Subpanel on a Subpanel

[Gary Chester]

Let's see if I can explain this properly... and this is kind of a concept question more than a what size breaker question.

I have a 70 amp breaker in my house that feeds a subpanel in the detached garage/shop. That's enough POWER for my one man operation, but there aren't enough slots in the subpanel for all the dedicated circuts I'd like to have.

But... what if I put, say a 50 amp breaker in the subpanel that fed a secondary subpanel with three 20 amp breakers... I would then have enough slots for the the dedicated circuts.

Of course each breaker in the entire circut would be sized properly for the wire it feeds, this follows the logic of "the breaker is there to protect the wire... not what is plugged into the circut".

Right off the bat I have a feeling that the code says there is a breaker limitation for the primary subpanel... maybe six 15 amp breakers is OK (total of 90 amps), but four 15 amp and one 50 amp is not (total of 110 amps).

This plan seems logical to me... but may not to the inspector.

Experts... what say you... and TIA

[Rollie Meyers]

Forget about adding up the combined rating of the circuit breakers, you need to compute the load as the the combined rating of the circuit breakers is meaningless/useless....

[Roger Frazee]

Gary

Are you sure your panel is full? Do you know if you can install single pole tandem breakers? If your not sure get the model number and maufacturer off the cover sheet on the panel door and post it for us.

[Robert Malone]

Gary

Are you sure your panel is full? Do you know if you can install single pole tandem breakers? If your not sure get the model number and maufacturer off the cover sheet on the panel door and post it for us.

That's what I did Gary. Replaced the single pole breakers with tandoms.
And also gives you the option for a couple of 20amp circuits.

http://www.lowes.com/lowes/lkn?actio...5CP&lpage=none

[Bob Abbott]

A few years my panel was full and the county inspector would not let me use tandems, because he said the panel was at max. I don't know if that was code or he was simply having a bad day. The final solution was to pigtail together wires from several under loaded breakers freeing up a couple breakers.

[Rollie Meyers]

A few years my panel was full and the county inspector would not let me use tandems, because he said the panel was at max. I don't know if that was code or he was simply having a bad day. The final solution was to pigtail together wires from several under loaded breakers freeing up a couple breakers.

A panel is marked as to the number of twin breakers allowed & that number may not be exceeded, if one looks at the label on the panel there be marking such as 12-,20-40 20-20, etc..., the 1st number is the number of full size spaces & the 2nd is the maximum number permitted overcurrent devices, some do not allow any twin breakers.

[Gary Chester]

Thanks Rollie and all,

I think we're getting closer to the bit that I am concerned about being a code problem. The primary subpanel is older and I don't remember seeing any designations as you suggest Rollie, but I'll look again.

I think the big suspicion I have is yet unanswered... If my primary subpanel is a typical 100 amp panel, does code limit the total possible amperage out? In other words, could I legally put 300 amps worth of breakers in that 100 amp panel? I think not... I'm thinking there is a ratio... maybe a 100 amp panel can legally support 150 amps worth of breakers since under normal circumstances less than 100 amps would normally be drawn... see what I'm getting at?

To the rest, I've rewired many times, with half space breakers etc without the success I'm looking for. The big problem is that I need more dedicated 240v circuts, and my panel only has room two of those. The sub/subpanel I suggested would be a 240v panel only.

Isn't electricity fun!!! Again, thanks for your suggestions and ideas.

[Roger Frazee]

Gary

Looks like you were up early during your post.

What we thought was that the addition of tandem breakers would free up enough space in your sub panel to give you the spaces you needed for your 240 volt branch circuits. Looks like your not going to be able to do that ......soooo.

My advice would be to hang a bigger sub-panel with more spaces than the one you already have in the detached shop.

Not wanting to do that then...I think maybe we should answer your big question then go from there.

When adding a sub panel there are a few things you consider to determine the maximum your feeder can be to the sub-panel from your first sub-panel. Every sub-panel has what is called a ' bus stab rating '. On older panels this may not be known.. so generally speaking the maximum rating of a 100 amp panel per bus stab is 70 amps. But most modern panels say from 1970 on will have the bus stab rating listed on the cover specifications sheet of the panel door or it will be a sticker on one of the side walls of the panel. For example it will say " sum of branch breakers not to exceed 70 amps per bus stab". What this means in your case is if I install a 70 amp double pole breaker and the maximum rating per bus stab is 70 amps then I cannot have any breakers adjacent to that 70 amp breaker fastened to the same bus stab.
I'll attach a drawing to illustrate. In the drawing the sum of the two single poles is 70 amps this is the maximum sum of breakers for the panel bus stab rating. The 70 amp double pole below it also equals the maximum bus stab rating...the difference is there cannot be any breakers in the adjacent two spaces or the bus stab rating will be exceeded.





