Combining to Identical XFMR's

Status
Not open for further replies.
aelectricalman

aelectricalman

Senior Member
Location
KY
Im in a situation where we have a 208/120V panel feed from a 480V system. This is an old warehouse/factory that was closed many years ago and the equipment we are using was originally in an area for the front offices. There is the 480V panel and a transformed 208/120V next to it. The transformer that feeds the 208/120V panel is only rated to supply max 80 amps. The 208/120V panel fed from it, is fused at 30 amps. Well, thats a dilema because we are now converting this to a fitness center where 2 tanning beds, 2 saunas and a clothes dryer will be required to run 208V. There will also be 7- 16 amp treadmills at 120V. So, at 80% use we are looking at somewhere around 150 amps. This XFMR also feeds a small panel that was once for "clean power", iso. Now the issue is that I have to put in a 150 amp disconnect and get 150 amps to the gym in place of the 30 amp. Next to the transformer in question (80amp), I have an identical transformer with fairly similar impedences. I dont think we will have any heating issue. These are older XFMRS, and my concern is that if one fails I might have trouble! Can I lock out one, when the other trips with a contactor and have a safe assembly. Are there any other major issues I should look at? Will I have any tap issues since these are older XFMRs? Is this a bad idea?
 
Last edited:
These people are eatting me up because I overlooked that small issue. I really dont want to come out of pocket for a 200 amp XFMR
 
Paralleling small transformers is not a good idea. Impedances must be 'exact' not just similar. And this would be similar to feeding a 200A panel from (2) 100A breakers (which is against code).

Why not just put in a second 208Y/120 panel?
 
jim dungar said:
Paralleling small transformers is not a good idea. Impedances must be 'exact' not just similar. And this would be similar to feeding a 200A panel from (2) 100A breakers (which is against code).

Why not just put in a second 208Y/120 panel?
Click to expand...

The problem is the building is a finished product and they are ready to open next week. It was oversight on my part, for the panel stated it was a 100A. When we acepted the job, we accepted it on the conditions that we had a 100 amp panel already intact as the nameplate reads. So, of course the company is trying to make us eat it. Just looking for an easier alternative.
 
30kva xfmr

30kva xfmr

There is a spare 30KVA XFMR but that is only gonna get me 80ish amp 3P. Thanks for input. I will try another route .
 
aelectricalman said:
There is a spare 30KVA XFMR but that is only gonna get me 80ish amp 3P. Thanks for input. I will try another route .
Click to expand...
The 30KVA 208Y/120 transformer is able to supply 83A continuously, this is the transformer usually used to feed '100A main breaker' panels.
 
How could a 100 amp panel serve a 150 amp load in the first place? Did you know the total load was going to be above the rating of the existing panel? If they changed the total load, by adding more requirements than you had originally accounted for, then that is a change in scope, and they should pay for the additional equipment.

Issues of payment aside, do I understand that you have two panels available, and two transformers? Can you serve the treadmills from the small iso panel, and use the spare transformer for that load, leaving the other panel and the other transformer to serve the rest of the facility?
 
jim dungar said:
The 30KVA 208Y/120 transformer is able to supply 83A continuously, this is the transformer usually used to feed '100A main breaker' panels.
Click to expand...

I have to use the panel I have. I can not tap into the old iso.
I need 150 AMPs continuous, but what Ive devised is this. In the building there is a 75KVA XFMR not being used. Im going to use it, and fuse it down to 150Amp Load side, so I can keep my conduit intact and not have to upsize the conduit. The conduit is 1 1/4 and there will be 4 - 1/0 and 1 number 6# ground. I have to do the calculations but I think we are ok with that. I thought it would be il-legal to do that since the fuses could technically be changed out to 200AMP. The inspector said as long as I put a metal nameplate stating there is 150AMP Maximum fuse, I would be ok.

Any thoughts?
 
You may use any size fuse on the secondary of a transformer as long as the primary protection does not exceed 125% of the primary FLA.

The secondary conductors of a transformer are protected using 240.24(C) not 450.3. As long as you fit into one of the transformer tap rules you should be good to go, a 'special' nameplate is not required.
 
Last edited:
jim dungar said:
You may use any size fuse on the secondary of a transformer as long as the primary protection does not exceed 125% of the primary FLA.

The secondary conductors of a transformer are protected using 240.24(C) not 450.3. As long as you fit into one of the transformer tap rules you should be good to go, a 'special' nameplate is not required.
Click to expand...

I understand what youre saying but my conduit is sized only for 150amp and all I have a 75 KVA transformer which supplies 208 continuous amps. I have to indicate that this set up can not be fused at 200 Amps on the load side because my wire is 1/0 therefore not rated for 200 amp.
This has nothing to do with Tap settings. Im looking at the wire size? Am I overlooking something?
 
Chris

Your feeder must be sized at 125% of the continuous load. See 215.3

The tap conductors Jim is referring to are the conductors between the transformer and first overcurrent device. See 240.4(F)and 240.21(B)
 
curt swartz said:
Chris

Your feeder must be sized at 125% of the continuous load. See 215.3

The tap conductors Jim is referring to are the conductors between the transformer and first overcurrent device. See 240.4(F)and 240.21(B)
Click to expand...

Im sorry , my continuous load is only 119A, so im ok. Im trying to say I need max 150 fusing. Im just trying to keep my wire size as close as I can to the conduit size limits that are already installed in attempt to not have to change the conduit.
I understand 125%, im talking about the fact that im using a 75KVA XFMR as opposed to a 60KVA which is not available to me. In my 75KVA XFMR Im using a 200 amp fused disconnect with 150 amp fuses, because on the load side of the fused disconnect, I can only use 1/0 feeders due to conduit restrictions. He was saying I dont need a sticker saying max fuse is "150 AMP, do not exceed". What if someone comes behind me and puts in 200 amp fuses and I have 1/0 cu?
 
Last edited:
Chris

If someone decides to increase the fuse size at a future time that's their problem. It's really no different than someone changing a 15 amp breaker protecting a #14 wire to a 30 at a later time. There is no requirement that you mark the disconnect with the fuse size. As long as your fusible disconnect meets the maximum distance requirements in 240.21(B) you are good.
 
curt swartz said:
Chris

If someone decides to increase the fuse size at a future time that's their problem. It's really no different than someone changing a 15 amp breaker protecting a #14 wire to a 30 at a later time. There is no requirement that you mark the disconnect with the fuse size. As long as your fusible disconnect meets the maximum distance requirements in 240.21(B) you are good.
Click to expand...

I really appreciate your help.
 
Status
Not open for further replies.
Top