208 volt to 480 volt transformer questions

Status
Not open for further replies.
8

87gn

Member
Location
Md
I have a 60 amp 3 phase 480 volt equipment. I have 3 phase 208 volt at the MDP.
Transformer sizing
208x60x1.73=49.82KVA
Purchase a 75 KVA transformer

Line side wire and OCP sizing
75KVA divided by 1.73x208=208 amp line side
4/0 THHW-2 copper wire size
OCP 208amp x 1.25=260 amp OCP/ 300 amp OCP?

Load size wire and OCP sizing
480x60x1.73=50amp load
#8 copper USE wire size
OCP 50amp x 1.25=63 amp OCP/ 70 amp OCP

USE is being used due to the equipment is in a shed away from the MDP. There will be trenching.
I stopped calculating the voltage drop at 250 MCM al = 3.4 voltage drop.
The manufactory warrantee requires no more then 10% VD. It was redundant to match 480 volt wire sizing with this knowledge in hand.
#6 al USE = 32.36 VD/ 6.74 VD percentage
#4 al USE = 20.36 VD/ 4.24 VD percentage
#4 al USE is a happy medium for me

TO SUM UP

WE have a 75 KVA trans former
We have 4/0 copper THHW-2 line side wire with 300 amp fuses
#8 al USE load wire traveling 500 feet with a 70 amp OCP
What's wrong with picture?
 
One thing, you initially stated a 480V 60A load. Then you did this:

480 x 60 x 1.73 = 50 amp load ??? You calculated kW, but called it amps.
 
other than the above (post #2),
240.4 requires you protect conductors at their ampacity. Your calculations appear correct but you need to upsize your conductor to the rating on your breaker or downsize your breaker to match your conductor.
 
First is 60 amps @ 480 volts actually the load of the equipment or is it the recommended overcurrent protection?

You possibly could get by with a much smaller transformer if it has 60 amp breaker sized to allow motor starting.

What is HP of motor and kW of heater, or any other information that might be on nameplate?

I could see a combination of a 25 kW heating element and 10 HP motor needing a 60 amp breaker to allow for motor starting while the heating element is drawing current, yet total load if motor is drawing FLA might only be 38-44 amps, and a 45kVA transformer would handle the load just fine.
 
augie47 said:
other than the above (post #2),
240.4 requires you protect conductors at their ampacity. Your calculations appear correct but you need to upsize your conductor to the rating on your breaker or downsize your breaker to match your conductor.
Click to expand...

I agree the initial design is not code compliant.
 
augie47 said:
other than the above (post #2),
240.4 requires you protect conductors at their ampacity. Your calculations appear correct but you need to upsize your conductor to the rating on your breaker or downsize your breaker to match your conductor.
Click to expand...
Also consider we are supplying at least one motor load, and the rules change a little bit when it sends you to art 430.

If the load were the 25 kVA heater and 10 HP motor I mentioned a few posts back, the secondary conductors could be 8 AWG protected by a 60 amp breaker. Primary side would need to have at least 96 amp of conductor but could have more then 100 amp overcurrent device to allow for motor starting, but you also need to factor in required transformer overcurrent protection, but if using a 45 kVA like I said you should be able to go up to 150 on primary side and still protect the transformer primary side.
 
kwired said:
Also consider we are supplying at least one motor load, and the rules change a little bit when it sends you to art 430.

If the load were the 25 kVA heater and 10 HP motor I mentioned a few posts back, the secondary conductors could be 8 AWG protected by a 60 amp breaker. Primary side would need to have at least 96 amp of conductor but could have more then 100 amp overcurrent device to allow for motor starting, but you also need to factor in required transformer overcurrent protection, but if using a 45 kVA like I said you should be able to go up to 150 on primary side and still protect the transformer primary side.
Click to expand...

I get a load of 14A for a 10 HP 3 phase motor at 460 and 30+ amps for 25 KVA heater. 44 amps without any other multiplier. #8 AL is good for 40 amps. What am I missing that an 8 AL USE can be used for this installation?
 
ptonsparky said:
I get a load of 14A for a 10 HP 3 phase motor at 460 and 30+ amps for 25 KVA heater. 44 amps without any other multiplier. #8 AL is good for 40 amps. What am I missing that an 8 AL USE can be used for this installation?
Click to expand...
would have to be copper, I knew that all along but never mentioned it. This was also a hypothetical example of what OP might have. So far we don't know if he has a 60 amp load or just equipment that specifies a 60 amp breaker but an actual lesser load. My guess is it specifies a 60 amp breaker though with a minimum conductor ampacity of a more precision figure like XX.X amps.
 
Status
Not open for further replies.
Top