maybe this is too hard to answer without lots of details, but i'll try anyway, might need an ebngineer even.

i'm looking at a job that replaces a 3-phase 800a main switchboard that has 3 200a panels downstream, one feeds a 200kva transformer that feeds 4 more downstream panels and one of these 4 is downstream of a transfer switch.

the engineers plans puts a 225a 3-phase main breaker ahead of everything. then theres a transfer switch to a 75kva generator one way, and the other way goes to all of the others (the 800a main switchboard with the 3 panels, one of which feeds the 200kva transformer which now feeds 5 downstream panels)

how can a 225a main breaker with four 4/0 thwn coppers feed all of the panels that used to be serviced by a 800a switchboard?

this is what i think. tell me if i'm thinking wrong. the new 225a main breaker is only feeding 3 panels so the load may calculate out to fit within that 225a feeder. the one that feeds the step-up transformer now takes the increased loads downstream so those loads dont need to be calculated with the main service feeder to the first set of 3 panels. you just recalculate the feeder downstream from the transformer for those panels.

ok you can wake up now, i'm done.

I can't quite visualize the system you described.
Is there any chance you could post a diagram?

Ed

Click pictures thumbnail to see full size.

<IMG SRC=\"http://g0atnet.com/misc_images/cw/firesta1a.jpg\">

<IMG SRC=\"http://g0atnet.com/misc_images/cw/firesta2a.jpg\">

It would be difficult to calculate the load for this service. There must be some incredibly low demand factors (or under loaded) on the distibution circuit breakers. I'm glad you posted the drawing, since I thought from your original description that the 800A main board was being repalced, which it isn't.
Unfortunately this design is faulty, in that the circuit breakers can never be coordinated for a distribution breaker to trip before the main. The main breaker is smaller than two of the three distribution breakers. The other thing to be sure is, that there is some type of shunt trip activation on some of the dirstibution to accomodate the smaller generator breaker as compared to the main breaker.

[This message has been edited by Ron (edited 01-24-2003).]

The designer must have used 220.35 (measuring maximum demand) to determine the service size. I don't think you could verify the service size based on the information on the drawing.

Cindy,

Man, I just love those Thumbnail Links!!!
I know that you explained how to create them before (in some old threads about ... who knows what we were all talking about then!), but could you E-mail me with a little info???

Anyhow, to the subject at hand.

Took a look at the plans - the Single Line Diagrams in Image "firesta1" aren't too bad (IMHO!).

Apparently the EE has figured no more than 75 KW will be needed in the event of Power Failure from the PoCo.
The Apparent Power rating of the Genny is 96 KVA, but that is only Apparent Power. The True Power (KW) is what the complete Gen Set can deliver within a given KVA / inside a given Power Factor.

Not sure as to how this figure of 75 KW was obtained, but most likely it would be the highest level of power needed to keep "Crucial" stuff running - such as egress lighting, F/L/S systems, certain machines which will cause some major problems if they are not stopped in a certain sequence, any servers which need to be powered down properly, and so on.

You might be able to find out what items are crucial loads by "Diving Into" the complete planset + Project's Specs Manual.

Simply, the Genny may be sized according to what loads are needed or desired when input power from the PoCo is lossed.
It could be sized Liberally - making it possible to run as much stuff anyone wanted during a power loss event (everyone runs around and turns all the lights on, or fire up all the machinery!).
Or it could be sized Conservatively - with intentions to drive the Necessary loads as figured to be crucially needed.

Even though the Genny is rated for 75 KW, the crucial loads might only total up to no more than 50 KW (or even 50 KVA). This would be a good planning idea for the Genny, as to not load it up to 100% Capacity for extended periods of time and allowing for some load diversity / variable load fluctuations.

I noticed there is a 150 Amp breaker between the Genny and the ATS. This is to protect the feeder between the Genny and the ATS in the event of a Ground Fault or an L-L / L-L-L Fault, not to limit the Genny's output.
To trip the 150 Amp breaker, the Genny would need to crank out >125 KW (or >125 KVA needs to pass through the breaker - which ever comes first).

The Prime Mover (in this case, the Diesel Motor) is rated for " × " horsepower, so once the demand of True Power (Wattage) starts to exceed the HP of the Prime Mover, the Prime Mover gets overloaded - resulting in a drammatic loss in speed.
This continues until one of three things happens:

1: Newly obtained speed results in reduced output True Power to connected equipment, which allows the Prime Mover to continue producing HP (resulting in horrible things on the "Electric Side"!!!),

2: The excessive loads are removed and Prime Mover returns to Full Governed Synchronous Speed,

or,

3: The Prime Mover stalls - eliminating the conserns for #1 and #2 above

I hope this makes sense and explains why the Genny seems to be small.

Make sure the EE has the crucial load requirement figured correctly before accepting my reasoning as to why the 75 KW Gen Set should be sufficient!!!

Otherwise you must bomb me with messages!

Seriously though, see if you can find out what equipment is considered to be crucial (or active 24/7 and uninterrupted), then you can determine how much back-up KW is needed as a minimum.

Scott35 S.E.T.

Cindy,
How are you planning to land 2 350kcmil per phase on a 225 amp breaker?
Don

Bet a nickel the genset is gonna’ be re-used. For the plain-vanilla NEMA-standard 12-lead stator, the coil pairs are connected in parallel for 208Y, {120V/coil} and in series for 480Y with the voltage regulator jacked up a bit to 139V/coil {corresponding to 277V ø-n.}





[This message has been edited by Bjarney (edited 01-24-2003).]

rescapt has an excellent point—given a typical “firehouse” 250A-frame molded-case breaker here out west, the 2x350kcmil/ø callout looks like trouble. An example might be www.cutler-hammer.eaton.com/unsecure/cms1/SD29120J.PDF pg.7 allows 1x350kcmil cable/ø, and 2AWG maximum for the stock multiconductor terminal. [That may be slightly complicated in that they’re service-entrance conductors?] It is possible details in drawing “Note 5” cover this, but I'd be asking the local engineering consultant who did the drawings. Cut-in may have been sorta’ glazed over for the bidding contractor(s) to work out at their own expense and coordination effort.




[This message has been edited by Bjarney (edited 01-24-2003).]

Oooops.

[This message has been edited by electric-ed (edited 01-24-2003).]

thank you for your thoughts on this, i'll be looking at the calcs and job site this next week.

but from the 1-line it looks wrong that the 800a s-board that was fed with 8-350's is now going to be fed from a 225a main breaker
for the whole system and 4/0 coppers to feed 3 panels rated 300a, 300a, and 200a. how can the designer do that? [not to mention the size issue of landing them/don's question]

I would look very closely at the load calcs. Also check with the poco and see what the historical load/month has been. This may have been over built for a future expansion that did not occur. I suspect that those areas will most likely have the answers-if the designer did not screw up his/her load calcs.
Sometimes owners want to do things as cheaply as possible an try to hard to save money. If designers cannot or will not tell- them sorry that will not work- you get drawings that are questionable.
Good luck on this job

[This message has been edited by nesparky (edited 01-25-2003).]

It is a code violation to have a 225A main serving a 300A distribution breaker, since they can never be coordinated.

>>>It is a code violation to have a 225A main serving a 300A distribution breaker, since they can never be coordinated.<<<

i know i'm going to sound stupid, but where does it say that? 240.12 tlaks about coordination for orderly shutdown, 110.10 says to cordinate to rpotect equipment. but i dont see why a smaller ocpd cant protect larger ocpd's or equipment. inconvenient to the user, but is it a violation? thanks

Ron
What code section requires OCPD coordination for general occupancies?
Don