Engineering Reasoning ??

[texassparky]

Just wondering if some one can enlighten me to some pros and cons that I might not be thinking of regarding an engineers reasoning for the following design. Our on staff engineer prides himself on being overly technical in his answers when explaining his designs, so before I go discuss it with him thought maybe someone here could shed a little light on it so I will know what he is talking about when I do go talk to him.

In a health care facility - design is to use new Square D 84 ckt panels. From a design stand point it saved a lot of wall space in the electrical rooms, from an install view I now have to get conduits into what used to be 40" of panel space now into 20" of panel space.

Second - spec is no more than 6 C.C.C. / per homerun so he did me no favors in being able to combine circuits in a homerun to get them into fewer conduits increasing the problem with the use of the 84 ckt panels.

Third - the design is for use of MWBC, using a shared neutral and use 3 pole breakers. from a design point this saved a ton of white and grey wire. However in a hospital - where turning off a single circuit takes a near act of congress, now you will have to turn off 3 circuits, just doesn't seem like a good design for this type of facility to me.

Last, and maybe this should be a separate thread but, original design was for use of 15kva transformers, after completion of the coordination study, these were all changed to 30 kva ( no change in load). So I assume this has to do with fault current but what does a 30 do or have that a 15 doesn't with regard to fault current ?

any opinions are welcome please...

 

[augie47]

Just wondering if some one can enlighten me to some pros and cons that I might not be thinking of regarding an engineers reasoning for the following design. Our on staff engineer prides himself on being overly technical in his answers when explaining his designs, so before I go discuss it with him thought maybe someone here could shed a little light on it so I will know what he is talking about when I do go talk to him.

In a health care facility - design is to use new Square D 84 ckt panels. From a design stand point it saved a lot of wall space in the electrical rooms, from an install view I now have to get conduits into what used to be 40" of panel space now into 20" of panel space.

Second - spec is no more than 6 C.C.C. / per homerun so he did me no favors in being able to combine circuits in a homerun to get them into fewer conduits increasing the problem with the use of the 84 ckt panels.
...

sounds like a perfect spot for a wireway

Third - the design is for use of MWBC, using a shared neutral and use 3 pole breakers. from a design point this saved a ton of white and grey wire. However in a hospital - where turning off a single circuit takes a near act of congress, now you will have to turn off 3 circuits, just doesn't seem like a good design for this type of facility to me....

Agree..poor design

Last, and maybe this should be a separate thread but, original design was for use of 15kva transformers, after completion of the coordination study, these were all changed to 30 kva ( no change in load). So I assume this has to do with fault current but what does a 30 do or have that a 15 doesn't with regard to fault current ?

any opinions are welcome please...

I wouldn't think fault current would be an issue on either....
future load capability would have been my guess...
15 kva on a 84 ckt panel seems a bit small.

 

[steve66]

1. The engineer probably wasn't thinking about this since we have the newfound freedom to put more than 42 OCP devices in a panel.

2. Do you mean 6 CCC per conduit. That sounds kind of excessive. Healthcare facility circuits are often lightly loaded since tripped breakers can be a major issue.

3. I agree, a MWBC can be a PITA in a hospital, but sometimes budget's override everything reasonable.

4. A 30 KVA might have enough current capacity to trip a breaker in it's instantaneous region, where a 15 KVA wouldn't (at least on paper.) It sounds reasonable, but without seeing all the details, I'm a little skeptical that it provides any benefit.

 

[texassparky]

sounds like a perfect spot for a wirewayQUOTE]

Initially I would agree, but for 84ckts - after derating it will be several wireways - which in-turn defeats the whole purpose of saving the wall space, I can use it up with wireways or I can use it up with panels.

And sorry just to clarify the 15kva transformers are not serving 84 ckt panels just 42's.

 

[texassparky]

2. Do you mean 6 CCC per conduit. That sounds kind of excessive. Healthcare facility circuits are often lightly loaded since tripped breakers can be a major issue.

yes the max is 6 CCC per conduit. when you say excessive you mean to be that restrictive right ?

 

[ultramegabob]

does everything have to be fed from above, or can you run conduits under the slab and come into the bottom of the panel?

 

[charlie b]

Items two and three seem to me to be contradictory in their philosophy. Item two seems to favor separation over cost, and item three does the opposite.

As to item four, as Steve said, without studying the plans, I can only offer a guess. But I will say that if you want a downstream breaker to trip before an upstream breaker, then it is easier to make that happen when the frame size and rating of the upstream breaker is higher.

I shouldn't think that going from 15 to 30 would help the cause, however. That is because an opposing factor comes into play. The one area in which it is most difficult to get selective coordination is the instantaneous trip region. If the available fault current is high enough, then two molded case breakers in series are often going to both get the command to trip at the same time, and either one could wind up winning the race. A way to avoid this problem is to arrange for the available fault current to be low enough that neither breaker gets the "instantaneous trip" command. But upsizing the transformer will double the available fault current, so that is a step in the wrong direction.

