Next higher standard ocpd

[Dsg319]

We have a building which we a supplying power to and will be fed from a 200amp sub panel. 208/120. I know in residential setting we can use 2/0 copper (which is below 200 Ampacity) wire because of the 83% rule.

But being that it’s a separate building and not residential would the wire Ampacity have to be 200 such as 3/0 which is greater correct? Or does this go into effect?

 

[roger]

It would have to be 3/0.

Roger

 

[Dsg319]

That’s what I was thinking.

 

[Dsg319]

Could someone give me a good example of the night higher size ocpd that you would see in the real world. I have never personally had to use it.

 

[Greentagger]

500 MCM rated at 380A can be used on 400A overcurrent if calculated load is less than 380 amps. Very common installation.

 

[petersonra]

Could someone give me a good example of the night higher size ocpd that you would see in the real world. I have never personally had to use it.

You don't have to use it. You can make the wire ampacity equal to or larger than the rating of the overcurrent protection device.

 

[Dennis Alwon]

You could use 4/0 aluminum if the calculated load is 180 amps or less.

 

[Cow]

500mcm cu for 400 amp and parallel 500 mcm for 800 amp is a standard install for us usually. Or 750 AL rather than 500 cu.

 

[frofro19]

You could use 4/0 aluminum if the calculated load is 180 amps or less.

On that note, could 2/0 CU be used on a 200 amp service if the calculated load is under 175 amps?

 

[Cow]

On that note, could 2/0 CU be used on a 200 amp service if the calculated load is under 175 amps?

175 amp is a standard overcurrent device size per 240.6, so no.

 

[Dsg319]

You could use 4/0 aluminum if the calculated load is 180 amps or less.

The sub panel ocpd is way over kill, could probably get by with way less, but leaving room for future. So with that being said is 2/0 copper still compliant?

 

[Carultch]

The sub panel ocpd is way over kill, could probably get by with way less, but leaving room for future. So with that being said is 2/0 copper still compliant?

If the ampacity of either the terminations or derated wire is an exact match the previous OCPD, 240.4(B) does not apply. This is what the issue is for 2/0 Cu, since it is rated for 175A at 75C terminations. You have to exceed the previous OCPD, in order to consider it protected by rounding it up to the OCPD you are really using.

There is no official threshold by just how much you need to exceed the previous OCPD to take credit for 240.4(B). Derate calculations might result in fractional amperes greater than a standard OCPD rating.

One example where 2/0 Cu is OK for a 200A feeder, is when it is a direct extension of 200A service conductors, where the 83% rule applies. In otherwords, if 2/0 was OK as a service conductor, it therefore should be OK as a feeder conductor if there is no reason for the current to be any different. All that happened is that it is now load side of the service disconnect instead of line side. By contrast, for a 200A feeder derived from a 400A dwelling service, it is no longer carrying the full load of the dwelling, and the 83% rule would no longer apply. It would need to be >175A, sized as a feeder in its own right.