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xformer

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Is it permissible by code to install a MLO panelboard, without a disconnect, to the secondary of a 480 primary / 208Y 120V secondary transformer?
 
post 1 states 208Y/120 post 3 states delta which is it ??
( somewhat a mute point as it can change the Art 240 rules, won't change the 408.36 rule)
 
augie47 said:
post 1 states 208Y/120 post 3 states delta which is it ??
( somewhat a mute point as it can change the Art 240 rules, won't change the 408.36 rule)
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I am trying to understand the validity of M.L.O. panels. I am familiar with the rules of Art 240 and 408.36. This is not an actual installation. Its more of just a brain storm.
 
Even if the primary OCPD protects the secondary winding of the transformer, the conductors to the panelboard would still be a tap and would have to terminate on an OCPD.
Or there would have to be OCPD to protect the panelboard at its bus rating. Maybe up to six handles of output breaker in the panelboard?
 
Because of the wording of 408.36, I don't think you will find an application that allows a MLO with greater than 6 throws of the handle and in terms of tap rules, 240.21(B)(1) is the only one, I believe, that allows termination in other than a singular overcurrent device.
 
jumper said:
I am unclear as to exactly what puzzles you about MLO panels.
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Are MLO panels designed to be used in conjunction with the requirement of 408.36?

I have observed M.L.O. panels installed with conductor termination on the main lugs leaving me to just wondering where the requirements of 408.36 went...
 
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xformer said:
I am trying to understand the validity of M.L.O. panels. I am familiar with the rules of Art 240 and 408.36. This is not an actual installation. Its more of just a brain storm.
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The sticking point in your example is the upstream transformer. If a panel is fed from an ATS, it would be perfectly fine to be MLO, and in some cases mandatory.
 
augie47 said:
can you elaborate
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I assume you mean on the 'mandatory' comment - I was pretty sure that hospital life safety and critical branch panels downstream from their respective ATS's were not allowed to have main breakers. But of course looking through Article 517 right now I can't seem to find it.

Edit: looks like I was mistaken - it doesn't appear to be in any codes or standards and just seems to be a good design practice.

I have half a mind to petition this to the NFPA 99 committee, though it should be mandatory for any emergency transfer switch.
 
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Pharon said:
I assume you mean on the 'mandatory' comment - I was pretty sure that hospital life safety and critical branch panels downstream from their respective ATS's were not allowed to have main breakers. But of course looking through Article 517 right now I can't seem to find it.

Edit: looks like I was mistaken - it doesn't appear to be in any codes or standards and just seems to be a good design practice.

I have half a mind to petition this to the NFPA 99 committee, though it should be mandatory for any emergency transfer switch.
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I don't see a problem one way or the other, the feeder to the downstream panel will have OCP anyways even if it's before the ATS

As far as the need to make a proposal, why would a designer put a main breaker in a panelboard that doesn't require it? ;)

As an aside, Isolation panelboards have mains even if supplied by the Critical Branch.



Roger
 
xformer said:
Are MLO panels designed to be used in conjunction with the requirement of 408.36?

I have observed M.L.O. panels installed with conductor termination on the main lugs leaving me to just wondering where the requirements of 408.36 went...
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Yes. See bold.

408.36 Overcurrent Protection. In addition to the requirement
of 408.30, a panelboard shall be protected by an
overcurrent protective device having a rating not greater
than that of the panelboard. This overcurrent protective device
shall be located within or at any point on the supply
side of the panelboard.
 
roger said:
I don't see a problem one way or the other, the feeder to the downstream panel will have OCP anyways even if it's before the ATS

As far as the need to make a proposal, why would a designer put a main breaker in a panelboard that doesn't require it? ;)

As an aside, Isolation panelboards have mains even if supplied by the Critical Branch.



Roger
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An OCPD immediately after an ATS is a single point of failure can will compromise the reliability of the loads served by the ATS. In a hospital this can cause devastating outages - trust me I've seen it happen. I was the senior electrical engineer for a major healthcare system in Boston for many years and after one such outage we changed our design standards to eliminate these OCPDs where ever possible.
 
Maders said:
An OCPD immediately after an ATS is a single point of failure can will compromise the reliability of the loads served by the ATS.
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The OCP ahead of the ATS is the same thing and can create the same problem. The question is, why did the OCP open in the first place? Was it an overload or a fault? But, back to my earlier post, why would a designer want to put a main where one is not wanted or needed?

Roger
 
roger said:
The OCP ahead of the ATS is the same thing and can create the same problem.
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Not quite the same problem if the opening of that breaker on the service input side of the ATS triggers the generator to start and pick up the load through the ATS.
You would need two points of failure to create the same outage as the single point downstream of the ATS.
 
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