MCB in MDP for Service

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Npstewart

Senior Member
Had a discussion with a colleague of mine. We have a 1600A service, all the panels down stream are being fed from the MDP with (5) 400A/3P circuit breakers. I am saying we don't need a MCB because we can shut the building down with the five breakers which is less then six. He is saying we need a MCB because the sum of the breakers is greater than the total service size. (ie. (5) x 400A = 2000A) which is greater then 1600A.

He is saying this is in the NEC. but I can't seem to find it anywhere. Ive been doing this a long time and never heard of this, has anyone else?

If hes right, what would be the code section?

Thanks Guys

PS. The load calc for each respective panel is less then 400A, far less in most cases. The total demand is also less then 1600A.
 

iwire

Moderator
Staff member
Location
Massachusetts
If this is really a service you are right, show him Exception No. 3 to 230.90(A)


Exception No. 3: Two to six circuit breakers or sets of fuses
shall be permitted as the overcurrent device to provide the
overload protection. The sum of the ratings of the circuit
breakers or fuses shall be permitted to exceed the ampacity
of the service conductors, provided the calculated load
does not exceed the ampacity of the service conductors.
 

Npstewart

Senior Member
Thanks iwire! I thought I was right. The demand calculation is the only thing that ever counts. Most times the sum of the breakers is greater then the main in any panel.

Some of the demand calcs are coming out to like 220 amps. We want to make sure they have room for future and the next standard size rating is a 400A anyway.

Thanks agin
 

iwire

Moderator
Staff member
Location
Massachusetts
230.95 Ground-Fault Protection of Equipment. Groundfault
protection of equipment shall be provided for solidly
grounded wye electric services of more than 150 volts to
ground but not exceeding 600 volts phase-to-phase for each
service disconnect rated 1000 amperes or more.
The
grounded conductor for the solidly grounded wye system
shall be connected directly to ground through a grounding
electrode system, as specified in 250.50, without inserting
any resistor or impedance device.
The rating of the service disconnect shall be considered
to be the rating of the largest fuse that can be installed or
the highest continuous current trip setting for which the
actual overcurrent device installed in a circuit breaker is
rated or can be adjusted.
 
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