emergency generator design 4

[Reesh14]

I am installing a new 50kw emergency generator for an existing office building. I am having trouble sizing the necessary wires for design. Inside the building there is an 80A emergency panel that will be wired to a 200A, 3phase, automatic transfer switch. The building uses 120/208V. The automatic transfer switch is wired to a 1200A Main Distribution panel as well. In design, I believe that the load breaker for the generator must match the size of the emergency panel, so it would be 80A/3p. I size the wire from the generator to the ATS for 80A. Do I size the wire from the ATS to the emergency panel for 80A as well? Now for the wire from the MDP to the ATS, I am not sure whether to size it for 200A or 80A? Right now I designed to have a 200A breaker in the MDP for the ATS and sized the wire for 200A. I was told that I do not need to size it for such a high amperage, so I am not sure what to do. I am sorry if this is really confusing to read, but I would appreciate any advice or recommendation of where to learn this design concept. Thank you.

 

[wareagle]

You need to determine how much load is on the emergency panel and size for that load. If the emergency is 80 amps, then the conductor from the MDP need not be much over that.
Here is a site that might assist with sizing tbe gen.

http://www.eatonelectrical.com/NASA...c=Apubarticles&cid=1016116480106&Sec=products

 

[EEJaime]

As wareagle states, the sizing depends on the loads to be served. However, the maximum your generator will support is 173.6 Amps at 0.8 pf and 208Y/120V, 3ph, 4w. If the building requires this level of backup power, then the 200A. ATS is about right and you should size your system accordingly. That is, all connections, panels, feeder breakers, feeders and so forth would be at the 200A. level, then you can sub-feed the 80A panel and have power available for the other loads. If your building will never require more than 80 Amps of emergency power then you can save some money by sizing your entire system at that level. The 200 Amp ATS will still work, you just have to make sure all the load sensors, etc...are set to the right levels. All this means is that you've bought too much generator and it will never see more than about 45% of it's capacity.

 

[fsmyth]

Wire the ATS for 200 amps. Wire the sub for 80/100 amps.
Do it right. You never know when someone will decide to
add more load.

IMHO, there is no such thing as "too much generator".
Oversizing (within reason) takes care of all sorts of
annoying little problems.
<als>

 

[waross]

Hello fsmyth;
I agree with you electrically.
Mechanically I have seen engine problems with two new sets that were oversized and had vendors representatives express serious reservations about two others. That said, I have installed quite a few sets that were oversized for motor starting duty.
Sometimes running a new diesel engine with a light load will glaze the cylinder walls. The engine will then pump its lube oil out the exhaust in a few hours. Typically the set shuts down on low oil pressure when there is not enough oil left in the crankcase.
I watch a new set for an hour or so for oil pumping. If a set starts pumping oil I set up a load bank for anywhere from a day to a week to make the engine work until the rings seat in.
I have never had to use it, but some mechanics will cure glazing by introducing a spoonful of Bon-Ami (Pumice powder) into the intake of a running engine.
A generator will not handle an over load nearly as well as a transformer. I see lots of sets running lightly loaded and once the rings seat in it's not a problem.
yours

 

[mpparent]

waross..

What you've described is called "wet-stacking". It can really become a problem. However, if you do monthly maintenance runs, ie run the gen at full load with a load bank, then it becomes less of an issue.

Mike

 

[ScottyUK]

In the UK the 'wet stacking' problem has the delightful nickname 'slobbering'. Not good news of the set has to run under full load after a long time under light load - bad cases develop into stack fires.


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