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Sam I
Sam I, Electrician
Category: Electrical
Satisfied Customers: 1718
Experience:  Maintenance Electrician, Electrical Contractor, Locomotive Electrician
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Okay, elevator company installed the wrong motor and

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Okay, elevator company installed the wrong motor and controls in a building I'm working on. Installed 480v Delta motor, and we have a 208 / 120 system. They asked me to install a Step up Transformer. I told them I would so they don't have to uninstall their equipment, and have another system made. Now I have a problem because I have to install 75kva Transformer to power there equipment, which according to my math would require 208 amps on the primary overcurrent protection device, and at least 90 on the secondary. Am I doing my math right? The motors full of current is 60 amps, can I undersized the breaker on the primary side? My problem is I already have a Siemens panel installed that takes BL Breakers, and it's protected by 150 amp main. And they don't make a 200-amp BL style breaker.

Hello, it does seem to be a problem. Under-sizing the breaker is not a good idea due to nuisance tripping from inrush current although with this being an elevator motor, is it not controlled by a VFD to decrease inrush and starting torque?

You don't have a distribution panel that you could run the transformer feed from?

Customer: replied 11 days ago.
that's the problem, it's ran from three phase commercial Siemens panel, that uses BL type Breakers. The BL breaker line only goes up to a hundred amps. I don't think 100 amps would cut it on my primary side
Customer: replied 11 days ago.
it's a 6 wire motor set up with soft starters, they can somewhat control the starting amperage.

What is the service size? Is this the main distribution for the building?

Customer: replied 11 days ago.
18 apartments above two commercial spaces below, 1200 amp service. Elevator is run off the house panel, which is 150 amp disconnect off the meter stack

I would say your only option is new run from the main 1200 or call the people back and have them install the correct motor/system

Customer: replied 11 days ago.
don't see how I could run off the main , I'd have to install a new meter, which I don't have room for. I could possibly step up the main breaker for the house panel to 250 amps, and get a commercial 3-phase panel that will accept a 200-amp breaker

You would need to do a load calculation to make sure your existing load and the new motor load won't overload even a 250

and a 250 amp feeder would mean a 400 amp panel.

After 225 amp rated panels the next step is 400

Seems like you're spending a lot of money and time to fix their mistake. Are they paying for this?

Customer: replied 11 days ago.
the GC made a mistake and didn't read the contract, so it's their mistake, but the GC signed off on it. what do you think about feed through terminals for my existing panel? And then install 150 amp disconnect for the Transformer primary side

No, not a good idea to under-size the transformer breaker and feed. Minimum protection for that transformer is 200 amps. Plus you have the other loads from the panel on a 150 amp feeder.

Customer: replied 11 days ago.
my meter base and disconnect is good for 225 amps, according to Siemens my main lug panel is good for 250 amps, if I change the main breaker in the meter stack to 200 amps, pulled in 4/0 aluminum. Then I could use feed through terminals and feed a two hundred amp disconnect.

Do you know the existing load on the panel?

Customer: replied 11 days ago.
the only thing that would be left in my house panel would be a 2 ton heat pump, and some common area lighting

If that adds up to 50 amps or less then it could work. The house panel is 3 phase?

Customer: replied 11 days ago.

Let me run some calcs for the transformer to see if it needs to be larger than 75 KVA. Do you have the data plate info? Is there a code letter on the plate?

Customer: replied 11 days ago.
30hp motor on elevator could have almost gotten away with 45 KVA

it's a 30 HP with 60 full load amps at 480?

Customer: replied 11 days ago.

Strange. Do you have a picture of the data plate or the code letter from the data plate?

Customer: replied 11 days ago.
Customer: replied 11 days ago.
Customer: replied 11 days ago.

ok, so the motor is 41 and the controls added to it total 60

Customer: replied 11 days ago.
that seems like a lot of control amps to me. the elevator guy said this thing could run at 60 amps , and wanted me to use that as the Possible full load current
Customer: replied 11 days ago.
even though the FLA says 41 amps

You would size the transformer for the motor inrush and operation frequency with more than one start per hour (20% added)

Then add the additional load

You really need the code letter from the data plate on the actual motor to make sure it's accurate. And the impedance from the transformer to make sure. I can forward you the calc sheet I use if you like

Customer: replied 11 days ago.
I know the Transformer impedance is 5%. I would like to have that worksheet.
Customer: replied 11 days ago.
given that the 75kva Transformer is the right size. Would you see any problem with using 225 amp main breaker, pulling in new feeder wires to my main lug panel , probably 250 MCM copper, using feed through lugs to a 225 amp fused disconnect before the primary side, then Transformer, then 90 AMP disconnect on the secondary side before the motor and controller?

I would use that to calculate for just the motor. Then use normal transformer calcs for the remaining 19 amps then merge the data

225 main sounds like that would be fine.

Customer: replied 11 days ago.
okay, I I'll mess around with that, and I'll have to get the motor code. that will ensure I got the right Transformer. Think I'm going to get the inspector down there too, just so he's on board. really appreciate you working through that with me. good to have somebody to bounce ideas off!

Glad to help. Let me know if you need further assistance

Customer: replied 11 days ago.
Thanks, I like to keep this open, and I can let you know how it works out

Yeah. no problem. I will get a notification when you post again

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