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Old 05-04-2011, 03:05 PM   #1
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Split Bus Panel


Hoping someone could advise me, as I have never had to deal with this type of panel before. The service coming into the house is 100amp. The panel is a split bust with 6 double pole breakers on top with 1 of them labeled main lighting which is a 60amp breaker feeding the single pole breakers/receptacles on the bottom of the panel.

Question: because the lower portion of the panel can theoretically be pulling 60 amps and the service in 100 amps service, can the upper portion of the panel, the 5 remaining double pole breaker spots, are they limited to a total of 40amps? Otherwise it would seem that you could be vulnerable to an over-current situation of pulling more than 100amps total. Is this a correct assessment? It seems 40 amps is pretty low for the double breaker positions where the range and furnace or hot water heater are attached?

Thanks for all your help

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Old 05-04-2011, 03:12 PM   #2
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You do not add up the circuit breaker handles to determine the service size. The 60 amp allows up to 60 amps per leg to flow into the lower buss. It does not mean that 60 amps is always flowing into the buss.

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Old 05-04-2011, 03:32 PM   #3
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Thanks Jim,

Since the lower portion of the panel can have an actual 60amps of current flowing at any given time, do I have to limit the upper portion so that no more than 40amps will ever be flowing (not handle values, nut actual current in use) on the upper bus?

Thanks
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Old 05-04-2011, 06:03 PM   #4
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, do I have to limit the upper portion so that no more than 40amps will ever be flowing (not handle values, nut actual current in use) on the upper bus?
No you don't, but I am curious as to how you think you would achieve this.
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Old 05-04-2011, 06:12 PM   #5
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I would make sure that all the loads attached to the upper panel if all were on at the same time would not exceed 40 amps. If I don't need to limit it to 40 amps like you are saying, doesn't that mean an over-current on the entry wires could technically occur?

Thanks so much for your help
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Old 05-05-2011, 12:47 AM   #6
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Split Bus Panel


This is actually an interesting question, and one which I wouldn't mind having a good answer to for future reference.

The main service wires must be protected by downstream OCPD, generally a main breaker sized appropriately for the wire feeding it.

So in the case of a split-bus panel with multiple mains, how does one ensure appropriate fusing for a given service size? Would a load calculation have to be done to ensure correctness? I suppose a load calc should be done on any panel prior to adding circuits, but in the typical case of a main-breaker panel, the worst that is likely to happen in an overload situation is a tripping main. With a split bus on the other hand, I could see how carelessness could create a hazard.

Add an electric drier, electric range, and electric furnace to a 100A split bus panel, and along with the general loads from the "lighting" section I could easily see the service conductors loaded to 200A or more.

So, for the pros: How do you determine if a split-bus service panel is overfused? I think my preferred solution if I ever had to deal with one would be "replace it!" but that's not always an option for everybody.
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Old 05-05-2011, 05:58 AM   #7
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So in the case of a split-bus panel with multiple mains, how does one ensure appropriate fusing for a given service size? Would a load calculation have to be done to ensure correctness? I suppose a load calc should be done on any panel prior to adding circuits, but in the typical case of a main-breaker panel, the worst that is likely to happen in an overload situation is a tripping main. With a split bus on the other hand, I could see how carelessness could create a hazard.

Add an electric drier, electric range, and electric furnace to a 100A split bus panel, and along with the general loads from the "lighting" section I could easily see the service conductors loaded to 200A or more.
So what would you consider this "hazard" to be? Do you think the wires will spontaneously combust if loaded to over 100A?

And if they were loaded to over 100A, how long do you think this would go on in a home??
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Old 05-05-2011, 02:22 PM   #8
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I'd imagine that with 200A of continuous load, #3 copper could overheat, though I honestly don't know if it would be enough to start damaging the insulation.

And after thinking about it a bit I do see that it would be pretty unlikely for a service to be that overloaded for very long at a time.

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