Transformer sizing

[MD84]

We are moving a machine shop with about 20 CNC, lathes, and various machines. The existing shop has a 400A 240 3P service. The new location has a 480v service.

The customer has requested we install step down transformers for each piece of equipment as necessary.

I performed the calculations and I am getting some questions on my results. I will provide nameplate information for one example. Please tell me what you would come up with for transformer size, primary OCPD, and primary wire size.

200/220v 3ph
223/203A
60hz

No other information on nameplate. The machine currently has a transformer with no legible nameplate. Voltage measured at 240 primary and 205 secondary.

 

[Ingenieur]

You need ~77kva
I would use a 75kva
you can go delta-delta 480/240 w120 center tap and tap down 7.5% to 222
or delta-wye 480/208 and tap up 2.5% to 213 or leave it at 208

the protection is per the nec table

 

[kwired]

We are moving a machine shop with about 20 CNC, lathes, and various machines. The existing shop has a 400A 240 3P service. The new location has a 480v service.

The customer has requested we install step down transformers for each piece of equipment as necessary.

I performed the calculations and I am getting some questions on my results. I will provide nameplate information for one example. Please tell me what you would come up with for transformer size, primary OCPD, and primary wire size.

200/220v 3ph
223/203A
60hz

No other information on nameplate. The machine currently has a transformer with no legible nameplate. Voltage measured at 240 primary and 205 secondary.

is your mentioned 203 amps one machine or the load calculation you came up with on all of them? My guess is all of them if you only had 400 amp 240 volt feeding them before. Seems like individual transformers is likely going to be more expensive then one that will run all of them, but really need to know more details to know for certain. If say only half of them for some reason can run at one time you only need half the kVA of a transformer then you need to run all of them. But still need individual units sized the same no matter how many can run at a time.

 

[MD84]

The 203 is nameplate FLA for the one machine. It is one of the larger machines. I gave the option of one transformer but the customer preferred individual.

I took a quick 5 minute current reading at the old service and got 310A max and 125A Avg on B phase. The customer never had any problems with the 400A. If I add it all the FLA running at 480 I get about 800A.

 

[kwired]

The 203 is nameplate FLA for the one machine. It is one of the larger machines. I gave the option of one transformer but the customer preferred individual.

I took a quick 5 minute current reading at the old service and got 310A max and 125A Avg on B phase. The customer never had any problems with the 400A. If I add it all the FLA running at 480 I get about 800A.

They obviously never ran at full capacity and/or all at same time in the past. You would need at least a 150 maybe a 200 kVA transformer to supply ~400 amps like you originally had - this not knowing much detail on these machines or even what other load there might have been, but it apparently was enough.

Now just based on that give the owner a price for one 200 kVA unit vs several units, and consider the fact the unit you did mention probably needs at least a 75 kVA.
You will need that 75 regardless with individuals, even if that large machine never runs while others are running.
Their cost will add up fast plus additional fuses, breakers, other accessories, and the labor to install all of them will be more then it is for just one. They will need more space, will likely contribute more heat to the facility if indoors, plus that heat is an inefficiency even if lost to the outdoors.

Depending on circumstances maybe individuals is still best, but you need to examine all aspects. Or maybe just two or three units instead of ten ends up making the most sense.

 

[Ingenieur]

Since the machines are computer controlled he may want individual xfmrs for isolation
give him what wants, he knows his equipment, is willing to pay
if you talk him into 1 xfmr and have problems you own them

if 400 A was fine at 240 (165 kva)
225 A should do at 480 (187 kva)
but go 400 A at 480 if you want

 

[kwired]

Since the machines are computer controlled he may want individual xfmrs for isolation
give him what wants, he knows his equipment, is willing to pay
if you talk him into 1 xfmr and have problems you own them

if 400 was fine at 240 (165 kva)
225 should do at 480 (187 kva)
but go 400 at 480 if you want

They weren't isolated before and apparently did not have any problems. He may know his equipment, but from the point of view of what production it does, not necessarily what it takes to supply it with power or what kind of glitches noise in the controls may cause.

That said the controls are probably already from separately derived systems within the machine(s)

 

[Ingenieur]

They weren't isolated before and apparently did not have any problems. He may know his equipment, but from the point of view of what production it does, not necessarily what it takes to supply it with power or what kind of glitches noise in the controls may cause.

That said the controls are probably already from separately derived systems within the machine(s)

It sounds like the example machine had one
he could not read the nameplate
he measured 240:205 prim:sec, sounds like isolation with small buck

 

[MD84]

Most of the machines had some sort of isolation transformer. Some have taps up to 460 so we lucked out on those. Others are like the one I mentioned.

We have a 400A panel. The 225A should be good but with one transformer per machine we will end up with an overall higher kVA sum than with one transformer.

For the subject transformer it was between a 75 and a 112.5KVA. The former supplying 180A. Less than nameplate but the machine will not likely be fully loaded.