480VAC Delta Supply on 277 Y Load

[Maro]

Hello,
I have a bank of 277VAc heaters that have the same resistance.
Can I wire them in groups of three from 480 VAC L1, L2, L3 on one side,
and terminals together on the other side, as a Y, and leave the Y neutral
floating (ungrounded)?

Is it acceptable without using a 480 Delta/ 480/277 Y transformer?

Thanks for your reply.

 

[LarryFine]

Hello,
I have a bank of 277VAc heaters that have the same resistance.
Can I wire them in groups of three from 480 VAC L1, L2, L3 on one side,
and terminals together on the other side, as a Y, and leave the Y neutral
floating (ungrounded)?

Is it acceptable without using a 480 Delta/ 480/277 Y transformer?

Thanks for your reply.

You're welcome, and welcome to the forum.

In theory, this would work, but the moment any heater leaves the circuit, whether due to thermostatic control, breaker opening, or even a heater burning out, the voltage at the "neutral" point would shift away from zero (to earth) and create an over-voltage on the remaining heaters.

In other words, don't do it without a Delta-wye transformer.

 

[winnie]

If these were 277V heating elements that you are assembling into a system controlled with 480V control elements, then I don't see a problem. In my mind this would be similar to a motor with an internal wye connection.

If these are 277V heaters with thermostats and the like, designed for use at 277V, then I see a big problem.

As Larry notes, if one heating element of the wye goes open for any reason, the potential of the neutral will shift. However this will _reduce_ the voltage on the other two heaters; the other two heaters would essentially be single phase in series at 480V, so each heater would only see 240V.

But the open circuit voltage that the remaining heater sees (in other words, the voltage between the supply for the 'open' heater and the center point between the remaining two) will be 416V, a problem if you have 277V control elements.

If _two_ heaters open, then the third would shut off, and the open circuit voltage would be 480V.

-Jon

 

[LarryFine]

As Larry notes, if one heating element of the wye goes open for any reason, the potential of the neutral will shift. However this will _reduce_ the voltage on the other two heaters; the other two heaters would essentially be single phase in series at 480V, so each heater would only see 240V.

I took the OP to mean each phase's load would be more than one heater, and that's what I meant, that it would subject the remaining heaters on that phase to over-voltage.

With only one heater per phase per group, Winnie's explanation would prevail. In other words, don't join the several "neutral-points" together.

 

[charlie b]

You don?t even need a failure (e.g., heating element fails open), in order to have problems. With all three turned on, there will be a balanced current in the three phases, and there will be 277 volts across each element. But when one element is turned off by its controller, there will be 480 volts across the other two (240 volts across each). They will then give you only 75% of their rated heat. When the second element is turned off by its controller, the third will be turned off as well, since there will be no complete path for current to flow in the third element (even though its controller has it turned on).

Would this be a code violation? I?m not sure. I?d have to look around for a while to be find out.

 

[GH_Vegas]

Without the neutral, the installation is unsafe. Winnie touched on this with the reference to 416V. When the 1st heater turns off, the contacts will have to disconnect 416V due to the neutral 'shift'. If a 2nd heater element turns off, that contact will have to disconnect 480V. The contacts will not be able to handle these voltages as well as the arcing danger of 480V.

In my mind, it does not matter if it is a code violation or not, It is NOT safe and IS dangerous. Don't do it!

You must use the Wye transformer.

 

[dbuckley]

Also if it was installed and did all work the first heater to fail would upset the balance, and some heaters would then be overvoltaged causing them to fail, and this would cascade till only one heater remained intact.

Approriately voltaged heaters would make more sense than a transformer on a new install...

 

[coulter]

Without the neutral, the installation is unsafe. Winnie touched on this with the reference to 416V. When the 1st heater turns off, the contacts will have to disconnect 416V due to the neutral 'shift'. If a 2nd heater element turns off, that contact will have to disconnect 480V. The contacts will not be able to handle these voltages as well as the arcing danger of 480V.

In my mind, it does not matter if it is a code violation or not, It is NOT safe and IS dangerous. Don't do it!

You must use the Wye transformer.

So one has to spec a 480V, 3ph contactor, not a 480/277 contactor. (I don't think I have ever seen a 480/277 contactor - is one even made?) Plenty of mfgs make 480V contactors - I rarely deal with anything else, unless it's 600V

After that, what is unsafe?

My only experience with this type of connection is two small electric boilers (20kW), both had 480V Wye connected elements - with no neutral. They worked fine. The controllers were a nema standard 3pole contactor on one and a 2pole scr on the other.

I'm not saying it is a good idea, I just don't see any reason it is inherently unsafe.

carl

 

[coulter]

Also if it was installed and did all work the first heater to fail would upset the balance, and some heaters would then be overvoltaged causing them to fail, and this would cascade till only one heater remained intact....

I don't think so. As charlie said, with all three heaters working, the voltage across each is 277V if one fails, the voltage across each of the two remaining is 240V.

I'm not seeing anything unsafe.

carl

 

[winnie]

So one has to spec a 480V, 3ph contactor, not a 480/277 contactor. (I don't think I have ever seen a 480/277 contactor - is one even made?) Plenty of mfgs make 480V contactors - I rarely deal with anything else, unless it's 600V

After that, what is unsafe?

One problem is that we are visualizing and discussing two different possible systems.

1) 3 separate heating units, intended for individual stand alone operation on 277V, which are connected wye, and then connected to a 480V supply.

2) 3 separate heating elements, each intended for use as part of a larger system with suitable controls for that larger system.

I see absolutely no problem with 2)

-Jon

 

[coulter]

One problem is that we are visualizing and discussing two different possible systems....

Yes that seems true. I'm trying to stick with the op statements and not add anything extra.

...1) 3 separate heating units, intended for individual stand alone operation on 277V, which are connected wye, and then connected to a 480V supply....

Humm, this doesn't quite match with your earlier post. In any case, I agree one would not want to use any 277V control equipment (eg. line thermostats)

My issue is with the statements of "UNSAFE" without any science explaining tthe reasoning.

carl

 

[winnie]

Humm, this doesn't quite match with your earlier post. In any case, I agree one would not want to use any 277V control equipment (eg. line thermostats)

I did swap order of possibilities, but in my original post I said:

If these were 277V heating elements that you are assembling into a system controlled with 480V control elements, then I don't see a problem. In my mind this would be similar to a motor with an internal wye connection.

If these are 277V heaters with thermostats and the like, designed for use at 277V, then I see a big problem.

But even having both made the distinction between heating elements and heaters, Larry and I presumed opposite configurations of multiple versus single elements per phase in each 'wye'. There are quite a few different possibilities that can be read into the OP's question.

-Jon

 

[masterelect1]

Code Violation - possibly @ NEC 110.3(A)(1) see FPN ? and last sentance of 110.4