Secondary OCP of 45kva Transformer supplying a 125A panel

Status
Not open for further replies.
D

dftmaint

Member
Location
TN
Occupation
Industrial electrician
We have a buss disconnect with 70A fuses supplying a 45kva 480D/208Y/120 transformer that is feeding a 125 amp 208Y/120 panel that is located right next to the transformer. The panel has a 100 amp main breaker. Also the primary conductors are under 25ft.
Is overcurrent protection besides the main in the panel require here? Also do i size the wire at 100 amps or the fla of the transformer secondary? Thanks.
 
Take a look at 240.21(C).
I believe you will find you are in compliance using the panel main for conductor protection due to the close proximity.
You would size the conductor to the breaker since with transformer secondary conductors, the OCP at the termination provides the necessary protection for the conductors.
 
dftmaint said:
We have a buss disconnect with 70A fuses supplying a 45kva 480D/208Y/120 transformer that is feeding a 125 amp 208Y/120 panel that is located right next to the transformer. The panel has a 100 amp main breaker. Also the primary conductors are under 25ft.
Is overcurrent protection besides the main in the panel require here? Also do i size the wire at 100 amps or the fla of the transformer secondary? Thanks.
Click to expand...

No additional overcurrent protection is needed other than the main you have in the panel, which is protecting the secondary conductors.

The secondary conductors from the transformer to the Overcurrent device is more or less a tap, so, your fine with 100a rated conductors to the 100a main.

JAP>
 
augie47 said:
Take a look at 240.21(C).
I believe you will find you are in compliance using the panel main for conductor protection due to the close proximity.
You would size the conductor to the breaker since with transformer secondary conductors, the OCP at the termination provides the necessary protection for the conductors.
Click to expand...

For someone who jokingly brings up his age in his profile, your awfully quick for an old school poster like myself. :)


JAP>
 
I was thinking that i didn't need to provide another ocpd. Sometimes the more i read the NEC the more confused i get :lol: Thanks again.
 
jap said:
For someone who jokingly brings up his age in his profile, your awfully quick for an old school poster like myself. :)


JAP>
Click to expand...

I learned how to swoop in quickly from watching the vultures who circle over me as I garden :)
vulture.jpg
 
I have another question about this install. We had an extra 60amp buss disconnect but I told management the transformer requires 70 amp fuses. So we had to buy a 100 amp disconnect ($1500) to put the 70 amp fuses in. If I would have used the 60 amp disconnect with 60 amp fuses, what are the chances that nuisance tripping wouldn't have been an issue? Has anyone ever tried this? The panel will have led lighting, around 10 fans @ 5 amps each, and the rest duplex outlets.
 
The primary current at 45 kva load wo0uld be 54 amps. I think you would have been fine with the 60s
 
dftmaint said:
I have another question about this install. We had an extra 60amp buss disconnect but I told management the transformer requires 70 amp fuses. So we had to buy a 100 amp disconnect ($1500) to put the 70 amp fuses in. If I would have used the 60 amp disconnect with 60 amp fuses, what are the chances that nuisance tripping wouldn't have been an issue? Has anyone ever tried this? The panel will have led lighting, around 10 fans @ 5 amps each, and the rest duplex outlets.
Click to expand...

Once you get past the initial throw of the handle and inrush current to energize the transformer, for which fuses are well suited, very likely would never miss a beat with 60a fuses.


JAP>
 
augie47 said:
The primary current at 45 kva load wo0uld be 54 amps. I think you would have been fine with the 60s
Click to expand...
Plus with 100 amp secondary protection even though 45 kVA = 125 amps is also putting some limitation on loading.

jap said:
Once you get past the initial throw of the handle and inrush current to energize the transformer, for which fuses are well suited, very likely would never miss a beat with 60a fuses.


JAP>
Click to expand...
I agree, energizing current is when it is mostly likely to trip outside of fault conditions, so if it holds that it is probably going to be fine.

I have used 25 KVA single phase transformers (because it was there and not being used a the time) for temp power where we would have otherwise purchased a 10 kVA. Rated load is 52 amps @ 480 v, but never had problem energizing with a 30 amp breaker. I'd guess many smaller transformers (under 112.5 kVA at least) you likely don't have too much trouble too often with ~ 50% of rated load on primary overcurrent protection when it comes to just holding inrush current. Closer you are to the source though the less line impedance you will have and the higher the inrush possibly may be.
 
kwired said:
Plus with 100 amp secondary protection even though 45 kVA = 125 amps is also putting some limitation on loading.

I agree, energizing current is when it is mostly likely to trip outside of fault conditions, so if it holds that it is probably going to be fine.