Any way this would be the maximum feeder you could run to your added sub-panel. BTW this is an often overlooked restriction....

Now in a more real world way...you can have as many breakers in the sub-panel as you like as long as the calculated load that those breakers serve does not exceed the feeder capability serving the sub-panel.. hope that makes sense. So in the above example if I was using the 70 amp breaker to feed my new sub panel from the existing sub panel I could have 200 amps of summed breakers in the added sub panel as long as the loads they serve do not exceed my feeder rating.

So in a nutshell you need to do what is called a demand load calculation on your main panel... then a demand load calculation on both your sub-panels add the three together and they cannot exceed your homes service panel rating of 100 amps or 200 amps whatever that may be. And the demand load of the new sub-panel and existing sub panel at the shop cannot require a breaker larger than the 70 amp breaker feeding the shop from your house panel.

Hope that makes sense.

One last thing .... it is permissible to hang a 100 amp rated sub panel next to your existing sub panel as long as you do not feed it with a breaker that is larger than the bus stab rating of your existing sub panel.

The second sub panel does not need to have a main breaker just simply an mlo (main lug only) panel. The detached shop should have a building disconnect located at the shop that kills all power to both sub-panels. This can be a main breaker in the existing sub or a remote disconnect located at the nearest point of entrance of the feeder into the shop. The breaker in the house panel will not satisfy the disconnect requirement for the shop electrical supply.

You will probably want a 100 amp second sub panel in order to get enough spaces for your needs. A 60 and 70 amp sub-panel will only have 4 spaces max. Or in other words all spaces would be used up by two double pole breakers.

If your shop is fed with 4 wires H-H-N-Grd from the house panel then you would feed your new sub panel with 4 wires from the existing sub-panel in the shop keeping the neutrals and grounds separate. The shop needs a grounding electrode system.

If your shop is fed with 3 wires H-H-N and no metal conduit has been used between the house and the shop to run the feeder inside then you will keep neutral and ground bonded in both sub panels. Also if you do have 3 wires then there cannot be any existing or added metallic paths like water lines or phone data that are bonded to the grounding electrode system at both the house and shop. If there is then a 4 wire feeder is required or if in the future you add a metal water line or add phone or data lines a 4 wire feeder will be required at that time.

[David Hostetler]

Dumb question here, but why not save the time, money, and effort you would spend on doing the sub of a sub thing, and just run one panel of sufficient capacity to your needs?

I'm not even sure what you are wanting to do would fly with an inspector...

[JohnT Fitzgerald]

My advice would be to hang a bigger sub-panel with more spaces than the one you already have in the detached shop.

+1 especially if the existing subpanel is 'old' and you have concerns about it...

[Tom Henderson2]

A panel is marked as to the number of twin breakers allowed & that number may not be exceeded, if one looks at the label on the panel there be marking such as 12-,20-40 20-20, etc..., the 1st number is the number of full size spaces & the 2nd is the maximum number permitted overcurrent devices, some do not allow any twin breakers.

For the slow kid in the class.... when you say "overcurrent devices" is that the same as "circuit breaker"?

Is the limit just on the number of breakers, and not the amperage of the breakers?

Thanks...

-T

[Rollie Meyers]

For the slow kid in the class.... when you say "overcurrent devices" is that the same as "circuit breaker"?

Is the limit just on the number of breakers, and not the amperage of the breakers?

Thanks...

-T

Yes, the NEC has had a limit of 42 spaces/breakers/overcurrent devices in a panel for many years this requirement was removed in the 2008 edition but the repeal does not apply to existing gear + manufacturers have to begin supplying them & not every area has adopted the 2008 NEC yet so still not allowed to use a larger panel until it has been adopted.....

[Don Jarvie]

+2 on replacing the panel.

Install a new 100 amp panel with 20 slots and use your 70 amp breaker as the main in the panel.

Panel will probably cost around 100 bucks and you will have plently of room for all your circuits.

[Bryan Rocker]

One other unpleaseant thing the 2008 code requires is arc fault CI for all circuits and tamper proof receptacles for 110v circuits. I just ran into that this week as I the electricians came out Monday to finish the rewire but I had the wrong receptacles.......The Arc fault breakers set me back $300 alone........