700.9 (nec 2008)

[charlie b]

Consider a large site with many buildings. In a Central Utility Plant (CUP), there are several diesel generators intended to provide Emergency, Legally Required Standby, and Optional Standby loads throughout the site. Conductors are run in a loop from the CUP to building to building to building, and back to the CUP. At each building, the main loop is tapped, conductors are brought into the building, and (through some appropriate distribution equipment) are connected to E, LRS, and OS transfer switches.

Question: Does the fact that the loop itself provides power to Emergency loads and to ?other than emergency? loads, as also does the feeder into each individual building, run afoul of 700.9, and in particular the new 2008 language in sub-paragraph (5)?

FWIW, I have one existing site with this configuration, and am designing another site with a similar configuration.

 

[charlie b]

Clarification: I am specifically talking about 700.9(B)(5).

 

[skeshesh]

Interesting question Charlie. I think if you're tapping the loop in each location and bringing each source to seperate transfer equipment before distribution you should be okay per 700.9(B)(5)(a), defining the transfer switch as an "individual disconnects mounted in seperate enclosures". Now, what I don't get is what you mean by "through some appropriate distribution equipment":

conductors are brought into the building, and (through some appropriate distribution equipment) are connected to E, LRS, and OS transfer switches.

Are you using a switchboard or something before going to said transfer switches?

 

[charlie b]

Now, what I don't get is what you mean by "through some appropriate distribution equipment." Are you using a switchboard or something before going to said transfer switches?

My thoughts were to bring the "backup" conductors into the building, go into a tap box, and supply two separate enclosed breakers. One of these would supply the emergency ATS, and the other would supply the LRSB ATS. That would get me into 700.9(B)(5)(a).

My problem is two fold. First, 700.9(B)(5)(b) speaks of having no OCPD at the source. But the central plant (where the generators are located) will have overcurrent protection, to protect the feeders that loop around the site.

Secondly, where exactly is the "source" to which the second sentence of 700.9(B) refers? If I have three generators in a central plant, each connected to the same bus via its own generator breaker, and from that bus there are breakers that protect the feeders that run around the site, and once I go into a building I include the wiring described at the top of this post, then does the "wiring from an emergency source . . . to emergency loads. . ." start at,

1. Each generator's breaker, or
2. The breaker that protects the loop conductors, or
3. The distribution equipment inside the building, or
4. The load side of the emergency ATS?

 

[chris kennedy]

Charlie, do you have the 08 Handbook? Exhibit 700.4?

 

[steve66]

Charlie:

Here is how I see it (right or wrong )


The intent is that no emergency loads share a breaker with any non-emergency loads. In fact, it goes even farther. No emergency loads shall share any any vertical switchboard section or panelboard area with any non-emergency loads.

So if you have any common switchboard vertical section that supplies both emergency and non-emergency loads, it doesn't comply. (And your setup has this.)

There is one exception, titled "exception to (5)(b). That exception allows (IMO) one common breaker to share EM & non-EM loads. That let's you have a generator with a single main breaker, or a single switchboard with a main breaker.

If any other breaker shares EM & non-EM loads, I don't think its Kosher.

I would run a separate loop for the emergency loads, and supply it from a separate vertical section in the main switchgear.)

Steve

 

[skeshesh]

My thoughts were to bring the "backup" conductors into the building, go into a tap box, and supply two separate enclosed breakers. One of these would supply the emergency ATS, and the other would supply the LRSB ATS. That would get me into 700.9(B)(5)(a).

My problem is two fold. First, 700.9(B)(5)(b) speaks of having no OCPD at the source. But the central plant (where the generators are located) will have overcurrent protection, to protect the feeders that loop around the site.

Secondly, where exactly is the "source" to which the second sentence of 700.9(B) refers? If I have three generators in a central plant, each connected to the same bus via its own generator breaker, and from that bus there are breakers that protect the feeders that run around the site, and once I go into a building I include the wiring described at the top of this post, then does the "wiring from an emergency source . . . to emergency loads. . ." start at,

1. Each generator's breaker, or
2. The breaker that protects the loop conductors, or
3. The distribution equipment inside the building, or
4. The load side of the emergency ATS?

It is certainly a bit of a hairy situation. First let me give you my thoughts about the definition of "source". I think the second sentence in 700.9(B) is not very clear in that it talks about "Wiring from an emergency source OR emergency source distribution overcurrent" I could see two possible interpretations: I. The intent is to establish that if overcurrent protection does not exist at the generator, the "source" is considered to start at the overcurrent protection in the distribution equipment. II. The second possible intent, and one that would help you out, is that if a large scale generation is distributed through a complex distribution system, the "source" can be considered to start at the distribution OCPD.

As far as the issue with having OCPDs at the generator, again I think the intent is probably similar to the second intent mentioned above. 700.9(B)(5) does start by saying "wiring from an emergency source to supply any combination of emergency, legally required, or optinal load...", but immediately afterwards in conditions (a) & (b), there's mention of verticle sections of switchboards. This leads me to believe they intend for the seperate wiring rule to begin at the distribution switchboard immediately downstream of the generator, so that you can run the feeders from the genset to the proper switchboard, and then run each section seperately to seperate E, LR, & OSB loads.

