There's an exsiting 3-section switchboard, consisting of a 1600A main fusible switch, and two 1600A feeder distribution sections on the same lineup. The main is fed from a 1600A fusible disconnect switch from a separate distribution switchboard. I am planning on removing and replacing the 1600A main fusible switch section with a new 2000A rated MLO section and keeping the two existing 1600A distribution sections. The 1600A feeder switch will be replaced with a 2000A rated switch and associated feeder conductors. Out of the new 2000A section, I will install a new 1200A feeder breaker to sub-feed a new 1200A rated distribution board. Does this arrangement comply with code?
Dissimilar Bussing
- Thread starter 2450
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Dissimilar Bussing
I?m not sure I am understanding the proposed arrangement in the same way you intended to describe it. What I read is that a single 2000 amp OCPD will have downstream a 2000 amp bus and a pair of 1600 amp buses. What I also read is that there will be no OCPD between the 2000 amp bus and either of the two 1600 amp buses. By contrast, the new 1200 amp bus will have a 1200 amp OCPD between it and the 2000 amp bus. If this is correct, then I would say that it is not code-compliant. Either of the two 1600 buses could experience a fault of 1800 amps, resulting in their (possible violent) destruction, with no chance of having the event terminated by a trip of the only available protective device (i.e., the 2000 amp OCPD).
Did I misunderstand your proposed alteration?
I?m not sure I am understanding the proposed arrangement in the same way you intended to describe it. What I read is that a single 2000 amp OCPD will have downstream a 2000 amp bus and a pair of 1600 amp buses. What I also read is that there will be no OCPD between the 2000 amp bus and either of the two 1600 amp buses. By contrast, the new 1200 amp bus will have a 1200 amp OCPD between it and the 2000 amp bus. If this is correct, then I would say that it is not code-compliant. Either of the two 1600 buses could experience a fault of 1800 amps, resulting in their (possible violent) destruction, with no chance of having the event terminated by a trip of the only available protective device (i.e., the 2000 amp OCPD).
Did I misunderstand your proposed alteration?
- Location
- Massachusetts
Re: Dissimilar Bussing
Charlie I think he has 1600 amp fuses upstream from the new 2000 amp section.
[ August 20, 2003, 01:02 PM: Message edited by: iwire ]
Charlie I think he has 1600 amp fuses upstream from the new 2000 amp section.
I am confused why you would use a 2000 amp section downstream from a 1600 amp fuses.
[ August 20, 2003, 01:02 PM: Message edited by: iwire ]
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Dissimilar Bussing
Maybe yes, probably no.
Reduced bussing is allowed in a switchboard but it must follow the NEC "tap rules" which effectively force each reduced bus to terminate in a single OCPD.
So, yes, if switchboard is laid out like this:
[1600A OCPD][2000A MLO][1600A OCPD]
No, if like this:
[2000A MLO][1600A OCPD][1600A OCPD]
or
[1600A OCPD][2000A MLO][1600A OCPD][1200A OCPD]
Maybe yes, probably no.
Reduced bussing is allowed in a switchboard but it must follow the NEC "tap rules" which effectively force each reduced bus to terminate in a single OCPD.
So, yes, if switchboard is laid out like this:
[1600A OCPD][2000A MLO][1600A OCPD]
No, if like this:
[2000A MLO][1600A OCPD][1600A OCPD]
or
[1600A OCPD][2000A MLO][1600A OCPD][1200A OCPD]
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