Oversized main disconnect

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rodriguezandy

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Cheetto10
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Electrician
Hi guys, I'm currently doing a project which according to the plans I'm supposed to run 225 amp service to a CAT scan disconnect. According to the specs sheets of the machine and equipment it's telling me the disconnect is 125 amp. My question is, is there any reason why the engineer would oversize the meter main disconnect by 100 amps? And if I run everything according to the drawings will it be an issue that everything is sized for 225 amps? The voltage is 277/480v 3p 4w
 
The little bit I been involved with medical imaging equipment, the equipment vendors were very particular about voltage drop and the specified conductor sizes would be overkill for most other applications with similar load levels. Unlike many other loads where they may not pay too much attention to voltage drop during starting where there may be higher inrush currents than operating currents, they do pay close attention to it with this type of equipment.
 
kwired said:
The little bit I been involved with medical imaging equipment, the equipment vendors were very particular about voltage drop and the specified conductor sizes would be overkill for most other applications with similar load levels. Unlike many other loads where they may not pay too much attention to voltage drop during starting where there may be higher inrush currents than operating currents, they do pay close attention to it with this type of equipment.
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Yes, you're right. That's why I thought I'd ask, just seemed so unreal to install a 225 amp meter main with 4/0 phase conductors and a 1/0 EGC for a 125 amp disconnect. And they laughed at me when I needed 750 mcm scrap wire to install a GFI at home.
 
rodriguezandy said:
I'm supposed to run 225 amp service to a CAT scan disconnect. According to the specs sheets of the machine and equipment it's telling me the disconnect is 125 amp.
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I'm assuming that the 125 amp "disconnect" is a circuit breaker? If so that will protect the equipment so installing a 225 feeder will be oversized but not relevant to the operation of the equipment. Seems odd to have a separate service for a small single piece of equipment.
 
I haven't seen the disconnect that comes with the ct machine but I'm assuming it's a breaker as well; a shunt trip breaker at that, since there's e-stop switches I need to install. Also, it's a separate service because the existing building only has 120/208 service. The tenants machine requires 277/480 so there's a new transformer coming in as well.
 
rodriguezandy said:
I haven't seen the disconnect that comes with the ct machine but I'm assuming it's a breaker as well; a shunt trip breaker at that, since there's e-stop switches I need to install. Also, it's a separate service because the existing building only has 120/208 service. The tenants machine requires 277/480 so there's a new transformer coming in as well.
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Just curious this is a new 480 volt service? Why the tranny?
 
The utility service only has 1 transformer and it's only 120/208. They have to tap into the line side of that transformer to supply a 480v transformer. It's a headache.
 
FWIW........Cat Scan machine that I went through here had a display that showed 120 kV @ 500 ma

So that's 60 kW, and at 480 / 3 phase works out to a bit over 72 amps per phase

Local electrician says they routinely do a 200 A @ 480 service for those.
 
infinity said:
I'm assuming that the 125 amp "disconnect" is a circuit breaker? If so that will protect the equipment so installing a 225 feeder will be oversized but not relevant to the operation of the equipment. Seems odd to have a separate service for a small single piece of equipment.
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I did similar thing in a small town hospital around 20 years ago. Facility had 208/120 service. Actually at first they were upgrading existing x-ray equipment and also installing a 200 amp pin and sleeve connector for mobile CT and MRI machines that travel the area and are in semi trailer. IIRC initially I installed a 200 amp 480/277 Fused disconnect as the service for these and then tapped two other feeders off that to each item. Each needed maybe 125 to 150 amps for overcurrent protection yet the specs were pretty stringent when it comes to voltage drop. This made for running possibly larger conductor than I would have for many other loads of same rating and also specs kind of made it a better deal to get 480 supply from utility to help assure voltage they wanted vs what kind of dips may occur if we were to supply it via existing 208/120 and transform it, and would also probably meant updating the already 600 amp 208/120 service that was existing to even larger or go with larger 480 service then transform for existing 208. POCO was fine either way, the hospital board likely had to decide how much they wanted to spend and the 480 service was likely the lesser cost.

Couple years or so later the mobile CT scanner was disappearing as many surrounding facilities had their own CT equipment, so they ended up doing some remodeling of an area and adding CT equipment. The pin and sleeve connector was still used for mobile MRI which still travels the area. Though the small town hospital I did this work for ended up closing maybe 10-15 years ago.

I ended up converting that 200 amp 480 service to 400 amps and an I line panel for service equipment and feeders to each of the items. I believe this CT equipment was similar rating to what OP has here seems was either 125 or 150 amp circuit supplying it. I had to provide my own shunt trip breaker in the vicinity of the equipment then feed main terminals of the equipment and connect the E stop to the shunt trip. Most the other electrical involving the equipment was done by the medical equipment people. I think i did install some two channel wireway, but they installed most of what went into it.
rodriguezandy said:
I haven't seen the disconnect that comes with the ct machine but I'm assuming it's a breaker as well; a shunt trip breaker at that, since there's e-stop switches I need to install.
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Like I said I had to install my own shunt trip breaker and connect the E stop the medical equipment company provided to it when I did a similar project. That may not always be the case, plus when I did that was maybe around 2010 or so. I haven't really done any work in healthcare facilities since that place ended up closing maybe about 2015 or so. Supposedly some the things that went with Obamacare sort of was a reason why they decided to call it quits, they were contemplating a pretty major upgrade of the facility and had already updated a few items that needed immediate attention to keep up with health care related codes before deciding to close for good.
 
It's always best to oversize the disconnect. I put 60A in where there are 30A loads. Just more electron surface area. They don't get hot or bottleneck voltage and current.
 
yup. I would land a disconnect rated at 80A or so there. Maybe even 100A based on common nominal sizing from manufacturers.
 
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