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Load Center Questions

Posted by Jeff (My Page) on
Wed, Dec 29, 10 at 9:01

I recently purchased an older home that still has a fuse box in it that I would like to upgrade to circuit breakers. It is a 100A service with a main disconnect breaker in its own box right where the service comes in to the garage. After the main disconnect it goes into the fuse box. I've been researching load centers and have a few questions.

- I understand the amperage rating on a main breaker corresponds to the tripping point of the breaker. What is the limiting factor in the amperage rating of a main-lug box? Lug size? Conductor size?

- I'd like to buy a load center bigger than 100A so I can fairly easily upgrade my service level in the future (probably to 150A). Is there a problem buying a main-lug or main-breaker box with a higher current rating and connecting it to a 100A service? The box will still be protected by the 100A main breaker upstream.

- Do the ground and neutral have to be bonded in the 100A disconnect box (the box closest to the service entrance) or can it be bonded in the new circuit panel?

Any help would be appreciated.


Follow-Up Postings:

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RE: Load Center Questions

Yes, in a main lug box the rating is a declaration that the components there in (the lugs, the bus bars, etc...) are capable of safely handling that amount of load.

You can put as big of a panel as you want in. The main breaker here is only protecting the panel as you are correct that your existing disconnect is providing the protection for the service and the feeder to current fusebox or your replacement.

The existing 100A *IS* your service disconnect. The equipment grounding conductor (ground) and the grounded conductor (neutral) must be connected there *AND NOT ANYWHERE ELSE* such as in your new panel. Depending on what manufacturer you are dealing with you may have to remove a jumper or screw, and in the case of Square D, install an additional bus bar.


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RE: Load Center Questions

You may find a panel with a main costs less than a lug panel.

Volume manufacture favors the panels with a main.


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RE: Load Center Questions

Thanks for the response. Another question:

Most load center manufacturers advertise their centers as something like 12 space / 24 circuit capacity. I assume this is because you can put two half-inch circuit breakers into one space. Occasionally I see a box where the number of circuits is not double the number of spaces (like 20 space / 30 circuits). Why is this? What prevents you from putting two half inch breakers in each slot?


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RE: Load Center Questions

What prevents you from putting two half inch breakers in each slot?

The geometry of the breaker and the bus bar


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RE: Load Center Questions

"Occasionally I see a box where the number of circuits is not double the number of spaces (like 20 space / 30 circuits). Why is this? What prevents you from putting two half inch breakers in each slot?"

Depending on the manufacturer, some tandem breakers can only be installed in bus bar slots that are notched to accept them. Where that's the case, a panel may have some slots that are notched and others than are not -- a design feature that serves to limit the total number of circuits.

If you look at the printed information inside the panel cover, you may see either a schematic or a table indicating that certain slots can be used to feed two circuits (i.e., will accept tandems) and others which cannot.

Also keep in mind that a normal two-pole 240V breaker takes up two slots but only counts as one circuit. Thus a 20-space box allowing for 30 circuits is at full circuit capacity when there are 5 two-pole breakers plus 10 tandems.


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Whoops!

One of those times I wished I could edit a post...

Thus a 20-space box allowing for 30 circuits is at full circuit capacity when there are 5 two-pole breakers plus 10 tandems.

Strike that. :-)

What was I thinking? Obviously, I need to give myself a remedial arithmetic lesson.

5 two-pole 240V circuits plus 10 tandems is only 25 circuits. DOH!


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RE: Load Center Questions

Also keep in mind that a normal two-pole 240V breaker takes up two slots but only counts as one circuit. Thus a 20-space box allowing for 30 circuits is at full circuit capacity when there are 5 two-pole breakers plus 10 tandems.

In a 20/30 box you have 20 full size positions but the capacity for 30 OCD's (NOT CIRCUITS!). The code counts a two-pole (e.g., 240V) breaker as two ocd'S for the limit purpose.


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