Incoming MCCB ?

[19781997]

I have a general question regarding incoming MCCB's in boards/panels.
In our company we face the following issue a lot. In order to save money, incoming MCCB to a board/panel is often omitted and an isolator is used instead. Is this the correct practice? What is exactly the purpose of incoming MCCB if there is already an MCCB upstream, on the other end of the panel's power supply cable.

 

[DRWeig]

Hi 19781977,

In general, if the upstream breaker and cable are sized to protect your panelboard in accordance with code, there is no need for the breaker at your panel.

If, for some reason, the upstream feeder is of higher ampacity than the panelboard and is protected at a higher level (say, a 400A breaker upstream feeding a 225A panel), you'll need a main breaker for the panel to protect it at its lower ampacity rating.

If the feeder to the same panel is protected at 225A, there is no need for a main breaker at the panel, it can be main lugs only. In fact, unless you have a special safety reason for including a disconnect switch at the panelboard, it's not needed either.

That's my experience and opinion. Best to wait for others to respond, I may be missing something. My experience is in facility engineering, in which I'm dealing with feeders to lighting and receptacle circuits, or air conditiong equipment, etc... Can't tell from your sketch what sorts of loads you may be powering.



Good on ya,

Goober Dave

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[ScottyUK]

You might need the local MCCB if you have a high fault level and you're relying on the series rating of an MCCB and MCB, unless the upstream device is acceptable and tested for series operation.


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If we learn from our mistakes I'm getting a great education!

 

[19781997]

Thanks for replies.

Dr.Weig, but is the cable protected in case without incoming MCCB? What happens in case of fault on panel busbars? Could the incoming MCCB minimize the damage and/or protect the cable?

ScottyUK, what exactly do you mean by "unless upstream device is tested for series operation"? Selectivity should be there, ofcourse.

 

[DRWeig]

Hi again,

Yes, the cable and the panel bus bars are protected by the upstream feeder breaker, as long as that breaker is sized appropriately to do so. Remember, the fault current originates at the source, it doesn't matter if you interrupt it at the panel or at the feeder connection upstream.

I'll let Scotty confirm his part of the thread, but I think it refers to a higher fault current than the panel is rated to withstand. In that case, you can cascade two breakers (as in your diagram) to reduce the let-through. Breakers arranged like this for that purpose must be listed for use in series.



Good on ya,

Goober Dave

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[ScottyUK]

Spot on Dave. Thanks.

19781997 (what the heck does that signify?) - The only tested combinations of MCB and MCCB tend to come from the same vendor. This means that unless the upstream protective device at the source happens to use an MCCB listed for use in a series combination with whatever type of MCB is in your panelboard then you can't assume that the series rating exists at all, and you therefore must assume that the MCB is on its own when assessing breaking capability.


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If we learn from our mistakes I'm getting a great education!

 

[eeprom]

Two breakers in series would be useful for service, and also possibly for safety. For example, if the upstream MCCB is not a lockable device (meaning you can securely lock it in the open position), then there would be no safe way to work on the electrical at the sub panel (the downstream breaker). In this case, it would be a code violation to omit the downstream sub panel breaker.

EE

 

[19781997]

Thanks to all three.
But, by eeprom's logic, I can just put an isolator and that's it. Why then, people use MCCB's as incomers???

PS: 19781997 are two years .

 

[eeprom]

That's not my logic, that's a code issue. As long as you can safely lock out the sub panel (at the sub panel or at the feeder breaker), and the sub panel is properly protected by the upstream breaker, then it is acceptable by code.

EE

 

[DRWeig]

You usually only find MCCB (panelboard main breaker) when the feeder is tapped from something nearby, or in a residential situation in which the utility service comes straight into the panel from the meter.

In the US, NEC allows you to tap a feeder with a lower-ampacity sub-feeder to a panel, as long as you don't exceed a certain distance. It's only allowed if there's a protective device downstream within 10 feet or 25 feet (the longer distance requires larger sub-feeder conductors than the shorter one).

Another common occasion is downstream of a dry-type transformer in a building (say, converting 480/277 3-phase to 208/120 3-phase to feed receptacle circuits. We usually stick our panelboard close enough to the transformer secondary to use a main breaker rather than a separate one (there's a distance limitation there, too).

Or, in the instance that Scotty outlined.

Hope this helps!



Good on ya,

Goober Dave

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