480VAC PDB Questions

kjd0323

Member
Location
United States
Occupation
Controls Engineer
Long time lurker first time poster. I am a controls engineer with a heavy background in programming and automation. My question is this:
We have a machine that has been requested to keep the 24VDC "UP" during maintenance periods. The team wants to put a PDB inside the main panel PRIOR to the main disconnect. This PDB will feed the 24vdc psu with 480. The idea being we need another machine to monitor the safety circuit on this equipment during maintenance intervention via PLC and other networking devices.

This project just seems like a bad idea, I do not know the NEC code that well. What are you guys thoughts?
 

kjd0323

Member
Location
United States
Occupation
Controls Engineer
I do not believe that is a option at this time. It's fed from a small mc. What are the concerns here with putting a PDB in the panel and feeding the power supplies? This is just my curiosity. And thanks for the reply.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I do not believe that is a option at this time. It's fed from a small mc. What are the concerns here with putting a PDB in the panel and feeding the power supplies? This is just my curiosity. And thanks for the reply.
Details.

I thought you were tapping ahead of the service equipment. Taps are allowed but there are a lot of details we don't know. You need an electrician involved.
 

jap

Senior Member
Occupation
Electrician
I do not believe that is a option at this time. It's fed from a small mc. What are the concerns here with putting a PDB in the panel and feeding the power supplies? This is just my curiosity. And thanks for the reply.

To me, by putting a PDB ahead of the control cabinet's disconnect, it would be the false sense that you've killed all the power inside the cabinet (other than the supply voltage to the line side of the disconnect) by shutting off the disconnect.


I could see something going very wrong very quickly in this scenario.
Especially with it being 480v power.

Jap>
 

kjd0323

Member
Location
United States
Occupation
Controls Engineer
I picked this project up from another engineer that retired.
1. 100 amp breaker is feeding the 480 volts to the control panel via #3
2. The control panel has a disconnect on the load side of the disconnect is 60 amp fuses
3. The engineer wanted to put a distribution block before the disconnect inside the panel for just the 24vdc power supplies
4. Coming off the block the tap conductor will land on a 10 amp MCB via #12
5. Load side of 10 amp MCB then feeds the individual breakers for the 24vdc power supplies ( 2 power supplies each on their own OCPD I think 2 amps or something )

Clear as mud I'm sure
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It is a common practice to tap the line side of the main disconnect of the control panel for this purpose. Many mccb lugs come with tapped holes on the lugs for this very purpose.

A label indicating this was done is in order and the wiring that remains live with the disconnect open should be run in yellow or orange.

Keep in mind it is a tap conductor and must follow the tap conductor rules.

Also, if you go with a distribution block you will likely find the sccr of the db might be an issue.
 
Last edited:

kjd0323

Member
Location
United States
Occupation
Controls Engineer
It is a common practice to tap the line side of the main disconnect of the control panel for this purpose. Many mccb lugs come with tapped holes on the lugs for this very purpose.

A label indicating this was done is in order and the wiring that remains live with the disconnect open should be run in yellow or orange.

Keep in mind it is a tap conductor and must follow the tap conductor rules.

Also, if you go with a distribution block you will likely find the sccr of the db might be an issue.
The DB spec'd out is 10ka SCCR / 350 amps

Disconnect is rated for 100ka and the load fuses after disconnect are 10ka so the DB has the same rating as the fuse holder
 

jap

Senior Member
Occupation
Electrician
Seems to me it would be much safer to have the power supplies outside the control cabinet and run the 24v control power inside the cabinet if need be.

oh well,,,

Jap>
 
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