Feed thru Lugs Help

[Electricalhelp]

I have a MDP with 208V/400AMP/3 Phase
But I need to connect (2) [Label Sub Panel as A and B] 208V/150AMP/1 Phase Panel to the MDP.
But the MDP only has one provision for a large frame breaker.
Would I be able to install a 208V/300AMP/1 Phase breaker on the MDP to connect to Sub Panel A.
From Panel A, could I use feed thru lugs of 208V/300/1 Phase to Sub Panel B.

I have never design a system like this.

Also instead of 300AMP/1 Phase, would I be able to do it with 300AMP/3 Phase breaker as well.

 

[infinity]

If you have 3 phase available why does the design have you installing single phase panels?

 

[Electricalhelp]

If you have 3 phase available why does the design have you installing single phase panels?

Since the other 2 Panels only have single phase loads.

If it did have 3 Phase or 1 Phase loads on the Subpanels, would I be able to use the feed thru lugs method I mentioned above.

 

[augie47]

It would make more sense for you to use 3 phase subpanels...there is no reason you can't feed single phase loads off a 3 phase panel.
You can feed either or both panels off a 300 amp breaker but each subpanel would need 150amp OC protection and if your feed conductor was rated less than 300 amps you will need to follow the 240.21 tap rules.

 

[Electricalhelp]

Would this riser diagram make sense?

 

[infinity]

Would this riser diagram make sense?
View attachment 2568390

You would need a main OCPD in Panel B and the tap rules would apply to the #1/0's. The #6 EGC run with the tap conductors is too small.

 

[augie47]

If you install a 300 amp breaker in MDP to feed MLO Panel A and a 150 amp breaker in Panel A to feed Panel B that will work.
The panels must be protected by an overcurrent device not greater than tne panel rating either at the source or internal with the panel.
IF you make Panel A or B M/B panels then the tap rules will apply.

(I thought infinity was off line so I answered..he's available and I will bow out and let you two continue)

 

[Electricalhelp]

You would need a main OCPD in Panel B and the tap rules would apply to the #1/0's. The #6 EGC run with the tap conductors is too small.

I can change Panel P to 300 AMP MCB
But when I look at Table 250.122, I went to 200 AMP since 1/0 is good for 150AMP. Hence how I get #6 EGC.

But should I size it to #4 EGC?

 

[infinity]

I can change Panel P to 300 AMP MCB
But when I look at Table 250.122, I went to 200 AMP since 1/0 is good for 150AMP. Hence how I get #6 EGC.

But should I size it to #4 EGC?

If you change the panel B to 300 amps and increase the feed-through conductor size to 350 kcmil you will not need a main CB. If you keep the #1/0's then you have tap conductors which must terminate in an OCPD. The EGC for the tap conductors is based on the OCPD ahead of the tap which is 300 amps so yes a #4 would be required.

So you have a few options based on your sketch in post #5:
1) 300 amp OCPD in service>350 kcmil to MLO Panel A>feed-through #1/0's (tap conductors) to 150 amp Main CB in panel B. Tap rules apply.
2) 300 amp OCPD in service>350 kcmil to MLO Panel A>feed-through 350 kcmil to 300 amp MLO in panel B.

 

[Electricalhelp]

If you change the panel B to 300 amps and increase the feed-through conductor size to 350 kcmil you will not need a main CB. If you keep the #1/0's then you have tap conductors which must terminate in an OCPD. The EGC for the tap conductors is based on the OCPD ahead of the tap which is 300 amps so yes a #4 would be required.

So you have a few options based on your sketch in post #5:
1) 300 amp OCPD in service>350 kcmil to MLO Panel A>feed-through #1/0's (tap conductors) to 150 amp Main CB in panel B. Tap rules apply.
2) 300 amp OCPD in service>350 kcmil to MLO Panel A>feed-through 350 kcmil to 300 amp MLO in panel B.

So if I use option 2, I don't have to worry about the tap rule and can use the same feeder for A and B.

Thanks for clarifying on how to use the tap rule for sizing for #4 EGC

 

[infinity]

So if I use option 2, I don't have to worry about the tap rule and can use the same feeder for A and B.

Yes, if every component after the OCPD that is ahead of the feeder is rated for 300 amps or more you won't need to address any tap rules and you can use MLO panels without a main circuit breaker.

 

[Rock86]

Just to add to the process... if the designer told you to use 1-phase panels originally, have them check their load calculations and see if you can come down to like a 200A.

(Rough numbers) 150A * 80% = 120A --> 120A * 208V 1ph = 25kW load --> 25kW / .360 (208V 3-ph) = 70A

 

[jap]

I'd first find out if the provisional space is 300a rated.
Some max branch sizes in 400a Panelboards are only rated up to 225 amps.

I'd also try to encourage you to use a 3p breaker and 3ph panels for the added panels A and B regardless if all the new loads are 1ph.

Yes, if you size the wire and whatever is downstream to whatever size Branch Breaker you decide to put into Panel MDP, then your dealing with a feeder and not a tap.

Jap>