MCB vs MLO + 10' tap rule

E

elecshop

Member
Location
FL, USA
Occupation
Electrical Engineer
I have this configuration in the back of house of a restaurant:
  • Panel H, powered from disconnect and meter in a separate electrical room in the building.
  • Panel H feeds transformer TX (480-208/120V)
  • Transformer TX feeds panel A
  • Panel A feeds panel B
My questions are:
  1. Can Panel H be MLO? I have seen panels away from their switchgear breakers / service disconnect and still configured as MLO without a main breaker.
  2. If TX is around 3' away from Panel A, I imagine the feeders from the secondary of TX going to A to be run above the ceiling and then down to A. So the run should not be more than 10'. Thus, conforming to the 10' tap rule, TX should not require an OCPD on its secondary, right?
  3. Can Panel A be MLO, or it should have an MCB? Since it has no direct OCPD ahead of it except for the sub-feed for TX in Panel H, I figured MCB would be reasonable.
I could not find an NEC article that addresses the above. Any reference would be appreciated.
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
In general all panels require an OCPD ahead of them so if panel A is MLO then you'll need an OCPD between the panel and the transformer.
 
E

elecshop

Member
Location
FL, USA
Occupation
Electrical Engineer
infinity said:
In general all panels require an OCPD ahead of them so if panel A is MLO then you'll need an OCPD between the panel and the transformer.
Click to expand...
What about panel H? The OCPD ahead of it is in a separate room i.e., the service disconnect in the electrical room relatively far from the space.
 
C

CoolWill

Member
Location
Alabama
Occupation
Electrical Contractor
As long as it has a disconnect somewhere in the building, panel H doesn't need a main breaker.
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
elecshop said:
I have this configuration in the back of house of a restaurant:
  • Panel H, powered from disconnect and meter in a separate electrical room in the building.
  • Panel H feeds transformer TX (480-208/120V)
  • Transformer TX feeds panel A
  • Panel A feeds panel B
My questions are:
  1. Can Panel H be MLO? I have seen panels away from their switchgear breakers / service disconnect and still configured as MLO without a main breaker.
  2. If TX is around 3' away from Panel A, I imagine the feeders from the secondary of TX going to A to be run above the ceiling and then down to A. So the run should not be more than 10'. Thus, conforming to the 10' tap rule, TX should not require an OCPD on its secondary, right?
  3. Can Panel A be MLO, or it should have an MCB? Since it has no direct OCPD ahead of it except for the sub-feed for TX in Panel H, I figured MCB would be reasonable.
I could not find an NEC article that addresses the above. Any reference would be appreciated.
Click to expand...
Most everything I am adding has been touched on or stated:

Panel H doesn't need overcurrent protection at the panel. The upstream breaker protects the feeders to the panel. As stated, the code reference is 408.36.

The 10 foot tap rule is acceptable when the ungrounded conductors are less than 10 feet long from termination to termination. However, 240.21(C) (6) which is a 25 foot tap rule can also apply.

You will notice that one condition of both the 10 foot and the 25 foot tap rule are that they terminate on an OCPD that is less than or equal to the ampacity of the conductors. Oddly , they are written differently but the results are the same.
 
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