Upgrading High Leg Service

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bpk

Senior Member
We will be doing a project (still in planning) to upgrade a service that is a 240volt high leg service. There are minimal 3 phase loads in the building ( ac compressors, roof fans, and little more). There will be an expansion of single phase loads in the future, currently the majority of the circuits are single phase so there are alot of unused B phases in the panel. The POCO doesnt want to change there configuration. Is it a bad idea to set a transformer with a second panel to allow for expansion of the new load, or does anybody else have a better solution, or should I push the poco to upgrade there end to a 120/208Y. The building is mostly office space with a repair and machine shop that has 3phase equipment. Thanks in advance
 

wireguru

Senior Member
i have seen on some of these, where the poco drastically undersizes the transformer providing the high leg based on the original load calcs with the minimal 3ph load provided when the service was installed. I would check this before installing a 240D-208Y120 transformer to get more 120v circuits.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I would recommend two panels, the first sized for the anticipated single phase, the second a small one for three phase only.
 

charlie

Senior Member
Location
Indianapolis
. . . where the poco drastically undersizes the transformer . . .
It is very seldom that the serving electric utility undersizes their transformers. We size to the expected demand which is normally about 40% of the actual service size. If we don't size our transformer correctly, we have to change it at no charge to the customer. Since we don't want to do that or waste kVA, we size them correctly most of the time.

If we have an existing bank of transformers already there and the bank is large enough, expect to pay for the replacement of the existing bank with three transformers and the installation of another primary phase if there are only two phases on the pole. My guess is it will cost somewhere north of $7k to get 208Y/120 volts. :)
 

wireguru

Senior Member
It is very seldom that the serving electric utility undersizes their transformers. We size to the expected demand which is normally about 40% of the actual service size. If we don't size our transformer correctly, we have to change it at no charge to the customer. Since we don't want to do that or waste kVA, we size them correctly most of the time.

If we have an existing bank of transformers already there and the bank is large enough, expect to pay for the replacement of the existing bank with three transformers and the installation of another primary phase if there are only two phases on the pole. My guess is it will cost somewhere north of $7k to get 208Y/120 volts. :)


I worded wrong, the point I was trying to get across. What I meant is that the transformer supplying the high leg may only be sized based on the original 3ph load, and will be undersized if he throws a delta to wye transformer in the building to get more 120v circuits out of the service. Had an entertainment venue learn that one the hard (expensive) way.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
BP, I have been faced with the exact same situation more than once. I agree with the general concensus as the others, and here's why:

Keep in mind that the 120/240 portion of your existing service (let's call it the 'main part') is exactly the same as a typcial 120/240v 1ph service. Identical. Just in case you weren't aware of it.

The odds are that your present supply has a lower high-leg ampacity than the main part, and perhaps is even an open Delta. You can confirm this by looking at the transformers.

As your existing 2-pole 1-phase and all 3-phase equipment is rated for 240v, and may be too old for 208v, you might be better off keeping what you have, as long as load calcs agree.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If we have an existing bank of transformers already there and the bank is large enough, expect to pay for the replacement of the existing bank with three transformers and the installation of another primary phase if there are only two phases on the pole. My guess is it will cost somewhere north of $7k to get 208Y/120 volts. :)
This is very close to the cost I saved a customer a few years ago, by re-working the service so they could retain the high-leg service and not go with the engineer's proposed new wye service.

They also had existing RTU's that were not rated for 208v.

(And, for the bean-counters, I didn't lose money on the change, since the new materials were the same anyway. Only the POCO would have made more money.)
 

jrannis

Senior Member
I would start with a load calculation, let the POCO know what you came up with and ask if the equipment on the pole will work for you.
If not, you will have to make some decisions. Usually, the POCO work is much cheaper than ours.
 
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