DISCONNECT ON UTILITY SIDE OF AN ATS 1

[TWW]

I have been trying to determine the NEC code requirement of a disconnect switch on the "utility" side of an automatic transfer switch. This is for a 701 legally required standby system with diesel generator as back-up source. I note that the handbook drawing of exhibit 700.1 does not indicate one (sorry, but drawings are "sometimes" worth a thousand words. My feeling is that there should be one to isolate the switch for maintenence. However would it be legal to instead use pole mounted fused disconnects on the primary side of the transformer? for a new design I would put one in, but in this case the construction is already started and I would prefer to not to introduce a mod unless it is strongly suggested.

Thanks and be nice.

 

[peebee]

NEC does generally requires not only a disconnect, but overcurrent protection on the utility side (as well as on the generator side).

The only exception I can think of is if the ATS is service-entrance rated, which means that it serves as its own disconnect, and generally implies that it is a two-breaker transfer switch.

There's no requirement for a LOCAL disconnect at an ATS.

Keep in mind that you'll usually have, at minimum, a utility disconnect, and in 99% of installations you'll have downstream distribution too. And your feeders to the ATS all require overcurrent protection per Article 240. And your generator requires overcurrent protection unless it's self-protecting (which is rare). And 701.6 requires that your entire emergency system have adequate capacity. Put all that together, and there's no way you could come to the conclusion that a disconnect is not required for the ATS.

And if you're still not convinced, see 230.82(4), which requires that even if you tap ahead of the service disconnect, you gotta provide another service disconnect.

230.70 requires that the disconnect be in a readily accessible location -- a pole mounted disconnect might or might not meet the Article 100 definition of "Accessible, Readily", it'd depend on the specifics of the installation (you definitely could not require a ladder to operate it).

Hope this helps.

 

[TWW]

The generator has a built in circuit breaker disconnect to protect the feeder. 150KW 208V-Y Gen w/400A 3p circuit breaker, that would provide overcurrent protection for this feed. The generator is sized adequately for the required load.

The ats is connected to this generator and a padmount transformer with OH fused cut-out protected primary lateral as well as the oil immersed primary fuses. On the transformer secondary side, currently there is no disconnects or fuses on the feed to the ats. The load side of the ats is tapped three ways feeding 3-buildings w/2 out of three having CB disconnects protecting these lower amperage feeds. The main building feed is not protected up to the main building CB disconnects. This was an interpretation of NEC 240.21 (5), outside taps of unlimited length.

Therefore I have met article 240 for conductor protection, primary conductors, secondary conductors and generator feed.
and I have met 230.82 by protecting the tapped feeders post ATS

I have met 230.70 by having readily accessible disconnects on the main and auxillary building's service entrances.

I am not saying this is a sound design practice, but nothing in the code I see yet says I can not do it. (A challenge to the code kings).

 

[TWW]

Just re-read 230.82 (4) (esp. note in the handbook). The ATS definetly needs a disconnect device. Thanks peebee for pointing me in the right direction. I will have to produce a new contract modification. Better me doit than the contractor!

 

[dpc]

Where are you taking service from the utility and where is your main service disconnect? It has to be before the ATS. The overhead fuse cutouts don't cut it as service disonnects. And the transformer fuses under the oil only provide backup transformer protection and are not designed to protect your secondary conductor. If all of this is on the utility's side, then it is a moot point, but you still need a main disconnect ahead of the ATS.

Why not just ask the local inspector for an interpretation and get it over with? You'll have to do it at some point.

 

[TWW]

yeh, yeh, I hear you. I will have a main breaker installed inside the secondary compartment of the pad mount transformer. This will be my main service disconnect. not exactly readily accessible for your average joe, but should suffice for a controlled environment (police/fire) with dispatch. My mistake was to assume that a giga-bucks ats would incorporate this function, (unfortunately I am not allowed to specify name, brand or make). bad assumption. As for the primary fuses protecting the secondary conductors, this is allowed for outside service entrance conductors that terminate on a breaker rated to protect the feed conductors. (just took this concept allittle to far).
As for inspectors, this is on a federal reservation, there is no inspectors per say. The contractors are responsible to provide there own quality control contractor CQC that can offen double as a super. It interesting to note that the
CQC's job is at the mercy of the prime who can fire him and get another. Welcome to the wacky world of federal work.

 

[dpc]

The Westinghouse (now Cutler-Hammer) Automatic Transfer switch is made from two molded-case circuit breakers with an interlocking actuating mechanism. These can actually be labeled as service entrance equipment as opposed to an ASCO-type switch that cannot. This is one of their main selling points when competing against a more traditional transfer switch. This would have been a good application.

I was burned on this once and had to install a 400A fused disconnect ahead of the ATS as a change order so I feel your pain.

 

[peebee]

All of the major ATS manufacturers offer two-breaker ATS's including GE/Zenith, Russelectric, and ASCO. Cutler-Hammer also offers 2-breaker units, and I'm pretty sure Square-D too.

The key thing is to get the service entrance rating. And yes, ASCO can provide such a rating.

I'd sort of already pointed this solution out to you: "The only exception I can think of is if the ATS is service-entrance rated, which means that it serves as its own disconnect, and generally implies that it is a two-breaker transfer switch."

 

[peebee]

If this involves a change, why not just tap the ATS downstream of the service entrance switch? That might be a zero-cost solution.

 

[asco1]

peebee is correct, ASCO does provide a service entrance rating. It is not a breaker pair though. ASCO uses an actual transfer switch which is bused or cabled (depending on the amperage) to a breaker to provide disconect and overcurrent.

 

[peebee]

Despite that being asco1's first-ever post on eng-tips, I suspect he knows what he's talking about when it comes to ASCO switches. . . .

 

[TWW]

As far as puting the ATS downstream of the service entrance switch, this would be a good idea normally. However in this case the ATS actually feeds three buildings, with a combined total of 6(3+2+1) service entrance switches.

And thanks, I like the idea of getting the switch with a service entrance rating, I will (try to) use this requirement to reject a non service entrance rated ats submittal.