I have read several posts on here about corner grounded delta systems being old. I have sucessfully been awarded 2 Tanning salon Jobs...Fitouts. The 1st was engineered the second I am doing a design build. For some unknown reason the beds are straight 240 single or 3 phase(3Ph being the uncommon). The incomming svc in both jobs is 120/208Y. The 1st job the engineer selected non off the shelf transformers which had midpoint of phase center tap... which is not used in any of the tanning bed pannels. (no line to Grounded conductor loads(05 code)). Why he did this is beyond me. The bld. allready has 120/208 SVC. For the design build job I am going to reverse feed a 240-120/208 transformer to generate the needed 240V. The cost of proceeding in this manner saves the customer at least $2K on the non-shelf item transformer. I was thinking that this application would be well suited for a corner grounded delta system and was wondering if there would be any drawbacks with this arrangement??
Corner Grounded Delta - old?
- Thread starter boptrop
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brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
And if you use buck-boost transformers you will save him even more and have a system that has a grounded neutral and are off the shelf, weigh less and take up less space. Cost savings all around.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The advantage of a 240/120 3PH 4W (center-tapped) delta system is the ease of grounding the secondary. Disadvantages to a corner-grounded system include non-standard panels/breakers and confusion to un-qualified electricians/people (they don't understand 240V L-G).
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
If you do not do the buck boost this is a PREFERED method. 300 kva at buck boost is around 20 kva (Watch out for Brian's quick math)
According to the vendor I would have to use (2) banks of buck boost transformers to meet the Required KVA. I dunno how to size buck boosts (not a standard cal. to me) so I have to trust them.
It seems to me that this would turn out to be real messy in the end. I don't understand the 20Kva buck boost option as stated. The Tanning bed panels have a 598 connected load amp. I haven't ordered material yet but need to do soon. Thanks for all help!
(Brian...are you saying watch out about yourself?)
It seems to me that this would turn out to be real messy in the end. I don't understand the 20Kva buck boost option as stated. The Tanning bed panels have a 598 connected load amp. I haven't ordered material yet but need to do soon. Thanks for all help!
(Brian...are you saying watch out about yourself?)
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For a buck boost, the voltage you use to calculate the transformer power is the difference between the input and output voltage.
So for 240 volts in, and 208 volts out you have a 32 volt difference. So for a single phase 240 volt tanning bed, to calculate the buck boost transformer power, you would multiply the tanning bed's current by 32 to get the transfomer power required.
For example, a 40 amp tanning bed on a normal transformer would require 240 volts * 40 amps = 9600 VA.
For a buck boost, it would be 32 volts * 40 amps = 1280 VA.
Steve
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Try to find 20KVA 120X240-16/32V single phase transformers. My guess is that they are "special".
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
I think 5 KVA are max (to lazy to look) but I did a job for another EC exacrt same thing, he did all the install I did all the connections.
I think I will just get the delta center tapped trans..With SqD bolt in pannels you can only use 2 pole breakers and use the netural bar for the grounded leg. As Jim said it is unfamillar to most. They say you can use I line with 3 Pole CB's but the cost defeats the purpose. The buck boost option is going to be messy given the quanity of transformer banks you need for this app. Not to mention 4 pannels, unless you can parallell, to attain 20 Kva. (I like clean installs) So .... Delta center tap is the cutting edge of old technology on this one. Thanks All!
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