Find more articles on: Transformers
There are practical industrial applications for wiring several buildings from one transformer, but does it meet Code?
A new controversy has come up on whether or not multiple buildings can be fed from a common transformer secondary, and if so, how. Assuming there are no problems with the computed load, and assuming a single disconnect and set of overcurrent devices at each building that will protect the outdoor lateral to that building from overload, could an arrangement similar to that shown in the drawing be used?
The EC&M panel's response
This question is a good follow-up to our discussion in the February 1996 issue. That issue addressed coordination of requirements in Articles 240 and 450, along with disconnecting requirements in Art. 225, when one building had multiple disconnects fed from an outdoor transformer.
In this case, although you need to remember the requirements for the conductors in Art. 240, the main issue will be protecting the transformer secondary. Although you now can use up to six overcurrent protective devices as secondary protection, they must be grouped, per the second sentence of Sec. 450-3:
The secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location.
Obviously, if the overcurrent devices are in separate buildings, they couldn't possibly be considered "grouped" in accordance with this rule. However, this wouldn't be a problem if secondary protection weren't required, or if secondary protective devices (up to six) were grouped at the transformer. If secondary protective devices are grouped at the transformer, and the feeders to each building are sized accordingly, then the controversy disappears. If the secondary protection can be omitted, however, using multiple feeders has become controversial.
Secondary protection not required
Secondary protection isn't required when the primary protection is sufficiently close to the transformer rating, and, in the case of medium-voltage primaries, there is expert supervision. For medium-voltage installations, the point of departure is Sec. 450-3(a)(1), which requires adherence to the size limits in Table 450-3(a)(1). That table requires secondary protection at all voltages. There is, however, an exception, Sec. 450-3(a)(1) Ex. 2, which allows the use of Sec. 450-3(a)(2) for "supervised installations."
A supervised installation is one in which "only qualified persons will monitor and service the transformer installation." This pointedly includes the word "monitor." In a commercial or residential environment, we doubt that many installations will qualify under this restriction, which implies continuing supervision; but, many industrial installations will qualify. The significance for this question is Sec. 450-3(a)(2)a. allows primary protection as the sole protection. Primary devices must be sized within 250% of rated primary current for fuses and 300% for circuit breakers, with allowances for the next higher standard-sized device.
Secondary protection also isn't required for installations at 600V and below, provided the primary protection is set within 125% of the primary current rating of the transformer, perSec. 450-3(b)(1). As in the case of medium-voltage installations, the next higher standard-sized overcurrent protective device can be used. There isn't any qualified supervision rule for applications at 600 V and below. Therefore, omission of secondary transformer overcurrent protection is clearly permitted for some supervised medium-voltage applications and many utilization-voltage applications.
Outdoor taps to the transformer secondary
If secondary protection isn't required for the transformer, then running multiple feeder taps from that secondary becomes possible. We think there is no limit to the number of such taps, as long as each feeder tap meets all the rules in Sec. 240-21(m):
(m) Outside Feeder Taps. Outside conductors shall be permitted to be tapped to a feeder or to be connected at the transformer secondary, without overcurrent protection at the tap or connection, where all the following conditions are met:
Following this, there are five conditions all of which we will stipulate to have been met in this case. In the drawing, each feeder has been connected at the transformer secondary, and we will assume adequate protection from physical damage (condition 1). The feeders run outdoors (condition 3) and terminate at a single disconnecting means and suitably sized overcurrent device (conditions 2, 4 and 5), as previously noted. There is no language in any of the conditions that prevents multiple feeder connections. The "conductors" referred to in the various conditions and the "single" overcurrent device are the conductors and the associated overcurrent device that comprise the feeder and its protection to a given building; a second feeder involves a different group of conductors and a different overcurrent device. As long as the transformer protection meets Code requirements, and each feeder tap has the prescribed protection, multiple feeders can be connected.
Remember, this tap rule, although seldom used in this way, also applies to taps from a large outdoor feeder. One could imagine a large feeder running through a handhole, with multiple taps originating in that handhole and running to separate buildings. Just as we can't conceive of any credible objection to this, multiple transformer connections are equally allowable. To say otherwise would be to say although two feeders originating from different handholes are allowable, the same feeders originating from one handhole are not. Although possibly unintended by the panel, this procedure is technically sound and squarely in accordance with the literal text.
RELATED ARTICLE: WHY AREN'T SERVICES RESTRICTED?
We received a letter responding to the analysis in the February 1996 issue noting that if the service point were at the building, the multiple disconnects would be allowed. Why, the letter asked, can utilities do this? In the same way, utilities constantly make installations as described in this month's question without restrictions. The reason goes to the divergence between applicable codes. The National Electrical Safety Code for utility work is less stringent because it assumes a higher level of specialized training on the part of those who maintain the work after the original installation.