480 V & Grounded Conductor

[eager2learn]

A little confused why in some applications we use a neutral and on others we don't. For some reason I'm having a hard time understanding this. Any type of info that can be shared will be greatly appreciated. Back to basics I guess

 

[coulter]

I'm not sure what you are asking. Maybe this:

Starting with the assumption you are discussing 480/277 grounded Y:

The neutral has to be brought to at least the first disconnect. It has to be bonded and grounded at least at one point. The reason for this is so the system can supply (and return) sufficient ground fault current to reliably, quickly trip protective CBs.

The NEC may also require the neutral to be bonded and grounded in two places. But this is a legal issue - has nothing to do with physics, good design practice, or safety

Any requirement for the neutral to go beyond the first disconnect depends on if the system has any 277V loads. If it does, then the neutral must be carried out to the last 480/277V panel.

A lot of industrial installations do not carry the neutral past the first disconnect. If there are any 277V loads out in the field, a small 480D/480Y xfm is installed to feed a local panel board. The 480Y secondary neutral is bonded and grounded just like it was a service.

Your questions are good - keep asking.

carl

 

[electricalperson]

we use the neutral whenever we need a 277 volt circuit such as lighting. ive seen 480 volt services without a neutral installed because it was just feeding 3 phase machines thatdidnt require a neutral. any new 277/480 volt service i install today always have a neutral even if its not used

 

[russ]

There are also delta transformers that have no neutral.
The three phase 277/480 volt wye is not much no different than three phase 208 volt wye, except for the fact that it is a higher voltage.

Check out this thread, there are some wye and delta drawings on the second page.


http://forums.mikeholt.com/showthread.php?t=91283&highlight=transformer+pictures

 

[eager2learn]

From the main swithgear we ran 480V with no neutral that tied into X-former on primary side. From the Secondary side of X-former we ran 120/208V where we picked up the neutral on The X/O. From there we ran to a disconnect which then feeds the buildings C.B. panel. So my question is, -
Does the nuetral from the X/O initiate inside the secondary side of the coil or where does it get picked up at. I hope that this is a question that you don't look at as being dumb I'm just new at understanding X-former's

 

[electricalperson]

when you feeda 3 phase transformer you dont need to run a neutral conductor. the secondary side makes its own neutral when you ground XO. just make sure to ground XO or you will get funny voltage. XO is always the neutral on transformers. no matter if they are several hundred KVA transformers feeding a building or a little 45 KVA transformer feeding a small panel.

 

[electricalperson]

thats how your transformer is probably wired
ps there is no stupid questions. i ask questions all the time

 

[eager2learn]

So this drawing shows a delta / wye connection. so lets say we wanted to pull a neutral on the primary side - as long as we terminate on the x / o tanking it to ground that would be o.k.

 

[weressl]

I'm not sure what you are asking. Maybe this:

Starting with the assumption you are discussing 480/277 grounded Y:

The neutral has to be brought to at least the first disconnect. It has to be bonded and grounded at least at one point. The reason for this is so the system can supply (and return) sufficient ground fault current to reliably, quickly trip protective CBs.

The NEC may also require the neutral to be bonded and grounded in two places. But this is a legal issue - has nothing to do with physics, good design practice, or safety

Any requirement for the neutral to go beyond the first disconnect depends on if the system has any 277V loads. If it does, then the neutral must be carried out to the last 480/277V panel.

A lot of industrial installations do not carry the neutral past the first disconnect. If there are any 277V loads out in the field, a small 480D/480Y xfm is installed to feed a local panel board. The 480Y secondary neutral is bonded and grounded just like it was a service.

Your questions are good - keep asking.

carl

There are also resistance grounded neutrals that CAN NOT use the neutral for any loads. It's sole purpose to allow the detection of ground faults without service interruption. Its use is restricted greatly by the Code.

The resistance grounded transformers are utilized on secondary unit substations and similar large distribution systems at the 480V and on higher voltage levels.

 

[eager2learn]

Thank you very much

 

[electricalperson]

So this drawing shows a delta / wye connection. so lets say we wanted to pull a neutral on the primary side - as long as we terminate on the x / o tanking it to ground that would be o.k.

if you are supplying a 480v 3 phase supply to a basic dry type transformer that supplies a 120v/208v panel the 480 volt side doesnt need a neutral. just run your 3 phase power and an equipment ground. the neutral on the secondary side needs to be grounded. if the transformer is in a metal building you can bond it to the building steel if you bonded it at the main service. if its in a wood building then you need to bond it directly to the grounding electrode conductor or the electrode itself. size the seperately derived bonding jumper in accordance to 250.66. im sure ill be corrected if im wrong