The building which contains our offices and shops is served by a 1200 amp 120/208 wye service. We are going to install a 300 KW standby generator to pick up the entire building. 2 years of demand and KW readings verifies our load doesn't exceed 210 KW (about 580 amps) so the 300 KW generator should be fine. My plan is to install an 800 amp service rated fused disconnect between the utility transformer and the ATS switch. The generator also has an 800 amp overcurrent device which will feed the other side of the transfer switch. I am planning on paralleling 3-300 MCM Aluminum conductors from the generator and transformer to the ATS as well as from the ATS to the building service. The one issue that I wonder about is the 1200 amp disconnect on the building service (which I plan to convert to 4 wire). Do you see any issue with the existing OC device remaining at 1200 amp? Although leaving the 1200 in place might be confusing, it appears all conductors and equipment will be adequately protected. Changing out this OC device would probably be around $5000 so I am trying to avoid that. Any thoughts?
standby power for entire building
- Thread starter NSTech
- Start date
- Status
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
No the 1200 now becomes just a switch.
3?300AL for a 800A service? How did you come up with that?
- Location
- Massachusetts
I see it like Chris.
3 sets of 300 AL is 690 amps, that would require a 700 amp breaker or fuse.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Why, OP states 800A fusible disco ahead of the 1200?
I misread... (read that the 800 was after the 1200)...
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I think 445.13 would require at least 600 kcmil @75? or 750 kcmil at 60? terminations for 3 parallel Aluminum. conductors per phase?
300,000 kva/208/1.732*1.15=957.65/3=319.21 amps per conductor.
Or am I getting this wrong?
300,000 kva/208/1.732*1.15=957.65/3=319.21 amps per conductor.
Or am I getting this wrong?
Last edited:
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
ceb58
Senior Member
- Location
- Raeford, NC
I think this may also be an area of concern. Do you have peak load demand numbers? Supplying a building that size must have several large motors for A/C and such. The starting capacity would need to be taken into account.
- Location
- Massachusetts
I agree with Chris, we are already past the first overcurrent device so 445.13 is not an issue.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I missed read the OP, thinking he said he was installing a 800 amp disconnect at the ATS for the generator feed. Whoops:roll:
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Leestarkweather
Member
- Location
- Los Angeles CA
(A) I'd leave the Utility to Main CB alone--someday you (or a future owner) might want the full 1200A capacity. Instead, install the new 800A OCPD after the 1200A and before the ATS. No utility disconnection or reconnection need to be scheduled. Feeder from the 1200A to the 800A can be sized at 800A (no less than the full 800A) if it's 25 feet or less. You should be using 300KCMIL CU, not aluminum, to get the 800A rating.
(B) Definitely check the starting currents of all motors in the building. If all AC's are 5 Ton or smaller compressors (or multi-stage staggered start), and no other large motors are present, you should be OK with a 300KW genset. But look out for bigger motors--you may be suprised at what a 7.5 or 10HP motor will do (at 208V) to destabilize your genset at startup. Also, remember that motors running at time of loss of power will start back up simultaneously upon the generator making power available (you can add timed delay relays to stagger the starting of larger motors). Also, buy the best (electronic) governor and voltage regulator offered by the genset mfg.
(C) If not sure, wire up the new ATS and cable in a rental 300KW genset and do an outage on the hottest day of the year (right about now). $$$ but less expensive that upsizing later.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
That happens.
That never happens. I'm going out to get a Lotto ticket.
- Location
- Massachusetts
Great point.
There are so many good reasons to do that. Along with future and shutdown issues you pointed out it would do away with having isolate neutrals in that switch as well.
Other than lack of space I can see no reason not to leave it if it is in decent shape.
ceb58
Senior Member
- Location
- Raeford, NC
Yes, Chris I agree that it allowable per 220.87. However you know as well as I 220.87 is a code minimum requirement. I think if it were you or I putting it on the line to size a gen. for a customer we would want to see a peak demand load to cover all bases. Cost nothing or very little to see the information now than find out the hard way later. Stay Cool
standby power for entire building.....
standby power for entire building.....
Thanks for the input.
I'm used to looking down the 75 degree copper column but because of the crazy pricing I ventured into the AL table in 310.16 and got offset. I see that sets of 300 is only good for 690 amps.
The demand of around 210 KW that I mentioned peaks in the winter when we have sequenced electric boilers heating coils in the floor with fractional HP pumps pushing the liquid around(Minnesota). Summer time cooling load is pretty diversified with several cooling units.
I was mostly worried about that 1200 amp switch and was curious about other's view of a 1200 A switch downstream from an 800 prior to when the inspector comes by and has a look at it.
What I didn't mention is the 30 HP fire pump that requires its own fire pump rated transfer switch, non typical overcurrent/motor running requirments and different wiring/location methods. But I think I have the fire pump stuff under control.
standby power for entire building.....
Thanks for the input.
I'm used to looking down the 75 degree copper column but because of the crazy pricing I ventured into the AL table in 310.16 and got offset. I see that sets of 300 is only good for 690 amps.
The demand of around 210 KW that I mentioned peaks in the winter when we have sequenced electric boilers heating coils in the floor with fractional HP pumps pushing the liquid around(Minnesota). Summer time cooling load is pretty diversified with several cooling units.
I was mostly worried about that 1200 amp switch and was curious about other's view of a 1200 A switch downstream from an 800 prior to when the inspector comes by and has a look at it.
What I didn't mention is the 30 HP fire pump that requires its own fire pump rated transfer switch, non typical overcurrent/motor running requirments and different wiring/location methods. But I think I have the fire pump stuff under control.
- Status