Transformer secondary conductors landing on chiller

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xguard

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Baton Rouge, LA
I have a transformer that's 3 phase, 225 Kva, 480: 208/120. The transformer secondary conductors are landed on an 800 amp molded case switch inside a chiller. On the load side of the switch it splits to two 400 amp circuit breakers.

The chiller nameplate isn't stamped with an MCA or an MOP unfortunately. Following the manufacture's procedure for calculating MCA and MOP I came up with 514 amps @ 208 VAC and 624 amps for the MOP with the next size down being 600 Amps.

Currently this transformer is connected with parallel (two conductors per phase) 300 kcmil (combined ampacity of 570 amps).

A few things I'm concerned about. In meeting the MOP requirement, the manufacturer provided the 800 amp switch with the two 400 amp circuit breakers so I'm assuming that's correct. I would have thought installing a separate over current device at the chiller's listed MOP as redundant. The manufacture already provided over-current protection. Is an additional external over current device needed to meet the NEC MOP requirement?

In this case with the parallel 300 kcmil conductors landed on the 800 amp molded case switch. I can't find a section this could be okay under considering the conductors are coming off the secondary of the transformer. Even though the conductors meet the MCA I'm thinking they need a minimum ampacity of 800 amps. Thoughts? Something I'm missing?
Just for reference the primary overcurrent protection is 600 Amps.
 
I think MOP is 400A breaker, this is why you don't see MOP label on equipment. Even if you bring 2000A, the chiller is still protected by 400A breakers. (Ignoring 800A unfused switch)

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victor.cherkashi said:
I think MOP is 400A breaker, this is why you don't see MOP label on equipment. Even if you bring 2000A, the chiller is still protected by 400A breakers. (Ignoring 800A unfused switch)

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And you are correct. I thought he had an 800A fused switch or breaker.

No matter, you are also correct about multiple loads off the tranny. 240.21(C) allows it.

Either way would work.
 
I have 2 concerns. I left my code book at the job site so I can't check myself:

1. Regarding xformer secondary protection, can one use both note 1 and 2 together? He has two ocpd's serving as secondary protection and needs to next size up to get to 800.

2. I don't think the requirements for transformer secondary conductors are being met. The 25 foot rule states they must terminate on a single ocpd. I'm trying to cogitate on whether this would be ok under the 10 foot rule....
 
electrofelon said:
I have 2 concerns. I left my code book at the job site so I can't check myself:

1. Regarding xformer secondary protection, can one use both note 1 and 2 together? He has two ocpd's serving as secondary protection and needs to next size up to get to 800.

2. I don't think the requirements for transformer secondary conductors are being met. The 25 foot rule states they must terminate on a single ocpd. I'm trying to cogitate on whether this would be ok under the 10 foot rule....
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Each breaker is a load, 240.21(C) permits multiple loads. Not really different than coming off a meter base and feeding 2 panels.
 
jumper said:
Each breaker is a load, 240.21(C) permits multiple loads. Not really different than coming off a meter base and feeding 2 panels.
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I know you can serve multiple loads off the secondary, but I'm not sure it's kosher to have one set for a distance and then split it into two.

I agree the conductors feeding each 400 ocpd may be undersized, but the op never really said what size they were.
 
electrofelon said:
I know you can serve multiple loads off the secondary, but I'm not sure it's kosher to have one set for a distance and then split it into two.

I agree the conductors feeding each 400 ocpd may be undersized, but the op never really said what size they were.
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They are parallel sets. All one size I am assuming. All sets land in switch and then split up, I guess.
 
xguard, In meeting the MOP requirement, the manufacturer provided the 800 amp switch with the two 400 amp circuit breakers so I'm assuming that's correct.
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The tap conductors to the 400A breakers, were likely provided and installed by the manufacturer, and wiring could be a AWM type.

But to protect the transformer secondary, and the chiller loads, breaker-ed at 400A ea, the parallel runs to the chiller 800A switch, should have been 600KCM instead of the 300KCM, or put in a OC device that protects the transformer secondary and the paralleled feeder conductors.

Seems the the chiller runs OK at the ampacity provided, we have no confirmation on it. So it seems that the solution to make it compliant would be to install a OC device at the transformer, to simultaneously protect the secondary and the feeder conductors at their rating.

No length was mentioned on the tap length, so which tap rule applies is undetermined. But a OC device at the secondary would cover either case.
 
MTW said:
The tap conductors to the 400A breakers, were likely provided and installed by the manufacturer, and wiring could be a AWM type.

But to protect the transformer secondary, and the chiller loads, breaker-ed at 400A ea, the parallel runs to the chiller 800A switch, should have been 600KCM instead of the 300KCM, or put in a OC device that protects the transformer secondary and the paralleled feeder conductors.

Seems the the chiller runs OK at the ampacity provided, we have no confirmation on it. So it seems that the solution to make it compliant would be to install a OC device at the transformer, to simultaneously protect the secondary and the feeder conductors at their rating.

No length was mentioned on the tap length, so which tap rule applies is undetermined. But a OC device at the secondary would cover either case.
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Agree. I wish that I had paid attention to the conductor sizes better.

I was doing the math on the tranny and skipped right over that. Worse-the OP mentions the potential problem and I missed that also.:ashamed1:

Edit: If the wiring in the unit was factory wired, then simply adding one more 300 kcmil would give the OP 800 amps of conductors to the switch.
 
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Yep, secondary protection or increased feeder, pick your poison. No reworks are good. But the tap rule for the 25', terminating on one OC device could be a deal breaker for the increased feeder.
 
But then you would be eliminating the Mfg provided main 800A disconnect.

Two 300Kcm would work just as good as two 3/0, but then you still have that missing main problem.
 
MTW said:
But then you would be eliminating the Mfg provided main 800A disconnect.

Two 300Kcm would work just as good as two 3/0, but then you still have that missing main problem.
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Man, you and e-felon are killing me here.:D

Why would I upsize the wires to second breaker, just because the ones to the first are in my scenario.

Working on the switch problem.
 
If you could get the Mfg to approve changing the 800A molded case switch, to a 600A molded case breaker, that could solve the problem, if it was within the tap rule limits.
 
MTW said:
If you could get the Mfg to approve changing the 800A molded case switch, to a 600A molded case breaker, that could solve the problem, if it was within the tap rule limits.
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$$$$

That breaker ain’t cheap, but it is a simple solution. Wonder about coordination of the that breaker and the other two though.
 
According to the OP the main is only a molded switch, and yes no rework is ever cheep. Might not be easy to get Mfg approval either.

We still don't know the tap length, that is a factor in what is suitable.
 
MTW said:
According to the OP the main is only a molded switch, and yes no rework is ever cheep. Might not be easy to get Mfg approval either.

We still don't know the tap length, that is a factor in what is suitable.
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Yeah, I give up trying to find a easy solution.

I only really got involved for the transformer protection calcs.
 
jumper said:
Agree. I wish that I had paid attention to the conductor sizes better.

I was doing the math on the tranny and skipped right over that. Worse-the OP mentions the potential problem and I missed that also.:ashamed1:

Edit: If the wiring in the unit was factory wired, then simply adding one more 300 kcmil would give the OP 800 amps of conductors to the switch.
Click to expand...

I like that idea.
 
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