480VAC PDB Questions

[kjd0323]

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?

 

[ptonsparky]

Sounds to me like you need an additional Service Disconnect. You may have up to 6 in one location. Not as simple as just adding a PDB.

 

[kjd0323]

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.

 

[zbang]

PDB?

May not be an NEC concern but more of an OSHA and LOTO concern.

 

[ptonsparky]

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]

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>

 

[ptonsparky]

The 24 volts isn't the issue so much as the the 480 supply to it. How is it tapped? Where? Is there physical room? Protection? Distances?

 

[kjd0323]

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]

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.

 

[kjd0323]

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]

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>