Wiring a building switchboard from an outdoor transformer.

Many rules must be correlated when a customer-owned outdoor transformer supplies a large switchboard.We received a deceptively simple question about the ampacity requirements for both the feeder and the main power bus rating on a commercial building main switchboard fed from an outdoor transformer. Quickly, however, the question started involving many different Code requirements, resulting in fairly

Many rules must be correlated when a customer-owned outdoor transformer supplies a large switchboard.

We received a deceptively simple question about the ampacity requirements for both the feeder and the main power bus rating on a commercial building main switchboard fed from an outdoor transformer. Quickly, however, the question started involving many different Code requirements, resulting in fairly complicated correlation problems.

The original question concerned a calculated load that we will assume can be properly supplied by a 2MVA transformer with a 480Y/277V secondary and fed by a medium-voltage service. The original engineering suggestion was to use a 3000A switchboard with six fused disconnects that, taken together, would substantially exceed this rating. Could this be done, avoiding a main disconnect? What if the feeder ampacity reflected the sum of the main fuse ratings? The service point was the 13.8 kV line terminals of a fused interrupter switch.

The EC&M panel's response

Our answer, in a word, is no, regardless of the feeder ampacity. There is no way to avoid a main disconnect when all the applicable rules are considered. A main over-current device might; however, be located at the transformer. In looking at a question like this, never allow yourself to be confused by mixing up the various requirements. Analyze each rule independently, and then correlate based on the worst case.

In this case, we will consider: 1) the disconnecting requirements for the building, 2) the transformer protection aspects, and 3) the status of the feeder and the switchboard busses and their required ampacities and protection.

Disconnecting the building

The building disconnecting means must meet the requirements of Sec. 225-8(b):

(b) Disconnect Required for Each. Where more than one building or other structure is on the same property and under single management, each building or other structure served shall be provided with means for disconnecting all ungrounded conductors.

The disconnecting means shall be installed either inside or outside of the building or structure served, at a readily accessible location nearest the point of entrance of the supply conductors.

Disconnects shall be installed in accordance with the requirements of Sections 230-71 and 230-72.

A remote disconnect at the transformer could not be used to comply with this requirement; however, up to six building disconnects could be used for this purpose. This is because the cross-reference to Sec. 230-71 incorporates the familiar six-service-disconnect rule for building disconnects. Therefore, in terms of applicable requirements for disconnecting the building, the building switchboard may include up to six disconnecting means. There are other elements to be considered, however.

The transformer protection

This is a medium-voltage transformer in a commercial occupancy that will not generally be judged to have the qualified personnel in place to "monitor and service the transformer installation" required to meet the allowances for supervised installations in Sec. 450-3(a)(2). Therefore, the transformer must be protected for unsupervised installations per Table 450-3(a)(1).

Assuming 6% or lower impedance in the transformer, the primary fuses cannot exceed the next higher standard size [see Sec. 450-3(a)(1) Ex. 1] above 300% of the rated primary current. In this case,

2000kVA [divided by] (13.8kV x 1.73) = 84A 84A x 3 = 251A

Therefore, primary protection will be limited to 300A, the next higher standard size fuse. The secondary must also be protected, at 125% of rating. The secondary current rating for this transformer is 2406A. Usually, for safety, this will be rounded to 2400A because 125% of 2400A is 3000A, a standard sized device. If the exception is applied (as the literal text does permit), then the next higher standard sized device is 4000A, which is 166% of the secondary current.

Sec. 450-3 permits up to six overcurrent devices to serve as this protection, provided the sum of the individual ratings does not exceed this value. Therefore, up to six overcurrent devices can be used on the secondary side of this transformer and still meet the protection requirements for transformers as well.

The feeder elements

The service conductors supply the medium-voltage main switch ahead of the transformer primary. The conductors connected to the transformer secondary are feeder conductors, as are the busbars within the switchboard. Whatever overcurrent protection arrangements are made, they must protect both the busbars and the feeder conductors in accordance with Article 240. There are two basic options, namely, to place the protection either ahead of the conductors, or at their load ends. In either case, since Sec. 240-3(c) does not allow the next higher size overcurrent device (above 800A) to protect a conductor above its ampacity, the ampacity of both the feeder conductors and the switchboard bus must not be less than that of the overcurrent device.

If the conductors are suitably protected at the transformer, then up to six disconnects may be installed in the building switchboard without an additional main device. These disconnects and their associated overcurrent protection may be sized to the load requirements of their derived feeders, and the sum of those ratings may exceed that of the bus. Of course, the calculated total load must not exceed the 3000A rating of the main bus.

On the other hand, protection at the load end, in this case the building switchboard, means that these are tap conductors and must meet one of the tap provisions in Sec. 240-21. Assuming the transformer is not immediately adjacent to the building (specifically, located so as to require secondary conductors more than 10 ft long), the only tap rules that could apply would be Sec. 240-21(d) for transformer taps in general, or Sec. 240-21(m) for outside feeder taps. As a practical matter, since the provisions in this latter subsection apply and are more lenient, this will be the applicable rule, and the first sentence of paragraph (2) turns out to be the limiting factor:

The conductors terminate at a single circuit breaker or a single set of fuses that will limit the load to the ampacity of the conductors.

This rule limits the number of overcurrent devices to one (or one set of fuses). Therefore, a single main device is required. If sized in accordance with the feeder conductor ampacity, then it would fully comply with the tap rule in Sec. 240-21(m), as well as more than meeting the six-disconnect rule in Sec. 225-8(b). Assuming it is sized to coordinate with the requirements in Sec. 450-3(a)(1), then it would properly protect the transformer secondary as well.

A valid restriction?

The single device restriction in Sec. 240-21(m)(2) is controversial, although it matches similar language in other tap rules, including Sec. 240-21(c)(3), (d)(5), and (e)(2). In this case, however, the tap conductors must be "installed outdoors, except at the point of termination." This makes them similar to service conductors in terms of hazard to the building, albeit somewhat less because there must be some overcurrent protection on their supply end.

There was a proposal for the 1996 NEC to correlate this requirement with the other applicable Code section and allow six overcurrent devices with a combined rating to that required for a single device. This proposal ultimately failed because of panel concerns about ready disconnection. The Correlating Committee noted; however, such concerns were outside the scope of Article 240. We agree with the Correlating Committee, and it is likely this rule will be correlated with Sec. 225-8(b) in the next Code.

TAGS: Design
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