code requirement for subpanel ampacity

[BKLELEC]

Is there a specific code for the ampacity of a subpanel?

 

[maghazadeh]

If I undrestand your question well, The ampacity of panel/subpanel is determined by the following factors.
Expected/ connected load calculation
Future expansion capacity desired and/or required
If the load consider to be continouse or noncontinouse
This is my best shot at it, hope it helps.

 

[iMuse97]

Is there a specific code for the ampacity of a subpanel?

must meet minimum requirements for connected and/or calculated load.

 

[jumper]

408.30 General. All panel boards shall have a rating not
less than the minimum feeder capacity required for the load
calculated in accordance with Part III, IV, or V of Article
220, as applicable.

 

[charlie b]

Is there a specific code for the ampacity of a subpanel?

The previous two posts have your answer. But let me just mention that it is inappropriate to use the word "ampacity" in this context. It is more to the point to call it the panel's rated current.

 

[jumper]

The previous two posts have your answer. But let me just mention that it is inappropriate to use the word "ampacity" in this context. It is more to the point to call it the panel's rated current.

LOL, I knew MR.B was gonna say this.

I made the mistake of using the word "ampacity" when speaking to a college physics professor.

He looked at me like I was from Mars.

 

[steve66]

If you happen to be refering to the ampacity of the feeder supplying the panel, 225.39 might apply if the subpanel happens to be in a detached garage, or some similar building.

Steve

 

[BKLELEC]

It doesn't seem to me the code would allow to just use the subpanel's rating or calculated load. This would leave the door open to continuously add multiple subpanels originating from just one main panel. For example, start with a 200amp panel in the house, then add a 150amp subpanel in a garage, then add a 100amp subpanel to the first subpanel to feed a shed, and then add a 60amp subpanel from the shed to feed a hot tub; or is there another code reference to prevent this?

 

[Dennis Alwon]

It doesn't seem to me the code would allow to just use the subpanel's rating or calculated load. This would leave the door open to continuously add multiple subpanels originating from just one main panel. For example, start with a 200amp panel in the house, then add a 150amp subpanel in a garage, then add a 100amp subpanel to the first subpanel to feed a shed, and then add a 60amp subpanel from the shed to feed a hot tub; or is there another code reference to prevent this?

You can do that as long as the calculated load for the entire place is 200 amps or less. The load is the issue not the number of cb's. I could have a circuit breaker (circuit) for each recep. that does not mean the load is the sum of the breakers.

 

[roger]

As the others have stated, it is the "calculated load". You could put 100's of panels downstream of the main if you chose to, which is the case in many facilities.


Roger

 

[malachi constant]

It doesn't seem to me the code would allow to just use the subpanel's rating or calculated load. This would leave the door open to continuously add multiple subpanels originating from just one main panel. For example, start with a 200amp panel in the house, then add a 150amp subpanel in a garage, then add a 100amp subpanel to the first subpanel to feed a shed, and then add a 60amp subpanel from the shed to feed a hot tub; or is there another code reference to prevent this?

Dennis is correct. There is nothing wrong with having a 200A service with numerous subpanels that add up to something much greater than 200A.

Restrictions:
1. Each subpanel must be rated greater than or equal to the calculated loads that will be put on it.
2. The main panel must be rated greater than or equal to the calculated loads that will be put on it (including subpanel loads).
3. Must be able to add a breaker with appropriate current rating in the main to feed the sub. For example I've seen an instance in the commercial world where you have an 800A main panel and want to add a 600A subpanel - but cannot find a 600A breaker that is listed for use within that 800A main panel.

I remember designing a police station, got a call from the electrician saying we had an 800A service, but the panel had (6) 200A breakers in it. He was convinced this was not allowed - that when you add up the breakers they need to be equal or less than the size of the main. So we needed a 1200A service or had to get rid of a couple of the 200A breakers. The total calculated load was only 400A or so, so in reality we could have got by with an even smaller service than 800A. He ended up installing it per plans. Cuckoo!

 

[iMuse97]

It doesn't seem to me the code would allow to just use the subpanel's rating or calculated load. This would leave the door open to continuously add multiple subpanels originating from just one main panel. For example, start with a 200amp panel in the house, then add a 150amp subpanel in a garage, then add a 100amp subpanel to the first subpanel to feed a shed, and then add a 60amp subpanel from the shed to feed a hot tub; or is there another code reference to prevent this?

Those who have already responded to your question here are correct.

To add to that: when speaking of the sub-panel rating or the calculated load it must be clear that these are two entirely different issues.

Code actually doesn't allow you just to use the sub-panel ratings; it requires you to use the "calculated load."

The real issue is that "calculated load" is a technical term describing the mathematical result of calculations done according to the NEC directives. A good way to start to learn to find this number would be to get an NEC Handbook and follow the examples given there. The get some MH materials on service sizing/etc. and continue your learning.

As an example of why the rating of a panel and the calculated load are two different animals, take this as an example:

I have a main panel: Rated at 200A. This panel was installed in the house based on requirements for that structure of that size. This does not mean that the panel will ever "see" a 200A load. I could turn on every electrical device at the same time and never total even 80A, even though my house has a 200A panel.

I also have a 100A rated sub-panel supplied from the 200A main above. This panel supplies a pool pump, some backyard lighting, and various power outlets in an attached garage. This panel never sees more than 20A total load. The reason I know this is that the feeders to this panel are #10AWG, and the breaker is a 2P 20A, installed on the cheap by an electrician I know who was unemployed at the time.

The total rating for the panels, in an additive function, is 300A; however, since one is downstream from the other, the 200A main would trip if we ever loaded these things to capacity.

On the other hand, as I have hinted above, I assure you that the "calculated load" for my house isn't more than 125A based on the NEC, and the actual loads are much less than that.

 

[BKLELEC]

Thanks all for the info. It does make sense to use the calculated load when you think it through.