Service breaker and another article 450 questions

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notmyspace

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Here is another overcurrent protection based on article 450 and 240 and service breaker panel

I have a 75kva transformer 480V delta to a 208/120 WYE. On the primary side, it?s connected to a service disconnect 100A with (3) #1 and (1) #8. On the secondary side, it?s connected to a 250A breaker service panel with (4) 250KCMIL + (1) #4 GND.
Based on article 450, I should be ok right?
saw article 450, there are two methods


Article 450:
Method 1: Primary protection only ? set no more than 125% primary current rating, and does not required secondary protection

Method 2: Secondary Protection set no more than 125% provided the primary current protection set less than 250%.

Question 2: Interpret article 450?

Question 3:
I have a 200A breaker panel that I want to up to a 250A. Can I just replaced the main breaker instead of replacing the whole panel?

Thanks
 
notmyspace said:
Here is another overcurrent protection based on article 450 and 240 and service breaker panel

I have a 75kva transformer 480V delta to a 208/120 WYE. On the primary side, it’s connected to a service disconnect 100A with (3) #1 and (1) #8. On the secondary side, it’s connected to a 250A breaker service panel with (4) 250KCMIL + (1) #4 GND.
Based on article 450, I should be ok right?
saw article 450, there are two methods


Article 450:
Method 1: Primary protection only – set no more than 125% primary current rating, and does not required secondary protection

Method 2: Secondary Protection set no more than 125% provided the primary current protection set less than 250%.

Question 2: Interpret article 450?

Question 3:
I have a 200A breaker panel that I want to up to a 250A. Can I just replaced the main breaker instead of replacing the whole panel?

Thanks
Click to expand...

I think the "ground" conductor on the secondary side should be #2 AWG (from T250.66), but the primary and secondary transformer & conductor protection look OK.

For the panel, Art 408 requires that a panelboard be protected by an OCPD having a rating not greater than the rating of the panelboard. If the panelboard is only rated 200A, you can't change the main breaker to 250A (unless for some odd reason it was protected upstream at 200A, but then you wouldn't gain anything be increasing the MCB size.)
 
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david luchini said:
I think the "ground" conductor on the secondary side should be #2 AWG (from T250.66), but the primary and secondary transformer & conductor protection look OK.

For the panel, Art 408 requires that a panelboard be protected by an OCPD having a rating not greater than the rating of the panelboard. If the panelboard is only rated 200A, you can't change the main breaker to 250A (unless for some odd reason it was protected upstream at 200A, but then you wouldn't gain anything be increasing the MCB size.)
Click to expand...

I thought based on tab 250.122, my ground cable from transformer should be #4 since the breaker panel is to served equipment not actually AC system coming from outside?

So, I can't upgrade the panel board from 200A to 250A? I wasn't clear on the question, (sorry about that). The panel board is feed from the 75KVA transformer. So in summary

100A (service disconnet primary) to a 75KVA transformer 480 delta with (3) #1 & (1) #8 gnd to a 250A panel board (assumed we can replaced just breaker not panel too) with (4) 250 KCMIL & (1) #4 gnd
 
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Yo are using the wrong Table for your secondary ground sizing. The correct Table is 250.66.
A good way to remember is to think if the corresponding conductors have OCP AHEAD of them. Transformer secondary conductors do not have OCP ahead of them so 250.66 applies
Your Code reference woild be 250.30 and 250.102(C)

As far as your panel is concerned, upsizing the main would probably be viewed as alteration to listed equipment. You might check the manufacturer specs and see what amperage buss is actually in the panel. If it has 225 amp bussing it might be looked at in a more favorable light.
 
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augie47 said:
Yo are using the wrong Table for your secondary ground sizing. The correct Table is 250.66.
A good way to remember is to think if the corresponding conductors have OCP AHEAD of them. Transformer secondary conductors do not have OCP ahead of them so 250.66 applies
Your Code reference woild be 250.30 and 250.102(C)

As far as your panel is concerned, upsizing the main would probably be viewed as alteration to listed equipment. You might check the manufacturer specs and see what amperage buss is actually in the panel. If it has 225 amp bussing it might be looked at in a more favorable light.
Click to expand...

Please forgive my stupidity and dumb questions
Thanks, you are right.

As for upsizing the panel, since the existing panel is 200A, and the total calculated load on the panel with 125% on continous load is 202A, Can I upsizing to 225A? Where in the manufacturer spec to show the bus bars? I can't find it. Do you have an example?
 
You would probably need to obtain panel specs thru your manufacturer rep.

If I might suggest, since you have complied with the primary 125% rule, you can add a second panel to the secondary and avoid the overload situation all together. Just be sure to comply with 240.21(C) rules.
 
How can you simply replace the 200 amp CB with a 250 amp? Does the existing 200 amp panel have 250 amp bus?
 
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