I have used 25 KVA single phase transformers (because it was there and not being used a the time) for temp power where we would have otherwise purchased a 10 kVA. Rated load is 52 amps @ 480 v, but never had problem energizing with a 30 amp breaker. I'd guess many smaller transformers (under 112.5 kVA at least) you likely don't have too much trouble too often with ~ 50% of rated load on primary overcurrent protection when it comes to just holding inrush current. Closer you are to the source though the less line impedance you will have and the higher the inrush possibly may be.
Click to expand...

Also today’s energy efficient dry type transformers only see about 3 times FLA for inrush current & for just a few cycles - e.g. Eaton Cutler Hammer


Sent from my iPhone using Tapatalk
 
The panel is finished and it's reading correctly and looks good. I want to thank everyone who has replied.


There were 3 other panels in the building with the exact model number. The system bonding jumper in all of them were installed in the panels and not the transformers. I did it the same way. . I remember seeing a video where Mike Holt talks about putting a ground rod at the bottom of a light pole and the cute little 6awg wire. :lol:https://youtu.be/mpgAVE4UwFw?t=3178


Now this panel is mounted on i-beam with four beam clamps and unistrut. The neutral bonding strap is bonded to the case of the panel. I used a lug and beam clamp to attach the GEC to the same i beam that the panel is mounted on. I got to asking myself, is this 6awg GEC wire really necessary since it's already mounted on structural steel?
 
dftmaint said:
The panel is finished and it's reading correctly and looks good. I want to thank everyone who has replied.


There were 3 other panels in the building with the exact model number. The system bonding jumper in all of them were installed in the panels and not the transformers. I did it the same way. . I remember seeing a video where Mike Holt talks about putting a ground rod at the bottom of a light pole and the cute little 6awg wire. :lol:https://youtu.be/mpgAVE4UwFw?t=3178


Now this panel is mounted on i-beam with four beam clamps and unistrut. The neutral bonding strap is bonded to the case of the panel. I used a lug and beam clamp to attach the GEC to the same i beam that the panel is mounted on. I got to asking myself, is this 6awg GEC wire really necessary since it's already mounted on structural steel?
Click to expand...

I think you still need to install it. The GEC should run unspliced to the nearest available electrode. We generally like to use CGB fittings or similar to exit the enclosure. Is the I beam a legitimate electrode per 250.50? It may not qualify unless bonded to a ground ring, rebar etc.
 
dftmaint said:
Now this panel is mounted on i-beam with four beam clamps and unistrut. The neutral bonding strap is bonded to the case of the panel. I used a lug and beam clamp to attach the GEC to the same i beam that the panel is mounted on. I got to asking myself, is this 6awg GEC wire really necessary since it's already mounted on structural steel?
Click to expand...

You cannot use a standard beam clamp to attach a GEC to the beam. They do make listed connectors for this purpose.
 
Dale001289 said:
I think you still need to install it. The GEC should run unspliced to the nearest available electrode. We generally like to use CGB fittings or similar to exit the enclosure. Is the I beam a legitimate electrode per 250.50? It may not qualify unless bonded to a ground ring, rebar etc.
Click to expand...

Dale001289 said:

B-Line makes a UL listed beam clamp that would qualify. It has 'teeth' to bite into the cable and beam.
Click to expand...


Is this it? https://www.rexelusa.com/usr/Root-C...6-AWG-to-250-MCM,-Tin-Plated-Aluminum/p/76316

It says for cable tray. I cant find one listed for beams.

Found these

https://www.cesco.com/Thomas-Betts-...tTQhJKprFz8_790oj-RYbIQNdVQlsYdBoCp_MQAvD_BwE

https://www.panduit.com/en/products...universal-beam-grounding-clamps/gubc5006.html
 
I believe from subsequent posts that the OP does have an overcurrent device in the secondary panel, but from the wording in the first post, my immediate thought was to clarify that 125 amp panel meant a panel with a 125 amp breaker not a 125A bus.
 
Dale001289 said:
I think you still need to install it. The GEC should run unspliced to the nearest available electrode. We generally like to use CGB fittings or similar to exit the enclosure. Is the I beam a legitimate electrode per 250.50? It may not qualify unless bonded to a ground ring, rebar etc.
Click to expand...

The former maintenance manager told me years ago an electrical engineer came and verified from drawings and tests that the building was grounded to earth at many locations to main i-beam and joined at some huge ground rod network. They had to do it because the powder paint system was acting as though the parts were not getting an adequate ground. They found the problem was not electrical.
 
Status
Not open for further replies.
Top