 

[texassparky]

does everything have to be fed from above, or can you run conduits under the slab and come into the bottom of the panel?

We are design assist contractor, so as typical the building is being built and we are still designing from where the owners engineer left it. I was able to get all my feeders in underground, and a minimal amount of branch on the first floor, but we were still on 75% CD's then and we were missing a whole lot of circuitry.

The elevated slabs - I got the structural engineer to allow me to put all the feeders thru the slab but he will not let me install the branch due to the amount of concrete displacement in such a small are like these electrical rooms. Not even as a penetration thru the floor and back up.

 

[ultramegabob]

Well.... somthing has to give somwhere, you can only get 5# of doo doo in a 5# bag.....

 

[LarryFine]

sounds like a perfect spot for a wireway

Initially I would agree, but for 84ckts - after derating it will be several wireways - which in-turn defeats the whole purpose of saving the wall space, I can use it up with wireways or I can use it up with panels.

I think he means a horizontal trough placed within 24" of the top of the panel (could be one below as well) to allow nipple-length conductor count rules to the panel.

The myriad wires passing through the trough would not have anywhere enough area to be too much for the trough when measured perpendicularly to the direction the wires travel. It would act like a big funnel.


I wonder if the MWBC mention is a requirement, or merely a permission. I'd use them as long as the engineer understands the limitations that handle ties cause.

6 CCC's per conduit gets you either three 2-wire circuits or two 4-wire MWBC's. Does the OP think that's too many or too few?

 

[texassparky]

So it seems as though it is agreed my questioning the design is not out of line. I hate to question the engineer, when it is his name and his stamp on the drawings, but sometimes I see the analytical side seems to take over the common sense side.

Thanks for each of your input.

 

[texassparky]

I wonder if the MWBC mention is a requirement, or merely a permission. I'd use them as long as the engineer understands the limitations that handle ties cause.

That is how they are shown in the panel schedules, so I took it as being a requirement, and that is how the Square D rep took them off switchgear BOM.

But maybe that is how I should ask him the question... requirement or permissible ?

 

[texassparky]

6 CCC's per conduit gets you either three 2-wire circuits or two 4-wire MWBC's. Does the OP think that's too many or too few?

Understood, and again I not the one with the stamp, but IMO with consideration of the loads on these circuits I feel it is too few.

 

[steve66]

yes the max is 6 CCC per conduit. when you say excessive you mean to be that restrictive right ?

Yes, too restrictive. I probably phrased that wrong.

Can you use 1/2" conduit, or is 3/4" specified?

If it were my project, I wouldn't mind at all if you called and said something like "I'm having trouble getting enough room for all the conduits I need. What did you have in mind when you specified 6 CCC's per conduit?"

Steve

 

[texassparky]

Can you use 1/2" conduit, or is 3/4" specified?

3/4" minimum. I have asked for a variance for use of 1/2" for single circuit branch rough in, for example critical circuit outlets for the in wall rough in, but no response as of yet.

Thanks for your input.

 

[petersonra]

Who knows why anyone designs anything the way they do.

I am all but convinced that often times things are decided by the flip of a coin.

 

[glene77is]

Texas,

It is always good to have questions, and to find answwers.

"...near act of congress..." is because of patient safety, and "...now you will have to turn(ed) off 3 circuits..."

Having worked in a hospital, I question the safety for patients whose medical devices may be connected to 'common trip' MWB circuits.

In 1979, I designed a patient area, and faced a similar problem, the solution to which was to provide individual circuits, clearly marked, for devices. Problems with #1 patient could not cause problems for #2 patient.

 

[VUGear]

Second - spec is no more than 6 C.C.C. / per homerun so he did me no favors in being able to combine circuits in a homerun to get them into fewer conduits increasing the problem with the use of the 84 ckt panels.

I believe the reasoning for restricting the number of circuits in the same homerun for healthcare work is to limit the number of circuits that are affected (i.e. taken out of service at a given time) in subsequent remodel work down the road. If an area gets renovated and a couple of circuits have to be repulled, the fewer other circuits in that homerun that get disrupted the better. A lot of healthcare engineers actually limit homeruns to no more than 3 circuits for this reason.

 

[VUGear]

But I will say that if you want a downstream breaker to trip before an upstream breaker, then it is easier to make that happen when the frame size and rating of the upstream breaker is higher.

Bingo. I believe this is correct because, yes, the fault current level is not the issue but rather the characteristics of thermal-magnetic breakers. For a 15kVA transformer, the typical overcurrent protection on the primary side (25A) will never coordinate with the 20A branch breakers in the 208V panel, right? So in order to provide selective coordination as required for healthcare, engineers end up increasing 15kVA transformers to 30kVA to get larger OCP (50A) that will coordinate with the branch breakers.