Automatic Transfer Switch OCPD

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rjken1969

Member
Location
Princeton, NJ
I'm installing a 1MW generator to service (2) 800A sub distribution panels. The panels are existing and both have main lugs, also they are fed from a switchboard that is located in the same elec/mech room. The new generator and ATSs will also be located in the same elec/mech room as the existing panels that will be serviced.

Is there a requirement for an ATS to have an OCPD after it before the MLO panel? It seems that the feeders are protected by the generator breakers and the main switchboard breakers. I can't find anything that calls for protection at the sub distribution level.

How about if the generator is outside and the ATSs are inside, is an OCPD required then?

Thx-
 

rcwilson

Senior Member
Location
Redmond, WA
Just one ATS feeding two MLO panels? As long as the normal power circuit breaker and the generator circuit breaker are sized to protect the feeders to the two panels, no seperate OCP is needed.

If this is 480V, the 1 MW generator will have a 1200A or 1500 A circuit breaker and the feeders to the two 800 panels have to be sized for 1200 or 1500 A, unless they meet the tap rules.
 

templdl

Senior Member
Location
Wisconsin
I'm installing a 1MW generator to service (2) 800A sub distribution panels. The panels are existing and both have main lugs, also they are fed from a switchboard that is located in the same elec/mech room. The new generator and ATSs will also be located in the same elec/mech room as the existing panels that will be serviced.

Is there a requirement for an ATS to have an OCPD after it before the MLO panel? It seems that the feeders are protected by the generator breakers and the main switchboard breakers. I can't find anything that calls for protection at the sub distribution level.

How about if the generator is outside and the ATSs are inside, is an OCPD required then?

Thx-

You omitted to state what type of an ATS you are refering it,
If the ATS is a breaker type then it is possible to include a TM feature with each breaker.
 

rjken1969

Member
Location
Princeton, NJ
These will be a standard open trasition ATSs. Not service rated. These are sub distribution panels not main.

There will be (2) ATSs, each one will be feeding an 800A sub distribution panel. Each will be fed from an output breaker on the generator, and the main distribution. These are main lug panels, I am unclear of an OCPD is required on the load side of the ATS, before the MLO 800A panels.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I see a couple of issues. 408.36 requires that a panleboard be protected by an overcurrent device having a rating no greater than the panelboard, so if the generator breaker is greater than 800 amps there is a violation.
As far as the generator being outside and the ATS inside, unless you meet one of the exceptions (and I imagine you do), 225.32 might give you a problem.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
These will be a standard open trasition ATSs. Not service rated. These are sub distribution panels not main.

There will be (2) ATSs, each one will be feeding an 800A sub distribution panel. Each will be fed from an output breaker on the generator, and the main distribution. These are main lug panels, I am unclear of an OCPD is required on the load side of the ATS, before the MLO 800A panels.

There should be an 800A breaker or fuse on the normal supply to each ATS, and an 800A breaker or fuse on the emergency supply to each ATS.

If you have that, you don't need any additional 800A breakers or devices on the load side of the ATS to protect the panels - they are protected by the breakers on the input side of the ATS.
 

rjken1969

Member
Location
Princeton, NJ
My concern isif the breaker on the main switchboard is opened to service the subdistribution board, the ATS will initiate the generator and reenergize the subdistribution board. Therefore, both the switchboard breaker and the generatorbreaker must both be opened to service the sub distribution board.

If cost, and space, weren't an issue I would just install a disconnect and moveon with life. But this client is really concerned with keeping cost down.

I was unsure if there was a code section that prevented a situation like this. I can't seem to find anything.

Other than marking the panel with a placard - 702.7.

 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
My concern isif the breaker on the main switchboard is opened to service the subdistribution board, the ATS will initiate the generator and reenergize the subdistribution board. Therefore, both the switchboard breaker and the generatorbreaker must both be opened to service the sub distribution board.

If cost, and space, weren't an issue I would just install a disconnect and moveon with life. But this client is really concerned with keeping cost down.

I was unsure if there was a code section that prevented a situation like this. I can't seem to find anything.

Other than marking the panel with a placard - 702.7.

No, there isn't any section in the NEC that says you have to have a single disconnect for the distribution panel.

I prefer not to put a main circuit breaker on the load side of an ATS. If there is a short on a branch circuit, and the main breaker isn't selectivly coordinated, the short could trip both the main breaker and the branch breaker. If that happens, the generator doesn't have any chance to pick up any of the load on the ATS. The generator doesn't even see an outage. And even if it did see the outtage and start, it can't supply any of the load on the ATS due to the tripped main breaker.
 
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