A final comment - I dont know how they really define the 'selective coordination' requirement, so couldn't you just download and toss together a few IT curves from the OCPDs between the generator and distribution board secondaries and call it a coordination study? Just a (silly ) thought that came to mind...

 

[charlie b]

Charlie, do you have the 08 Handbook? Exhibit 700.4?

I do. And I have used that configuration for a recent design for a building that will have its own generator. But my issue relates to a set of generators inside a central plant, with a distribution system inside the same plant, serving as the backup source for multiple buildings on the same site. If I don't have separate feeders leaving the central plant, one for an emergency loop, one for a LRSB loop, and a third for an OSB loop, will I have blown independence?

 

[chris kennedy]

First, 700.9(B)(5)(b) speaks of having no OCPD at the source. But the central plant (where the generators are located) will have overcurrent protection, to protect the feeders that loop around the site.

Doesn't say "shall not have OCPD".

700.9(B)(5)b.
The common bus or separate sections of the switchboard or the individual enclosures shall be permitted to be supplied by single or multiple feeders without overcurrent protection at the source.

Secondly, where exactly is the "source" to which the second sentence of 700.9(B) refers? If I have three generators in a central plant, each connected to the same bus via its own generator breaker, and from that bus there are breakers that protect the feeders that run around the site, and once I go into a building I include the wiring described at the top of this post, then does the "wiring from an emergency source . . . to emergency loads. . ." start at,

1. Each generator's breaker, or
2. The breaker that protects the loop conductors, or
3. The distribution equipment inside the building, or
4. The load side of the emergency ATS?

I vote the gutter or tap box in the building ahead of the ATS's. This is the ATS's source of emergency power.[/quote]

 

[skeshesh]

Doesn't say "shall not have OCPD".

If you continue reading exception to (5)(b) states "Overcurrent protection shall be permitted at the source or for the equipment, provided the overcurrent protection is selectively coordinated with the down-stream overcurrent protection". Since this exception specifies where it is permitted to have OCPD at the source or equip., I would think it follows that it is otherwise not permitted. I agree that we can't take 700.9(B)(5)(b) permitting connections without OCPD at the source to mean ones with OCPD is not permitted, but in the same way we cant take it to mean that it does permitt it.

I vote the gutter or tap box in the building ahead of the ATS's. This is the ATS's source of emergency power.

I dont know... from the generation to the main distribution equip can be combined, but I think the secondary of distribution to an ATS has to be seperate. Any code referrences, etc. to support this?

 

[chris kennedy]

Since this exception specifies where it is permitted to have OCPD at the source or equip., I would think it follows that it is otherwise not permitted. I agree that we can't take 700.9(B)(5)(b) permitting connections without OCPD at the source to mean ones with OCPD is not permitted, but in the same way we cant take it to mean that it does permitt it.

Are you a politician?:grin:

I dont know... from the generation to the main distribution equip can be combined, but I think the secondary of distribution to an ATS has to be seperate. Any code referrences, etc. to support this?

Distribution? The 3 gennys in parallel act as one and the feeder leaving is protected be a single OCPD. I see no where in 700 that says an alternate source can feed only one building.

 

[skeshesh]

Are you a politician?:grin:

Politician, engineer, lover & a fighter
But seriously, how does permitting it without OCPD automatically let you assume it is permitted with OCPD specially when its followed immediately with an exception detailing when you are permitted to use an OCPD?

Distribution? The 3 gennys in parallel act as one and the feeder leaving is protected be a single OCPD. I see no where in 700 that says an alternate source can feed only one building.

When did I say you have to feed only one building? One building or a 100, you either run straight to transfer equip or you run to distribution equip and then to transfer. I think that you dont have to have seperate runs if you use a distribution switchboard before you run to transfer equip since 700.9(B) & the subsections talk about OCPD at source OR at source distribution - if you're running straight to an trasnfer equipment, there is no OR, you just have OCPD at source and you cannot combine the run after that.

 

[steve066]

Forget about the circuit breakers for a minute. If we focus on the part about "vertical switchboard sections", I think we can clearly deduce that a common feeder to one (or more) buildings to feed EM & non-EM loads is clearly a violation.

The code clearly states that you can't have EM loads and non-EM loads fed from the same vertical switchboard section. No exceptions. (I don't have my book here, so I can't quote it verbatum.)

Any one feeder is clearly fed from one vertical switchboard section. So if a single feeder comes from a switchboard, and supplies EM & non-EM loads, it is a violation.

The only feeder that can supply both EM & non-EM loads is a single one from the generator to the main switchboard. That is allowed both by the exception to 5(b), and by the fact that the feeder is supplied directly from a generator or a main breaker. It's not supplied by a switchboard.