480V power wiring: wiring from hot to ground to get 277V - OK? 3

[emissaryx]

I have a situation where there is a 480V 3-Ph 3-Wire system feeding a location with some 480V motors and controls. There is no neutral pulled so the electrician wants to wire a 277V 4.5kW resistive load (electric heater) from HOT to GROUND to get 277V at the site since there is no neutral. The heater itself is rated for 300V.

I know this is not the usual or preferred way to get 277V out of a 480V 3PH system.

What are the negative effects of using this setup (480V to GROUND = 277V)?

 

[jghrist]

Load current will flow through the ground. This can cause various problems including misoperation of ground fault protection. I'm sure that this would violate the NEC, but I don't have a particular article to quote. Run a neutral or install a 480-277 volt transformer.

 

[dpc]

The main two disadvantages:

It's ILLEGAL.

It's DANGEROUS.

The green ground wire is required to be a non-current carrying conductor. Any use of it for carrying load current creates voltage drop across the conductor and more or less defeats its purpose. You may also have problems with any ground fault protection tripping due to load current in the green wire.

If you really have to use a 300 V load, you can install a local step-down transformer to convert the 480 V to 277 V.

If your "electrician" is a licensed journeyman, you should report him to the IBEW local.

 

[emissaryx]

Can you elaborate on "various problems"?

 

[davidbeach]

Various problems - how about the introduction of a deliberate ground fault on the system.

If it is truly a 480V 3 wire system, rather than a 4W system without the neutral pulled to this location, in other words if it is derived from a delta or a high impedance grounded wye, the 277V to ground is merely nominal and can range from zero to 480V (or more) depending on various system conditions. Your load would have a tendency to pull it toward zero under most conditions.

 

[emissaryx]

Thanks for the replies so for! Let me try to redirect my question.

Would this 480V/ground connection ever have the possibility of the voltage ever going above 277V? I mean way above 277V and for an extended period of time.

Would this connection pose any threat to the equipment connected to the system?

Disregard personnel safety, ground fault protection systems.

I asked this question because the heater that was connected like this failed after a very brief test. We are assuming a mfg defect, but I wanted to examine the possibility that this connection caused the failure. Besides the connection being illegal and dangerous, I want to be able to tell this person: “You have to pull a neutral because if you wire the heater backup like you did the first time – it is going to fail because”…

Because why?

 

[jraef]

I think davidbeach's post of 5 Nov 07 12:51 gave you the reason.

If you have a 3 wire service, you cannot use a 277V rated device without installing a new 4 wire service transformer or a dedicated smaller delta-wye transformer just to feed that load.

As as an engineer, you should not have to say things like "Disregard personnel safety, ground fault protection systems." and "Besides the connection being illegal and dangerous, I want to be able to tell this person...", you should stop right there and stand your ground based on professional conduct. I would also do it in writing to protect myself, because it sounds as though you are working for someone who is not concerned with doing things right, just cheap.


Just my humble opinion.

 

[jghrist]

Is the system grounded? If one phase is center-tapped and grounded, then you would get 415V phase-to-ground on the high leg. If corner grounded, you would get 480V phase-to-ground on the other phases. If ungrounded, the heater would just not work because there would be no voltage across it and the voltage to ground of the other phases would be 480V. If the source is 277/480V grounded wye, then you have the safety and ground protection problems. You also may damage sensitive electronic equipment because of the voltage you put on the ground.

I agree with dpc, you should tell the person "You have to pull a neutral because if you wire the heater backup like you did the first time – I'll report you to the IBEW local."

 

[Senselessticker]

Perhaps the heater failed because...

... the heater is equipped with its own internal ground fault protection to as a necessary means to preserve life of complete imbeciles...

 

[emissaryx]

Thank you.

 

[PH927]

By passing your comment “Disregard personnel safety, ground fault protection systems.” Let me make some comments. The fact that your electrician wants to use the ground as a neutral tells me you need a new electrician. Since they are asking you does that mean you get to take responsibility when something goes wrong now or in the future?

Most important and above all it could hurt or kill someone.

My 2 cents.

 

[WDeanN]

"I agree with dpc, you should tell the person "You have to pull a neutral because if you wire the heater backup like you did the first time – I'll report you to the IBEW local.""

Where is he going to get a neutral from??!!! If this is a standard Delta system, like those that commonly feed motor loads, there is no possibility of a neutral.

You need another source. Either a transformer, or look elsewhere for your power.

 

[dpc]

OP did not say it was a delta system. There are a lot of grounded wye systems that do not bring out the neutral.

 

[sslobodan]

If you need a neutral you need to create one. It is simple system. You ground a starting neutral point for entire system and than run neutral around where you need it. You should not ground neutral on some other place and you should not connect grounding to neutral at any other place. This is called TN system If I remember the title correctly.

 

[sslobodan]

One addition this means that you need to have symmetrical 3 phase system at beginning so you can have neutral point (star) on it grounded, and I suppose you have symmetrical system.