Sub Panel

[shorttermmem]

Add a 100A sub panel to a 2 section 3 phase 120/208 panel that is full.

1. Commercial installation / office building.
2. Existing panel (section 1&2) is located in a parking garage electric room.
3. New Sub Panel would be located in the same room as existing panels.
4. New proposed sub panel type: 3p4w...208y/120v...100A...MLO...w/1-2p20 and 16-1p20 (18 ckt panel)
5. Use 1-3p100 in LPA section 1 to feed new sub panel (relocate 3-1p20 receptacle ckt's to sub panel).
6. Red is the proposed sub panel. View attachment 13097

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I'm concerned that the sub panel would be to much for the existing panel to handle. I did what I think is a workaround if you don't know all the existing panel information. I just based everything existing off of 80% but I feel like I'm way off and a few things don't make sense to me. Below is the breakdown.

• Existing panel (LPA section 1) has a 225A main and is a 42 ckt panel.
• LPA section 1 has the following breakers. 1-3p225 Main, 25-1p20, 15-1p15, and 1-2p40
  • 20A*(25-1p breakers)*80%*120V = 48,000VA
  • 15A*(15-1p breakers)*80%*120V = 21,600VA
  • 40A*(1-2p breaker)*80%*208V = 6,656VA
  • Total = 76,256VA

• Existing panel (LPA section 2) is MLO and is a 42 ckt panel.
• LPA section 2 has the following breakers. 14-1p20, 12-2p30, and 2-2p40
  • 20A*(14-1p breakers)*80%*120V = 26,880VA
  • 30A*(12-2p breakers)*80%*208V = 59,904VA
  • 40A*(2-2p breakers)*80%*208V = 13,312VA
  • Total = 100,096VA

• New Sub Panel.I was thinking to just use the 100A sub panel as the design load. Exactly what this will be used for is unknown.
  • 100A*80%*208*1.732*1.25 = 36,025VA

• Total
  • 76,256VA + 100,096VA + 36,025VA = 212,377VA
  • 212,377VA / 208V / 1.732 = 589.51A

​With this total it doesn't look to me like there is any way I can add this panel? Am I even in the ballpark on this?
This brings me to my second question. If you take the LPA Section 1 total on its own it looks like a 225A main is correct but when you add LPA section 2 into the mix it doesn't seam to jive. On top of that LPA Section 2 alone looks more than a 225A Main can handle. I understand it was probably originally calculated at the demand load but with both panels together something doesn't look right.
Is there a solution for this?

LPA Section 1

• 76,256VA / 208 / 1.732 = 211.67A

LPA Section 2

• 100,096VA / 208 / 1.732 = 277.85A

Total

• 211.67A + 277.85A = 489.52A

 

[infinity]

Did you perform an actual load calculation based on the existing loads and Article 220?

 

[iwire]

I did what I think is a workaround if you don't know all the existing panel information. I just based everything existing off of 80% but I feel like I'm way off and a few things don't make sense to me.

Going by the handle rating minus 80% is meaningless. It really is, you would do as well by rolling a set of dice.

The only way to do this is per article 220.