main breaker or not?

[royta]

I've always been confused on whether I need a main breaker for a sub-panel.

Situation 1 - Residential main service with a feeder breaker feeding sub-panel mounted within or on home. Is there a situation when the sub-panel needs a main?

Situation 2 - Residential main service with a feeder breaker feeding sub-panel mounted on seperate structure or somewhere else on property. Sub-panel only needs a main IF there are more than six breaker handles (twin counts as two handles), correct?

Situation 3 - 400A residential main service with first 200A main supplying the built-in distribution. Second 200A main feeding 20/40 sub-panel located on other side of house. Sub-panel has conduit provisions to supply future sub-panels located on other structures or areas of property.

Situation 4 - Commercial main service on exterior of building. Individual feeder (main) breakers feeding sub-panels located in each commercial suite. Every sub-panel I've ever seen in these situations always has a main breaker.

 

[charlie b]

I've always been confused on whether I need a main breaker for a sub-panel.

It?s actually simple. You never need a main breaker. OK, there is probably some obscure exception that someone will find and point out to me. But until then, I?m sticking with ?never.?

In your situation #2, you may need a disconnecting means either outside or inside the separate structure. A main breaker in the panel may or may not count as the required disconnecting means. But you don?t have to use a main breaker as the disconnecting means.

In your situation #4, each tenant must have access to the overcurrent devices serving their space. But they are not required to have access to the main breaker. It is just a convenience, and I think I would use main breakers here. But you don't have to.

 

[royta]

Good deal. So...

In 100% of residential applications, where the sub-panels are located on/in the same structure as main service, a disconnecting means is not necessary.

In residential applications where sub-panel is detached from structure that holds main service, then a disconnecting means is only necessary if sub-panel has more than six individual breaker handles.

In commercial applications, tenant just needs to be able to disconnect power from the busses on their respective panel.



In commercial applications and residential sub-panels that are detached from structure having main service, main breakers may not be necessary, but they do happen to be a convenient means of disconnecting, as it takes up no additional equipment mounting space.



Thank you for the explanation.

 

[charlie b]

Not so fast, if you please.

In 100% of residential applications, where the sub-panels are located on/in the same structure as main service, a disconnecting means is not necessary.

What I said was that the sub panel does not have to have a main breaker of its own. The feeder from the main panel to the sub panel will have to have overcurrent protection. It is usually a breaker in the main panel that serves this need. That same breaker also serves as a means for disconnecting power to the sub-panel.

In residential applications where sub-panel is detached from structure that holds main service, then a disconnecting means is only necessary if sub-panel has more than six individual breaker handles.

No. A separate structure does need a disconnecting means.

Please read 225.30 through 225.33. In a separate structure, you must have a means to disconnect all power to the structure. In certain cases, you can use up to six breakers (and/or disconnects) to count as the required means. In other cases, you need an additional disconnect (inside or outside), even if you have only six breaker handles in the panel that serves the structure. For example, if the enclosure with the six breaker handles is not located near the point at which the conductors enter the building, then the six breakers do not qualify as the required disconnecting means.

In commercial applications, tenant just needs to be able to disconnect power from the busses on their respective panel.

I mis-spoke on that one. My apologies. Each tenant must have access to the main disconnecting means, so that they can shut off (or turn back on) power to their spaces. See 225.35. But that does not mean their panel has to have a main breaker. It?s convenient if it does. But if they can have access to a disconnect or breaker located outside their own spaces, that will count. It would also count if the disconnect to each tenant space is in a room that is locked, but that can be opened at any time (i.e., 24/7/365) by an authorized representative of the building management.

 

[rlMutch]

The disconnecting means for a seperate building or structure is required to be suitable for use as service equipment per 225.36. Most main lug loadcenters have a label on them that says something like this:

A) UL Listed as suitable for use as service equipment with field-installed service disconnect.

B) UL Listed as suitable for use as service equipment when not more than six service disconnecting means are provided and when not used as a lighting and appliance branch circuit panelboard.

So...IMHO, If a main lug loadcenter is equipped with a backfed main breaker, it qualifies as the building disconnect. If not, even with 6 or less breakers installed, it is probably still being used as a lighting and appliance branch circuit panelboard, would not be suitable for use as service equipment, and therefore would not qualify as the building disconnect.