Article-517

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anbm

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We have a main emergency distribution panel 'A' fed out of an emergency generator.

Distribution panel 'A' feeds existing panel 'B'. Panel 'B' feeds (4) ATSs (critical, life-safety and two equipment branch ATSs).

Is it legal to feed above ATSs from a panel downstream of main emergency panel?
 
Is the maximum demand 150 KVA or less?

Roger
 
The load is more than 150kVA, this is not a good design I think, in emergency mode (generator runs)... because if we loose the breaker that feeds the panel, we loose all ATSs, but I am not sure if it still comply with NEC or other code rules.
 
anbm said:
The load is more than 150kVA, this is not a good design I think, in emergency mode (generator runs)... because if we loose the breaker that feeds the panel, we loose all ATSs, but I am not sure if it still comply with NEC or other code rules.
Click to expand...
The answer is in 517.30 (B) and since the load is more than 150 KVA, there is no question that the design is no good.

Roger
 
I am not sure I understand your answer. Since the max demand load is more than 150kVA which meant we can have more than one transfer switch to serve each emergency load as defined per code, we have (4) transfer switches here.

The question is whether we can feed all above (4) ATSs from a single panel and this panel is not the main emergency panel (I call the main emergency panel is the first panel downstream of E.G).

ATS are also fed out of normal power source.
 
IMO, there are to many chances for failure with numerous panels upstream of the ATS's. When i was mentioning the 150 KVA or less it was only because the requirements are less stringent.

In reality, I don't think there is anything actually prohibiting multiple OCPD's upstream of the ATS's regardless of how many panels they are in.

Roger
 
dkidd said:
Have a look at

http://static.schneider-electric.us/docs/Electrical%20Distribution/0110DB0901.pdf

Also, you need to make sure you have selective coordination of overcurrent devices.
Click to expand...


That references the 2008 NEC. The 2005 was very different - no explicit requirement to separate the emergency supply breakers although different people had different ideas on what exactly what was required.

I do agree that selective coordination would be required for the "A" and "B" panels.

But I actually like to put the ATS's closer to the actual load (both physically and electrically) - that means there is less chance for a single breaker to trip on the load side of the ATS. On the load side of the ATS, if a breaker trips it takes out both the normal and emergency power.

Same idea if a feeder is damaged by fire or something else. The closer the ATS is to the load, the less chance the common feeder from the ATS to the load is going to be damaged.

It can be pretty embarassing to have one breaker on the load side of an ATS take out the entire critical branch when the utility power is still running.

But putting the ATS's closer to the load often means more circuit breakers in series on the emergency and normal sides of the ATS.

So I don't see anything wrong with the setup you described.
 
I believe the addition of 700.10(B)(5)(d) in the 2011 code now specifically allows this. Under the 2008 code I am not so sure.
 
steve66 said:
The 2005 was very different -
Click to expand...

Where did 2005 come up in the post?

The ROP that clarified this issue for the 2008 revision made it clear that the ATS feeders for the different emergency, legally required or optional loads could originate in switchgear, but not panelboards.

Panel Meeting Action: Accept in Principle
Revise existing section 700.9(B) by adding a new Number (5) to (B) to read as
follows:
(5) Wiring from an emergency source shall be permitted to supply any
combination of emergency, legally required, or optional loads in accordance
with (a), (b) and (c).
(a)From separate sections of a vertical switchboard, with or without a
common bus, or from individual disconnects mounted in separate enclosures.
(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.
(c)Legally required and optional standby circuits shall not originate from the
same vertical switchboard section, panelboard enclosure or individual
disconnect enclosure as emergency circuits.
Exception to (5) (b). 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.

Emphasis added

Panel Statement: The panel has placed revised text in a new list Item (5) in
Section 700.9 (B) to clearly identify that the original separation requirements
from the source to the loads or from the source distribution overcurrent
protection to the loads is to remain unless modified by (1) ? (5).
The revised text will further clarify that it is permitted to supply any
combination of emergency, legally required or optional loads from a single
feeder or from multiple feeders or from separate vertical sections of a
switchboard that are supplied by either a common bus or individually.
The use of an overcurrent protective device at the source or for the equipment
is a matter of reliability and design. While the requirements in (5) (b) maintain
the highest degree of reliability, the exception to (5) (b) will permit the use of
an overcurrent device at the source or for the equipment. The coordination of
the overcurrent protection at the source or for the equipment with the
downstream overcurrent protection requirement in the exception will maintain
the highest degree of reliability possible while allowing protection for
conductors and equipment. The revised text in the main paragraph should also
make it clear that circuits supplying emergency loads are not to be combined in
panelboard enclosures with circuits supplying other loads.
 
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