2 wire, 208V from 3-ph delta -wye TX 3

[Sn00ze]

I am questioning my coworker and he hasn't been able to give me a satisfactory answer either.

So, i am hoping to get an answer from here in hopes to understand it better and explain it or have it explained to me.

We have a system being fed from a station service TX. 120/208V 3-ph to our AC panel.

We are feeding a circuit breaker from this panel with 240V heaters so they had to install an aux Tx to step the voltage up from 208V (phases A and B).

So, up to the TX the load is 5.4kVA. but the transformer is 208V to 240V. So what is the current in each of the legs coming from the AC panel?

is it simply kVA/208V ? Iin=Iout?

or do we need to kVA/208V*1.73.

thanks

 

[cranky108]

If you have no single phase load, why would you need 1.73?

Why do you need 5.4kVA of load in a circuit breaker? Unless you have that much in heaters, in which case you should ask why you need that much heat. If you live far North, then maybe, but otherwise, why not feed the heaters from the 208?

 

[waross]

"is it simply kVA/208V ? Iin=Iout?"
Yes, plus transformer losses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Most 240V heaters in switchgear are fed at 120V anyway. No need for a transformer of any sort. Heater effective kW is 25% of nameplate kW. All is good.

 

[waross]

Further to David's post;
Condensation may occur at any ambient temperature.
Anti condensation heaters are typically connected at 50% voltage as David points out.
On generators they often last for the life of the machine.
That said, in colder regions, space heaters may be installed in addition to anti condensation heaters.
The additional heaters are typically connected at rated voltage and are controlled by thermostats.
Rather than adding the expense of transformers I would oversize the heaters by 25% to 35% and connect them directly to 208 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

See the following older post:

https://www.eng-tips.com/viewthread.cfm?qid=383971

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Sn00ze]

Thanks for your replies everyone.

I am in canada so these would be Tank heaters for 138kV SF6 CB. So i would not recommend running them lower than rated by the manufacturer. Do you guys have experience with this not being a problem though? Doesn't give me the warm and fuzzies. (no pun intended )

 

[crshears]

NOW we're getting someplace!

Where in Canada?

In northern Ontario we've had it get so bitterly cold that even with all installed heaters fully in service @ rated voltage the temps still fell low enough that the possibility of SF6 liquefaction became all too real and breakers [by design] started tripping themselves open on low temp while they still could, just to ensure they didn't get to a state where they would be unable to interrupt fault current if called upon.

Needless to say the customers who lost power in the middle of the night and didn't get it back until halfway through the next morning when the sun had re-warmed the breakers enough to allow the control centre to close them were none too happy . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[cranky108]

Now that's different. In the past we have had engineers order the extra tank heaters when we don't need them. Sort of create a problem where none exists.

I would think you should be able to order the breaker heaters with the voltage you want to operate them at. But if someone wanted a standard design, a transformer might be required.
Many small aux. transformers allow voltage taps so you can raze the output voltage by 5%, to maybe cover losses.

But if you were talking about just condensation heaters, I would have no problem running them at a lower voltage. Provided they were not also connected to outlets, or lights.