can you tap a 600V:208V transformer to 480V:208V?

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malachi constant

Senior Member
Location
Minneapolis
We are engineers redesigning a distribution system to a large school facility. Long story short there is an existing 208V service that steps up to 600V in order to distribute around the building. We had assumed this was 480V but fortunately caught it a couple months before we finalize the design. Questions:
1. I would love to reuse the 600V feeders to distribute from a new 480V board. The feeders are sized in such a way I am not concerned about load size or voltage drop. Conduit reuse would likely not be an issue. However the feeders are aluminum, and at least 40 years old. I assume I should be recommending conductor replacement to the Owner? There are a lot of cons to reusing, with the only pro I can think of being "you save money".
2. I would love to retap the step-down transformers to 480V:208V. But looking at nameplate, taps bottom out at 540V. So I assume reuse of these on a 480V system is not an option - am I correct?
3. I've been doing this 20 years and had never heard of a 600V system installed in the US. I assume this is not unheard of, but I'm correct that it's pretty weird, right?

We have the option to keep it, but it would mean that all our ~new~ distribution and mechanical equipment would have to be 208V, which would add quite a bit of cost to the project. Also we are adding a generator and creating an emergency distribution system - that was planned to be at 480V. Most all lighting is being replaced, so we were assuming new 277V lighting throughout. Wouldn't be the worst thing to keep lighting systems at 120V (could reuse all their home runs & branch circuitry) but would make all new em lighting be stepped down to 120V.

If anyone has answers to the conductors and transformers question, or general thoughts on 600V distrubution, I'd appreciate hearing them. Thanks!
 

iceworm

Curmudgeon still using printed IEEE Color Books
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EE (Field - as little design as possible)
I don't have a good picture yet.
The new service is 480V? How big is it? 500kva?, 1000kva?
The existing 208/600V step-up is removed?
The 480V incomer panel/MCC feeder CBs are then connected to the existing 600V conductors?
Well, you could test them, however, they are 40+years old. Does the customer want to have the new system to last another 40 years? The existing conductors probably won't do that.​
Then reconnect the existing 600V/208V xfm to the 480V feeders? (Yes, you are correct - No, this one won't work)


No comment on using 208V equipment instead of 480V

600V distribution:
Yes I have seen a few, all 575V and all industrial - not weird, but uncommon. All the equipment can be bought for 575V. Lead time will be longer for motors and transformers. Cost per kw should be about the same. Lighting and other hotel loads will be fed by 575/208 xfm instead of 480/208V xfm. Panelboards will be a bit more since the CBs will be 600V instead of 480/277. Conductors are a bit smaller. Gen set is the same, possibly a longer lead time.

the worm
 

augie47

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State Electrical Inspector (Retired)
I would suggest you check your lighting circuits closely. I would expect some load problems if you converted 277v lighting circuits to 120v.
 

malachi constant

Senior Member
Location
Minneapolis
I would suggest you check your lighting circuits closely. I would expect some load problems if you converted 277v lighting circuits to 120v.

We're going the other way - 120V lighting now, moving everything to 277V. Will cost a little more since we won't be able to reuse branch circuitry. But since all the ceilings and lights are coming down, and there are hundreds of thousands of SF of corridors and classrooms, and we are getting rid of battery packs and adding an emergency distribution system...it seems like the right thing to do to fix it for the next generation.
 

malachi constant

Senior Member
Location
Minneapolis
iceworm -- I think you answered my questions, thank you. Here's a little more information anyway.

The new service is 480V?
yes

How big is it? 500kva?, 1000kva?
Not sure without asking the guy who just left for the afternoon. Switchboard is probably 1600-2000A at 480V. Utility will provide whatever size transformer they please - probably 750 or 1000kva.

The existing 208/600V step-up is removed?
Yes, that is what we are proposing. Get rid of all the 600A stuff and move as many new and old systems as make sense to the more standard 480V.

