Sub Panel - Electrician Talk - Professional Electrical Contractors Forum

Electric pole · 10-24-2009, 05:24 PM

I have two questions

1) Is it against code for a 100 amp Main panel to feed a 100 amp sub panel?

2) Is it against code for a 100 amp sub panel to feed a 100 amp sub panel. In this case the first sub panel is being fed from a 200 amp main panel.

My friend and I are having a difference of opinion on these 2 questions. We are not finding anything in the code to explain it one way or the other. What do you think? Thanks

rewire · 10-24-2009, 05:45 PM

Quote:

Originally Posted by Electric pole

I have two questions

1) Is it against code for a 100 amp Main panel to feed a 100 amp sub panel?

2) Is it against code for a 100 amp sub panel to feed a 100 amp sub panel. In this case the first sub panel is being fed from a 200 amp main panel.

My friend and I are having a difference of opinion on these 2 questions. We are not finding anything in the code to explain it one way or the other. What do you think? Thanks

you can feed all the sub panels you want as long as you do not exceed your service load or the overcurrent device for the sub panel.

Electric pole · 10-24-2009, 06:22 PM

Thanks iwire. It makes sense. although I do believe there should be a limit. It's not something I would practice.

even though everything would be per code when I finished the job, I would worry about the customer adding circuits on there own in the future without doing a load calculation. I think it would be a disaster waiting to happen. thanks

Bob Badger · 10-24-2009, 06:41 PM

Quote:

Originally Posted by Electric pole

Thanks iwire. It makes sense. although I do believe there should be a limit. It's not something I would practice.

even though everything would be per code when I finished the job, I would worry about the customer adding circuits on there own in the future without doing a load calculation. I think it would be a disaster waiting to happen. thanks

I think it would be a disaster waiting to happen.

A little dramatic don't you think?

If you have a 40 circuit 200 amp panel filled with 20 amp breakers is that also a disaster waiting to happen?

If you look at a large commercial building do you think the service size exceeds the total rating of all the subpanels?

The ONLY thing that matters is the calculated load, when people add circuits in the future it is their responsibility to do load calculations.

Magnettica · 10-24-2009, 10:32 PM

One time I fed a 200 MB panel with #6 and 40 AMP overcurrent protection. Is that a code violation?

TOOL_5150 · 10-24-2009, 10:34 PM

Quote:

Originally Posted by Magnettica

One time I fed a 200 MB panel with #6 and 40 AMP overcurrent protection. Is that a code violation?

Nope. [Possibly] poor design, but its OK with the NEC.

~Matt

Magnettica · 10-24-2009, 10:42 PM

Very poor design.

Magnettica · 10-24-2009, 10:44 PM

Only a #10 cu for the equipment grounding too. Not a violation.

TOOL_5150 · 10-24-2009, 10:56 PM

Quote:

Originally Posted by Magnettica

Very poor design.

I got to ask, why would you do such a thing anyway?

~Matt

Magnettica · 10-24-2009, 11:01 PM

Doing what I was told to do by the last EC I worked for is all.

william1978 · 10-25-2009, 12:01 AM

Quote:

Originally Posted by Magnettica

One time I fed a 200 MB panel with #6 and 40 AMP overcurrent protection. Is that a code violation?

One time at band camp.

Electric pole · 10-25-2009, 12:55 AM

Bob, I guess I got a little carried away by calling it a disaster waiting to happen. I agree with you that the load calculation is the number one concern.

I also understand and agree that who ever adds to the load in the future should do a load calculation first. Of course, we both know that will not always happen.

I just don't think it's a good idea to push it to the limit.

wingz · 10-25-2009, 06:46 AM

Quote:

Originally Posted by Electric pole

I just don't think it's a good idea to push it to the limit.

Depends on what you mean by pushing the limit.

I have a 200 amp main panel with MB and feed through lugs. I have a 60 amp breaker in that panel that goes to a transfer generator panel. The lugs feed a 200 amp MB sub-panel. I have a 100 amp and a 50 amp sub-panel fed from the 200 amp sub-panel.

Bob Badger · 10-25-2009, 07:01 AM

Quote:

Originally Posted by Electric pole

I just don't think it's a good idea to push it to the limit.

What is being pushed to the limit?

Consider this.

Imagine a single family home, this home has 100 'outlets' divided evenly on 10 circuits so each circuit has 10 outlets.

Now imagine the exact same home rewired with each outlet on a home run so now we have 100 circuits feeding the 100 outlets.

Which set up draws more power from the utility?

Electric pole · 10-25-2009, 12:43 PM

Hi Bob, as I said before, I understand and agree that the load calculation is the main concern.

If I had a 100 amp main panel, I would not feed a 100 amp sub panel that would feed another 100 amp sub panel that would than feed another 100 amp sub panel. even if the numbers checked out for the load calculation. especially if the third sub panel had available spaces when I finish the job. If the spaces are available, The wrong guy will probably use them without a load calculation. I understand at that point it is no longer my concern. It may be overkill, but that's how I roll.

One more example- I also would not send the same circuit to a microwave, dishwasher, and disposal even if the customer promised me he would not use all three at the same time. It's just an example

Have a good one.

Magnettica · 10-25-2009, 01:28 PM

Bob's point was that the load is still the same regardless of how many panel are used.

Quote:

Originally Posted by Electric pole

One more example- I also would not send the same circuit to a microwave, dishwasher, and disposal even if the customer promised me he would not use all three at the same time. It's just an example

Because that would be a code violation. NEC 210

Bob Badger · 10-25-2009, 01:44 PM

Quote:

Originally Posted by Electric pole

If I had a 100 amp main panel, I would not feed a 100 amp sub panel that would feed another 100 amp sub panel that would than feed another 100 amp sub panel. even if the numbers checked out for the load calculation.

Unless your adding load, there is no reason not to, none at all

Quote:

especially if the third sub panel had available spaces when I finish the job. If the spaces are available, The wrong guy will probably use them without a load calculation. I understand at that point it is no longer my concern. It may be overkill, but that's how I roll.

Of course your free to roll that way.

But the bottom line is it is not prohibited by the NEC, it is not unsafe or necessarily a bad design.

Quote:

One more example- I also would not send the same circuit to a microwave, dishwasher, and disposal even if the customer promised me he would not use all three at the same time. It's just an example

Again you may be free to do that but that has nothing to do with what the code may or may not require.