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split bus rating


John Dirks Jr
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I'm not sure what you mean by the two main breakers? One would likely be feeding the "lighting" section, but the others are not really "mains" even though they are in the "main disconnect" section and are all part of the service disconnect. I suspect the other 50 is for the range. With a 150-amp panel and 2/0 AL SEC's, you have a 150-amp service.

Have a photo?

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Consider a GE split bus panel with a rating of 150A max. It is being fed by a 2/0 AL SEC. The two main breakers are each 50A.

Does this mean that the rating of this service is 100A?

If there are two main breakers of 50-amps each, then yes, it's a 100-amp service.

Are you sure that there weren't other main breakers? Most of the 150-amp split bus panels I've seen have at least 4 mains.

- Jim Katen, Oregon

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Hi Jim,

I was assuming there were more breakers on the upper bus than just the two. I've never seen a panel with just two "mains". But...here's a question. Supposing the other "main" spaces just aren't being used and he does in fact just have the two 50's in the main. Is that a 100-amp service, or a 150 service with unused spaces? I'm thinking the latter.

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Hi Jim,

I was assuming there were more breakers on the upper bus than just the two. I've never seen a panel with just two "mains". But...here's a question. Supposing the other "main" spaces just aren't being used and he does in fact just have the two 50's in the main. Is that a 100-amp service, or a 150 service with unused spaces? I'm thinking the latter.

The latter.

- JK in OR

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Consider a GE split bus panel with a rating of 150A max. It is being fed by a 2/0 AL SEC. The two main breakers are each 50A.

Does this mean that the rating of this service is 100A?

If there are two main breakers of 50-amps each, then yes, it's a 100-amp service.

Are you sure that there weren't other main breakers? Most of the 150-amp split bus panels I've seen have at least 4 mains.

- Jim Katen, Oregon

The labeling was poor but there were 5 240v's on top. The top two were 50's. Again, with poor labeling I'm not sure but the other 240's on the top were to electric island cook top, electric wall oven, and AC.

I'm feeling comfortable rating it at 100amp after what you said Jim.

My photo archive on this panel is lacking.

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That looks like a 150amp split-bus panel, not a 100amp panel. Just because it has two 50 amp main breakers does not make it a 100amp panel.

The top six breakers comprise of the service disconnect. Actually, the lower left "main" breaker has been split off into two breakers. Therefore, you now have more than six hand movements in order to shut off the power to the home. I would have written this up as a defect and encouraged the client to upgrade the panel, since there is no more room and there are double-taps.

Kevin

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I think Kevin is correct.

The large pair of SEC on the left feeds power to this panelboard from the meter. The upper right breaker is the disconnect for a large appliance or a sub-panel elsewhere in the house and the 2-pole on the upper left is the branch lighting/appliance cutoff. You can see the cables that backfeed the lower half of the panel behind all of that crap on the left. One pole of that middle-left breaker is still being used to power something so it does violate the 6-throw rule.

That's a pretty crowded mess; plus the double-lugged breakers.

Jim,

Would you say that this violates the fill rule? I think it's time for a new panel or the addition of a sub-panel.

ONE TEAM - ONE FIGHT!!!

Mike

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The service is 100 amps, assuming the two 50 amp mains are correct. The panel and the SEC are rated at 150 amps but the service is limited by the capacity of the mains. If there was only one 50 amp main, the service would be 50 amps, regardless of what the individual components are rated at.

BTW, is this split bus configuration common? The only split I have seen, a CH from the 60's, had a factory installed 200 amp main that fed a bus for the heavy appliance loads and the disconnects for the two other busses.

Tom

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No Tom,

That's not correct. This is a split-bus enclosure rated for 150 amps being fed by SEC rated for 150 amps. If there is no single main disconnect ahead of the panelboard - and there'd better not be recause the grounded and service-grounding conductors share the same bus - it's a 150 amp service.

In a split bus configuration where there isn't any single main disconnect breaker as in your example, the top six breakers (Or bottom, depending on which way the panel is installed) are collectively considered the main disconnect. The size of the individual breakers used for various appliances, or for the sub-main feed, in a split bus panel do not decide its rating. It is rated by comparing the rating on the panel to the size of the SEC coming to it. Whichever is lowest determines the service's rating.

For instance, if you have a panel rated for 150 amps but the SEC are only rated for 125 amps then you have a 125 amp service. If you have a panel rated for 125 amps but the SEC feeding it are capable of sustaining a 150 amp load you still have a 125 amp service because you're not allowed to exceed the rating for that panel.

Six throw main split bus panels are very very common around here in homes built prior to 1984.

ONE TEAM - ONE FIGHT!!!

Mike

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So if it is not a split we use the smaller of the panel, SEC and Main ratings; but for a split we omit the main because there isn't one single disconnect?

The panel I described is a rare bird then with a single disconnect, but a 200 amp service in a late fifties early sixties house was probably not all that common either.

Tom

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That looks like a 150amp split-bus panel, not a 100amp panel. Just because it has two 50 amp main breakers does not make it a 100amp panel.

The top six breakers comprise of the service disconnect. Actually, the lower left "main" breaker has been split off into two breakers. Therefore, you now have more than six hand movements in order to shut off the power to the home. I would have written this up as a defect and encouraged the client to upgrade the panel, since there is no more room and there are double-taps.

I agree.

That's a classic 150-amp split bus panel. There aren't 2 main breakers, there are 7 -- and there should only be 6. It looks like the house has outgrown the panel.

- Jim Katen, Oregon

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Any chance of getting CEU approval for participation here? I learned more in this thread than in my last 6 hour class. I'd be willing to settle for one credit a month. [:D]

Tom

Man, I wish. After doing this for nearly 8 years, I'd never have to spend another dime on CEU's for the rest of my life.

OT - OF!!!

M.

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Thanks for the help and discussion everyone. I was planning on writing up the panel for being over crowded and the double taps as well.

I wanted to make sure the information about the rating and other stuff I use to convey the message is as accurate as it could be.

I'm finishing the report now. I promise you though, even if nobody had replied to this post, this panel was destined to be written up. I knew that as soon as I pulled the cover.

I must admit I'm still getting used to examining the split bus design. After this thread, I can't wait to see the next one.

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