Replacing old motor starter loads with VFD loads from existing MCC

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Junior_EE

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Working on the renovation of a 20-30 year-old existing chiller plant. The chiller plant has a bunch of motor loads connected to a 1200Amp MCC. We are replacing a few of the motors and installing VFDs for each of the new motors.

I usually do a lot of new construction work within the last ten years, so I am not used to renovation-type work and I'm not used to seeing many/any MCCs, so I don't really know what's possible.

Can I install a regular thermal-magnetic circuit breaker within each existing MCC cubicle that no longer serves a motor and connect the breaker to the new switch/VFD/motor?

Or am I better off tapping the main 1200Amp MCC bus and running the power to a new local distribution panel to feed each of the new VFDs/motors?
 
Generally, you are not allowed to "tap" the bus at all, short of adding a feeder breaker bucket.

There is no good reason you cannot remove the motor starter and overload from a bucket and reuse the existing TM circuit breaker to feed the new VFD if the rating is acceptable. You might get an inspector who thinks otherwise though.

Be careful though, you might have an MCP in the bucket which generally cannot be used with a VFD and would need to be replaced with a TM CB.
 
If what you have won’t work, It used to be that you could purchase MCC buckets fairly reasonably. Now I have heard there are supply chain issues. If you are not modifying existing bucket and using existing TM CB you should be ok.
 
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There are several shops where you can send your existing buckets to be modified or modernized under NRLT labeling procedures.
 
petersonra said:
Generally, you are not allowed to "tap" the bus at all, short of adding a feeder breaker bucket.

There is no good reason you cannot remove the motor starter and overload from a bucket and reuse the existing TM circuit breaker to feed the new VFD if the rating is acceptable. You might get an inspector who thinks otherwise though.

Be careful though, you might have an MCP in the bucket which generally cannot be used with a VFD and would need to be replaced with a TM CB.
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A feeder breaker bucket would be ideal, where I can install it and send the feed to a new downstream panel. Is this a common off-the-shelf product?
 
Junior_EE said:
A feeder breaker bucket would be ideal, where I can install it and send the feed to a new downstream panel. Is this a common off-the-shelf product?
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Best bet is to buy one from the MCC maker. Potentially you could also get one from a third party as Jim Dunger suggested. You will need to take a close look at the loading of the bus bars to make sure you are not overloading them.
 
ptonsparky said:
In the past it was cheaper to buy a whole column vs one bucket. IDK what things are like nowadays.
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Good point. Sometimes true.

However, if you buy a new section you will have to pour a new base for that section and then get it lined up with the existing MCC so they can bolt together. That generally involves having to get another contractor involved.
 
petersonra said:
Good point. Sometimes true.

However, if you buy a new section you will have to pour a new base for that section and then get it lined up with the existing MCC so they can bolt together. That generally involves having to get another contractor involved.
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Some times you can take the old out and slap a new in. Depends on what you need and what you have.
 
Depending on the brand of the old MCC, it may be difficult to find a new Feeder bucket ready-made. Allen Bradley are the only ones who have not changed their MCC design since inception, so if that's what you have, you can just order a new bucket(s) and they will fit. But some brands have changed a lot over the years and obsolete their old designs, others no longer exist at all. Eaton does make a lot of retrofit buckets for older styles of MCC though. The parts inside will be Eaton or course, but they make the bucket frames and stabs to match the old stuff. https://www.eaton.com/content/dam/e...market-buckets-quick-selector-ca04304001e.pdf

But it is often very possible to still get a replacement Thermal-Mag breaker that is the same frame as an older MCP used in a starter, so it just comes down to how comfortable you are at disassembling and reassembling the mechanical parts. It's not hard, just tedious sometimes.

Side note; my "rule of thumb" on buying replacement buckets vs full MCCs, is 3 buckets will cost you the same as a full stack MCC (6 12" high buckets will fit in a full stack) . I refer to it as ordering buckets and having them shipped in a "handy steel shipping container". You can then strip the bus bar out of it and sell it for scrap.
 
Jraef said:
Depending on the brand of the old MCC, it may be difficult to find a new Feeder bucket ready-made. Allen Bradley are the only ones who have not changed their MCC design since inception, so if that's what you have, you can just order a new bucket(s) and they will fit. But some brands have changed a lot over the years and obsolete their old designs, others no longer exist at all. Eaton does make a lot of retrofit buckets for older styles of MCC though. The parts inside will be Eaton or course, but they make the bucket frames and stabs to match the old stuff. https://www.eaton.com/content/dam/e...market-buckets-quick-selector-ca04304001e.pdf

But it is often very possible to still get a replacement Thermal-Mag breaker that is the same frame as an older MCP used in a starter, so it just comes down to how comfortable you are at disassembling and reassembling the mechanical parts. It's not hard, just tedious sometimes.

Side note; my "rule of thumb" on buying replacement buckets vs full MCCs, is 3 buckets will cost you the same as a full stack MCC (6 12" high buckets will fit in a full stack) . I refer to it as ordering buckets and having them shipped in a "handy steel shipping container". You can then strip the bus bar out of it and sell it for scrap.
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Buying panelboards isn't all that much different. Especially something like I-Line. need one breaker , might have to eat a somewhat high cost. Need 3 or more, likely can get a small panelboard with the breakers you are needing either installed or at least on same order and remove them and install in your existing. Sell cabinet and bus as scrap metals.

I also agree in many instances there is a thermal-mag that is same frame as the MCP and isn't any different of a task than if you needed to swap the MCP.
 
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