Backing up 480V to building with generator
- Thread starter ddderek
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chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
The neutral current from your 227 loads needs a path back to the source. (Generator)
Curious how you would do that?
Curious how you would do that?
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
The generator is almost definetly wye output, so bring the neutral out with you to the transfer equipment for the L-N loads.
If there are no L-N loads, then you can leave it at the first disconnect (be sure to bond it (N-G) if the transfer equipment is four pole).
If there are no L-N loads, then you can leave it at the first disconnect (be sure to bond it (N-G) if the transfer equipment is four pole).
In the situation where you may be feeding a service transformer from a generator feed you would establish the neutral there and in the back of my mind I was comparing it to a single phase generator feed in a residential situation where the neutral would be established at the house...
kwired
Electron manager
- Location
- NE Nebraska
How are you ensuring that there is no power accidently fed back into the utility?
A transfer switch or some kind of interlock preventing both sources from being connected together needs to be used.
We were looking at feeding the switch gear direct while opening and locking out the main that connects to the system transformer.
The building is a rental so we were looking at possible back up scenarios without investing in a transfer switch application. We have a lot of refrigeration equipment for food storage and upon power loss the management was requesting the feasibility of bringing in rental generators and hooking them up to preserve stock. Being that it is a rental and they are looking at building a new facility, they do not want to invest in the obvious best choice of permanant stand by generators and automatic transfer switches. We have the question posed to an electrical engineer but we were just trying to brainstorm feasible possibilities in the mean time...
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
You have reviewed and are confident your proposed install will meet the requirements of 702.6 exception?
I had not, and if I read that correctly it would appear that a transfer switch is required even for a tempoary portable generator to hook up to the building?
Do they make manual transfer switches with cam lok bulk head adapters to feed the gen side? Or would that be a fabrication issue we would have to look into?
?Exception: Temporary connection of a portable generator without transfer equipment shall be permitted where conditions of maintenance and supervision ensure that only qualified persons service the installation and where the normal supply is physically isolated by a lockable disconnecting means or by disconnection of the normal supply conductors.?
Yes, now that I have found the exception, this would allow us to operate without a transfer switch, as we would be locking out the utility transformer main breaker and on stand by the entire time the building was running on generator...
kwired
Electron manager
- Location
- NE Nebraska
The thing that may get you in trouble is how does your AHJ interpret "where conditions of maintenance and supervision ensure that only qualified persons service the installation"
We do have a good working relationship with our local inspectors and have already discussed that if we need to go on generator back up, we would have at least one electrician on site 24 - 7 while the building was being fed that way.
The other interesting scenario we need to address is the buildings fire pump.
The fire pump is fed on the LINE side of the main switchgear disconnect, so could NOT be fed temporarily without a transfer switch. So our inquiries are to have two generators one for the building gear and a seperate one for the fire pump. Then you get into the fact that since it is fed on the line side of the gear it is fed at a far greater ampacity than it draws. And it needs to be fed at full locked rotor current. That is quite substantial even though the conductors feeding it currently are not sized for that from the load side of the pressure rated switch feeding it. This is why we are truly relying on a engineer to decipher this to assure calculations and protocol is followed....
Anyone have any experience backing up a fire pump? I know I need to read up on the code sections but have not gotten through it all yet.
kwired
Electron manager
- Location
- NE Nebraska
I haven't read that section in a while but if I recall correctly the conductors only need sized according to full load current (x 1.25) but the generator needs to b e capable of supplying locked rotor current indefinately.
We have also gone back and forth over neutral bonding and I am not quite sure I am clear on the code appropriate requirements when using generator back up.
Situation: 480V 3ph 4wire Gen feeding switchgear... main opened, locked out... neutrals brought in from gen connected to neutral bus... does the bond to ground have to be lifted in the gear and established at the generator?
When inquireing, our local "go to answer man" told us that the bond should be left in place at the gear and that the connection to that is through the ground going to the generator... hence no bonding at the Gen. Reading an article from E.C. magazine tonight I seem to be reading that it depends on whether you are calling the Gen a seperately Derived system or not? When or why would the Gen NOT be a seperately derived system??? I would think it always is?
The article then said if the Gen is considered a seperately derived system the bond should be at the Gen... if not, the bond is at the gear.... I am missing something in understanding this explanation.... Can anyone clarify?
And along the same lines... When you are dealing with these large truck mounted Generators, do they have an internal ability to bond or not bond the neutral to ground on them? Or is this something you need to do or can do with cableing when deemed necessary?
Situation: 480V 3ph 4wire Gen feeding switchgear... main opened, locked out... neutrals brought in from gen connected to neutral bus... does the bond to ground have to be lifted in the gear and established at the generator?
When inquireing, our local "go to answer man" told us that the bond should be left in place at the gear and that the connection to that is through the ground going to the generator... hence no bonding at the Gen. Reading an article from E.C. magazine tonight I seem to be reading that it depends on whether you are calling the Gen a seperately Derived system or not? When or why would the Gen NOT be a seperately derived system??? I would think it always is?
The article then said if the Gen is considered a seperately derived system the bond should be at the Gen... if not, the bond is at the gear.... I am missing something in understanding this explanation.... Can anyone clarify?
And along the same lines... When you are dealing with these large truck mounted Generators, do they have an internal ability to bond or not bond the neutral to ground on them? Or is this something you need to do or can do with cableing when deemed necessary?
This would make sense with how it is currently wired, because it is fed to the line side of the switch with 5 sets of 500 KCMIL yet from the load side of the switch to the pump with 2 sets of 250 KCMIL...
Soooo, to be done correctly, we would need to install a transfer switch and install Gen feed to mirror the Utility feed.... we couldn't get away with going from a gen directly to the pump with the load side rated feed? As code would still require the Gen feed to the pressure rated switch at locked rotor current... In other words we could NOT, for the back up purposes, get away with only feeding it to the pump controller and avoiding the pressure rated switch entirely?
kwired
Electron manager
- Location
- NE Nebraska
A generator is considered a separately derived system if the transfer equipment also transfers the neutral. If both utility neutral and generator neutral are connected together it is not a separately derived system.
Generator feed to fire pump would not necessarily need to match service. The conductors only need sized to 125% full load current of motor. 695.3(B)(1) only says the generator needs to be of sufficient capacity to allow normal starting and running of the motor(s) driving the fire pump(s) while supplying all other simultaneously operated load. Load shedding is permitted to comply with this. Overcurrent devices need to be able to carry locked rotor current indefinately.
Generator feed to fire pump would not necessarily need to match service. The conductors only need sized to 125% full load current of motor. 695.3(B)(1) only says the generator needs to be of sufficient capacity to allow normal starting and running of the motor(s) driving the fire pump(s) while supplying all other simultaneously operated load. Load shedding is permitted to comply with this. Overcurrent devices need to be able to carry locked rotor current indefinately.
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