The 480V incomer panel/MCC feeder CBs are then connected to the existing 600V conductors?
Well, you could test them, however, they are 40+years old. Does the customer want to have the new system to last another 40 years? The existing conductors probably won't do that.​
I'm guessing they never want to touch this again until they are all long retired. I can't recommend reusing but if they want to I will require testing first.

600V distribution: Yes I have seen a few, all 575V and all industrial - not weird, but uncommon. All the equipment can be bought for 575V. Lead time will be longer for motors and transformers. Cost per kw should be about the same. Lighting and other hotel loads will be fed by 575/208 xfm instead of 480/208V xfm. Panelboards will be a bit more since the CBs will be 600V instead of 480/277. Conductors are a bit smaller. Gen set is the same, possibly a longer lead time.
Asking the Owner and future mech/elec maintenance contractors to use 600V motor and distribution components just seems like a mistake. I'm glad to know it exists but this doesn't seem like the right facility to have it.
 
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jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
1. I would love to reuse the 600V feeders to distribute from a new 480V board. The feeders are sized in such a way I am not concerned about load size or voltage drop. Conduit reuse would likely not be an issue. However the feeders are aluminum, and at least 40 years old. I assume I should be recommending conductor replacement to the Owner? There are a lot of cons to reusing, with the only pro I can think of being "you save money".

There is no such thing as 480V conductors. So there is no reson you cannot supply your existing conductors with 480V.

40 years is not too old for conductors, there are plenty of 60 year old buildings that have not been rewired yet.
Aluminum feeder conductors are still pretty common.
All reused conductors should be evaluated.
 

Jraef

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Staff member
Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
I've seen this done for voltage drop issues on large campuses. It's not as uncommon as you might think, but doing it for one building would be unusual (although we don't know the size of that building).

Re-tapping the transformer could, in theory, be done by a transformer reman facility but I would suspect that unless you have one nearby, the cost to remove, ship both ways and reinstall them would exceed the cost of new 480-208V transformers.
 

kwired

Electron manager
Location
NE Nebraska
I've seen this done for voltage drop issues on large campuses. It's not as uncommon as you might think, but doing it for one building would be unusual (although we don't know the size of that building).

Re-tapping the transformer could, in theory, be done by a transformer reman facility but I would suspect that unless you have one nearby, the cost to remove, ship both ways and reinstall them would exceed the cost of new 480-208V transformers.
My understanding is service was 208/120 and will be changed to 480/277. Therefore originally there would have been at least one step up transformer involved as well. Now all is needed is step down transformers - best option IMO is likely to plan to replace them with 480-208/120 transformers. If lighting is changing from 120 to 277, might actually be able to use smaller capacity transformers for remaining 208/120 loads than what was originally there, may help with feeder ampacity some if existing feeder capacity was questionable, that plus new lighting is likely more efficient than old lighting.
 

Fnewman

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Location
Dublin, GA
Occupation
Sr. Electrical Engineering Manager at Larson Engineering
A Few Comments

A Few Comments

Having just completed the design for a fairly large industrial project in a plant that utilizes 600 v systems, I will offer the following comments - not necessarily to disagree with others posted earlier.
1. 600 v systems (nominal, also called 575v) were originally found mostly in Canada, as I understand, then were eventually adopted by some large industrial sites in the US.
2. 600 v equipment is generally not specified today in the US unless it is an expansion of an existing system.
3. 600 v equipment is readily available from pretty much all equipment manufacturers.
4. Could you "tap" the winding of a 600/208 transformer? Sure, it is technically possible, but probably not too economical. call the original manufacturer or one of the many transformer rebuild companies.
5. Lighting : If you are planning to use LED lighting for the upgrade (as I assume you would today), choice of voltage and circuit load will likely be insignificant because most fixtures automatically accommodate a wide range of voltages and require comparatively little power. Always evaluate voltage drop as the guiding factor
6. Overall, you (and your client) might be best served by subcontracting this evaluation and design to an engineering firm with 600 v experience.
 
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