violates any connector listing I'm aware of; I think the maximum listing is two cables. If you use more, you do violate the listing, and the inspectors are correct to enforce it. See the second paragraph of Sec. 300-15, which expressly prohibits violating such a listing. There are listings on connectors larger than 1 in. for Type NM cable, since the Code allows this wiring method to be manufactured with up to No. 2 conductors. However, I doubt this question focuses on large conductor makeups.
That said, the 1999 NEC will, for the first time, allow you to use an oversized raceway nipple out of the top of a cabinet or cutout box. This would include the panelboard enclosures directly addressed in the question. You need to look at the new exception in Sec. 373-5(c). There are very specific details and restrictions, which are too complicated to go into here. For more information, review the analysis in the October 1998 issue of EC&M and in our 1999 Illustrated Changes Books.
Regarding the number of wires in a terminal hole, you need to follow the directions on the panel cover. In general, a manufacturer can get a listing for multiple small equipment grounding conductors, but not grounded circuit conductors. Generally, the directions reflect this difference. The reason is: Grounded conductors are current-carrying conductors, subject to heat cycling. It turns out to be problematic to get a connector used for multiple connections to pass the requisite tests under a heat-cycle condition. The equipment grounding terminations are okay because, justifiably, UL doesn't impose a heat cycling test on these terminations.
Q. Are there any simple knife bladed disconnect switches designed to be opened/closed under load? My understanding of the NEC is "disconnects" were to be provided for local lockout/isolation purposes, but not to be operated under load.
A. Actually, they are tested under load, because the NEC allows them to be used for this purpose up to 100 hp. See Sec. 430-109(e).
I have always believed a controller should be used to control a motor. I made the motion on the floor of the 1992 NFPA Annual Meeting that kept a rule mandating insight disconnect switches for all motors out of the 1993 NEC. I argued if there was a reason for providing for emergency stoppage, then the control circuit should provide for safety chains, deadman controls, etc. I still feel controllers should do what they're designed to do: start and stop motors. Still, within Code limits, you can use these switches in an emergency, and they will function properly.