Installation of Auto Transfer switches

[timothy.collins]

The Situation:

A facility wishes to have a emergency generator installed (1.1Mg). The installation also includes two Auto Transfer switches. The facility is fed with two large transformers, each from a different riser pole. These transformers feed into a mechanical room to an A bus and B bus with a Tie Breaker. This allows for the building load to be carried by either transformer (max 2000amps) This facility currently has no backup capability ie., Generator. From the A & B breakers, power is then distributed to multiple distribution panels with no reasonable access to break into the load side of the A & B bus to place in the two transfer switches. However, with removal and relocation of the transformers the switches can be placed over the load conductors conduits going into the building, allowing for the automatic transfer of failed power from either transformer. Are there any provisions in the NEC that would allow this installation. Under artical 230-82 it lists equipment permitted ahead (supply side) of the service disconnect. However, in the handbook it does include this statement: Systems such as emergency, fire alarms, fire pumps, standby power and sprinkler systems are permitted to be connected ahead of the normal service disconnecting means if such systems are provided with a seperate disconnecting means and overcurrent protection. Please advise if this applies to my situation... Thx for your time

[ February 27, 2003, 03:52 PM: Message edited by: timothy.collins ]

 

[tom baker]

Re: Installation of Auto Transfer switches

First off, is this an article 700, 701 or 702 applicaiton? It makes a huge difference on the rules.

 

[paul]

Re: Installation of Auto Transfer switches

There are ATS's that are rated for service or even a service disconnect. The way you describe what you want to do is very plausible. Just get the right ATS.

 

[Nick]

Re: Installation of Auto Transfer switches

This would have to be an article 702 installation. You can?t put the entire building on article 700 & 701 covered installations without multiple transfer switches. I say this because Tim says the owner wishes to have the back up. Not the AHJ. The building is existing so it must already meet life safety codes without the generator. I could be wrong.

There are a couple of issues here. Let?s assume it is an article 702 installation.
Paul is right about the service rated transfer switches. What you need to look at is the fact that they will be located outside. Article 230.91 requires the Service equipment overcurrent protection to be located as an integral part of the service equipment or located immediately adjacent thereto. Your overcurrent protection is located inside at your existing services. Maybe the service rated ATS? are available with OC protection. I am not sure. The other issue here is grounding. You already have a grounding electrode system at the building mains. You will have to have a GES at the service rated ATS? now too. Will you have to remove the main bonding jumpers in the building, or treat it like a separate building from the new service transfer switches? Something to look at.

Is the facility primary metered? I don?t think the owner would want to pay the utility for power he is generating himself!

Another big one is ground fault protection. Based on the loads you describe I assume you have a 480/277V system? Article 702 does not relive you from having equipment ground fault protection at the generator. A 1 Meg genset is going to have about a 1600A breaker. The ground fault system in the generator is going to have to be designed to work with the existing ground fault system in the building.(and the ats? if they have OC protection integral to them) They are going to have to talk, even if you have a 4 pole ats (which you will have to have because of the GFP at the generator) they are going to conflict due to the location in the circuit you are connecting the generator feed. Seeing how you have a tie beaker set up you should be part way there already. The existing building should have an elaborate system already to make that set up work. It can be done. I have done it. But it is going to take compatible components and an engineer that understands ground fault protection circuitry to design it.

I think it will work, it just sounds expensive and a lot of design will have to go into it. To tell you the truth, if the place is primary metered I would bet it would be cheaper to install one ATS on the primary side and a medium voltage genset and be done with it.
Nick

 

[ron]

Re: Installation of Auto Transfer switches

Regarding the ground fault protection comment:
Article 230.95 indicates to provide the GFI "for each service disconnect ...". The generator output breaker is not a service disconnect. Where is GFI required on generator output in the code?

 

[Nick]

Re: Installation of Auto Transfer switches

215.10
The load side of a generator is a feeder and subject to this section.
You don?t see a lot of GFP on large feeder breakers due to exception 3. However, exception 3 does not apply to the first overcurrent device on a generator for obvious reasons.
By 90.3 the provisions of chapters 1, 2, 3 and 4 apply to chapter 7 unless specifically amended by chapter 7. There is nothing in article 702 relieving you of the ground fault protection requirement of 215.10. Article 700.26 and 701.17 specifically amend the GFP requirement in those chapters.
Nick

 

[hbendillo]

Re: Installation of Auto Transfer switches

Do you need to have the entire facility connected to the generator? Could you possibly put a couple or three automatic transfers switches on the load side of the switchboard and feed a few panelboards that would keep your critical loads running? You would have to have a planned shutdown to "insert" the transfer switches in the feeders for these panelboards but that is usually possible. What is your service voltage?