Hospital (150kVA or less) single transfer switch separation requirements

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Pharon

Pharon

Senior Member
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MA
NEC 517.31(B) states the following:

One transfer switch and downstream distribution system shall
be permitted to serve one or more branches in a facility with a
maximum demand on the essential electrical system of 150 kVA.
Click to expand...

And NEC 517.31(C)(1) states this:

Separation from Other Circuits The life safety branch
and critical branch of the essential electrical system shall be
kept entirely independent of all other wiring and equipment and
shall not enter the same raceways, boxes, or cabinets with each
other or other wiring....

Wiring of the life safety branch and the critical branch shall
be permitted to occupy the same raceways, boxes, or cabinets of
other circuits not part of the branch where such wiring complies
with one of the following:
(1) Is in transfer equipment enclosures
(2) Is in exit or emergency luminaires supplied from two
sources
(3) Is in a common junction box attached to exit or emergency
luminaires supplied from two sources
(4) Is for two or more circuits supplied from the same branch
and same transfer switch
The wiring of the equipment branch shall be permitted to
occupy the same raceways, boxes, or cabinets of other circuits
that are not part of the essential electrical system.
Click to expand...

The way that sounds to me is that if your EES is 150kVA or less, you can have one transfer switch to feed the three different branches (LS, CB and EB), but with the separation of circuit limitations, you can't actually connect the EB to the load side of the transfer switch -- not without violating 517.31(C)(1). There is an exception to allow combination between LS and CB "in transfer equipment enclosures," but this exception does not include EB.

So my question is this -- where are you legally allowed to tie in your EB wiring/feeder on this type of system? The Informational Note Figure 517.31(b) shows it diagrammatically, but the associated text does not seem to allow it this way.
 
I've run into a situation recently where the AAAHC is retroactively enforcing single transfer switch installations as violations of 517.31(C)(1). {Technically the enforcement is a violation of 517.30(C)(1). This is because the 2011 NEC is referenced by the 2012 NFPA 99. Since July of last year, CMS started using the 2012 Edition of NFPA 99 for healthcare facility continuous compliance life safety audits}
 
I get what you saying- and I do interpret it the same way- but I can't see why they would then include that graphic. To me the graphic is the ultimate intent as its harder to miss structure a graphic then already muddy linguistics.
 
Pharon said:
The way that sounds to me is that if your EES is 150kVA or less, you can have one transfer switch to feed the three different branches (LS, CB and EB), but with the separation of circuit limitations, you can't actually connect the EB to the load side of the transfer switch -- not without violating 517.31(C)(1). There is an exception to allow combination between LS and CB "in transfer equipment enclosures," but this exception does not include EB.

So my question is this -- where are you legally allowed to tie in your EB wiring/feeder on this type of system? The Informational Note Figure 517.31(b) shows it diagrammatically, but the associated text does not seem to allow it this way.
Click to expand...

Your are allowed to tie the equipment branch together with the life safety and critical branches IN the transfer equipment enclosure, per 517.31(C)(1).

You quoted the relevant text, but appear to be misinterpreting it...

Wiring of the life safety branch and the critical branch shall
be permitted to occupy the same raceways, boxes, or cabinets of
other circuits not part of the branch where such wiring complies
with one of the following:
(1) Is in transfer equipment enclosures
Click to expand...
 
Wow, it appears you are correct - not sure how I missed that the first 700 times that I read it. Thanks so much. :)
 
david luchini said:
Your are allowed to tie the equipment branch together with the life safety and critical branches IN the transfer equipment enclosure, per 517.31(C)(1).

You quoted the relevant text, but appear to be misinterpreting it...
Click to expand...

This interpretation appears correct however, there still are other conflicting interpretations of single transfer switch system architecture. For example;

In 2014 517.30 (B) was modified to read: "One transfer switch and downstream distribution system shall be permitted to serve one or more branches in a facility with a maximum demand on the essential electrical system of 150 kVA."

The added words "and downstream distribution system" appears to imply that a single downstream distribution system is allowed serve all three branches. (as shown in the Informational note figure 517.30 No.2)

Additionally, the commentary included in the 2012 edition of NFPA 99 HB under section 6.4.2.2.1.4 (B) One transfer switch shall be permitted to serve one or more branches in a facility with a continuous load on the switch of 150 kVA (120 kW) or less.

reads as follows:

"Another frequent question regarding the 150 kVA (120 kW) and smaller facilities is whether the systems need to be segregated downstream of the single transfer switch. The implication of having a single transfer switch is that all loads served by that transfer switch can be considered to be part of a single system, and, thus, need not be segregated."​

Handbook commentary or explanatory material is not Code however, it is vetted by NFPA staff prior to publication so one would think that it should be in agreement with the intent of the TC.
 
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