277 VOLT NUETRAL CONNECTION 2

[TONSOFKVA]

I design 3 phase 3 wire motor controls. I must admit I rarely get away from my desk anymore and into the field. Could anyone tell me what is done in the real world with these 277 volt branch circuits?

My concern is I never see 3 pH 4 wire specs because most of my circuits are so far from the panel boards that the 4th wire is not run. I know I never supply an insulated terminal for the contractor to land it on but yet I am told from customers that a 277 volt heater or lighting circuit was added to the motor controller.

My guess is they land the 277 volt heater nuetral on the equipment ground terminal and get the 277 volts.

If someone would be kind enough to tell me what happens in the real world and what problems occur by not running the nuetral back to the panelboard I would really appreciate it.

In my younger years I did work in maintenance on lighting circuits and am familiar how lighting is connected to 480 and nuetral to acheive 277. Just want to know more about other applications.

 

[Senselessticker]

Sounds to me that you are confused about neutral and ground. For 277 Volt systems the neutral will be a current carrying conductor (return path). Tieing the neutral and ground terminals at the equipment will result in ground acting as the return path for the current (VERY BAD). Problems associated with not running a neutral back to the panelboard may include: ground current, electric shock, fire, ...death!!.

Here's a link to a quick and dirty summary that may help clarify.

http://www.sea.siemens.com/step/default.html

 

[TONSOFKVA]

Thanks senselestiker. Shock and Death. Just what I thought I was wondering how often it is done in the field.

I have heard a lot of people say 277 is 480 volt to ground.

 

[rbulsara]

Since the neutral is grounded at the source, you will always measure 277V to ground in a 480V system. But as mentioned earlier you connect a 277V load between L and N and not between L and G.

 

[briand2]

tonsofkva,

Are you sure the contractors aren't just installing 480 / 277 volt transformers between two of your phases? That's what we do if we don't have a neutral running to the motor control centre. (But I'm in the UK, so it's 415V / 240V (or 400V / 230V these days, as we've recently become Europeans...)).

Brian

 

[Senselessticker]

I believe I misunderstood a bit. In any case, if your feeder to the 480V MCC does not have a neutral but you want to run a 277 Volt load from that MCC, you'll need a 3 phase wye transformer. Then you'll have a 480/277 3 phase 4 wire system. Someone please let me know if i'm out in left field here.

Thanks

 

[rbulsara]

For small single phase loads, normally smalls single phase or 3 phase step down transformer would be employed. A good contractor will not connect a load between L and G. If it is done, it may work for small loads such as heater, but will be in grave violation of Code and safety!!

You may want to ask the customer the clarification as to how that 277V is acheived.

 

[RalphChristie]

If you have not designed a panel with a 277V supply point from 480V, there can then just be one of 4 scenarios how the customer can obtain it:

1. Customer install an extra neutral insulator and supply the panel with 3ph 4 wire sytem,
or

2. Customer install an transformer in the panel to obtain 277V,
or

3. Customer use the earth as a neutral conductor. Code and safety violation, could end in a fatal.
or

4. Customer use two 277V bulbs, or two heaters in series. (Connected in series between two phases) Not a very good way of doing, but there will be 240V across each element.

 

[advidana]

Yes I have see what you have described, after someone has been hurt or from a lot of damaged electrical equipment.

 

[RalphChristie]

advidana:

I think most of us have seen it, or at least heard of something similar that happened. There can be a lot of reasons for such violations: inexperience, trying to save money, shortcuts, not enough time, etc, etc, but as if this are not enough, a lot of this are qualified artisans. Maybe due to decreasing standards? Lack of pride in their work? I do not know what the reasons are, but it is totally unacceptable.