Have a facility in the middle of nowhere, literally, fed by genset only. Two 600kW gensets in an N+1 setup, only run one at a time. Each genset connects to a manual transfer switch with a single feeder to an MDP. Run one genset one period, manually shift, run second set the next period. My thoughts were bonding grounded to grounding in the MDP, with EGC run back to each genset, ensuring there is no N-G connection in either genset, and calling it a day. Each genset has an output CB. What am I missing? This would require 4P transfer switch to isolate neutral in offline genset
Separately derived system with multiple gensets
- Thread starter Sdr96
- Start date
- Status
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
If you have have no n-g connection at the gensets, then 3 pole transfer switch is the way to go.
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
If the two gensets are sharing a common neutral (3P transfer switch ) the system would not be considered separately derived.
mcelweec@gmail.com
CMAC1
- Location
- NYC
- Occupation
- Electrician
is there utility power?
From the OP: "Have a facility in the middle of nowhere, literally, fed by genset only."
Sdr, The n/g bond has to be made at the first disconnecting means ( gen output cb ) of
both gen sets, 4p TS would be required to open n/g loop. GE at each gen set.
Frank, can you call the gen sets seperately derived when the transfer switch has a solid neutral bar.
Rick
mcelweec@gmail.com
CMAC1
- Location
- NYC
- Occupation
- Electrician
apparently 4p ts's reek havoc with ups equipment,so if 3p ts is rated as service equipment and containing the main service switch you could bond there.otherwise you could treat one genset as an sds and the other non sds, performing your bonding in the sds and isolating n-g in the non sds genset.
mcelweec@gmail.com
CMAC1
- Location
- NYC
- Occupation
- Electrician
please disregard last sentence,it makes no sense to me either.
Actually that is an option that could be used as well, but can the jumper be installed in the MDP down stream of the transfer switch. I don't see where the equipment bonding jumper has to be continuous.
I think 250.35(B) and (B)2 could be applied to this setup.
Rick,
If all the neutrals land on the same bar in the TS, it is not seperately derived. That is why
I said a 4p TS would be needed?
Rick,
The N/G bond of each gen set is required to be made in the first disconnecting means, that
is the c.b. at the generator(s). Ok, tell me how you can use 3p TS without regrounding the
neutrals. Also, each gen set will need a grounding electrode system.
mcelweec@gmail.com
CMAC1
- Location
- NYC
- Occupation
- Electrician
neutrals are grounded at one point only,either one of the gensets or at the main building switch,with a 3 pole ts there is only one path back to "ground",whereas with a 4p there is 2 which ultimately end up at the same place,but apparently ups's are sensitive to this.
Last edited:
It is required but only for SDS's. What i was saying is you can't call the two gen sets seperately derived because each generator is tied into another system (the other generator)and each of the two systems have a common tie via the solid netral bar.
(Note;see definitions SDS)
mcelweec@gmail.com
CMAC1
- Location
- NYC
- Occupation
- Electrician
benaround if you have the handbook p204 is what i'm talking about,otherwise section 250.20(d),250.30,250.35a.
I've always found this situation quite perplexing. The code covers grounded service-supplied and separately-derived ac systems, but lacks the specifics for non-separately-derived source not interconnected to a service-supplied ac system.
Smart, Rick,
I'd like your opinion of 250.20(D), in particular the new text added in 2008 NEC. Oddly
it's under (D) Seperately Derived Systems, but, IMO, it addresses this situation by stating
( in short ) that to be a non-SDS the grounded conductor of the on-site Gen.(s) are to be
solidly connected to the "service grounded conductor " or it is a " derived system " and
grounded per 250.30(A), which is Grounding SDa/cSystems. There is no service at this site
, so no grounded service conductor, therefore must be treated as a SDS. Opinions?
- Location
- Massachusetts
I have no idea what you mean, 4 pole ATSs are very common with UPSs and work fine.
Frank i agree with you based on 250.20(D) because it doesn't give any other choices or options to make if it's not solidily connected to "a service supplied grounded conductor". It definitely says shall be grounded in accordance with 250.30(A).
Rick
- Status