Step down transformer

R

Roscoe

Member
Location
Oh
Occupation
Electrician
Pulling roughly 450 ft of cable at 480 volts for a 120 amp 3 ph service I will be using a step down transformer to step it down to 208v . According to to my calculations the 450 ft run should be using 2/0 aluminum which would give me a 2.91% voltage drop, I would use the same gage for the load side of the transformer which would be 45 kv does this sound correct ? Its for a score board.
 
I would think 2/0 aluminum would give you a much smaller voltage drop than 2.91%.

The size of the conductor on the load side of the transformer will depend on the Overcurrent Protective Device that it is terminated at.
 
zbang said:
I assume that's 45KVA; not a small scoreboard there (or some lighting is included).
Click to expand...
Correct it has video playback on it. It’s essentially what most of the high schools are going to nowadays so that they can do replays and they have the delay of time clock amongst many other things that all require high output, LED lighting. The scoreboard comes equipped with its own panel board were simply in charge of installing the service disconnect And getting their feeders from the disconnect to the panel board and obviously the service to my disconnect a 450 foot run isn’t crazy long but based on the normal three phase formula and being that I have 480 V leaving the building going underground to Meet the transformer and then get stepped down from the transformer to the disconnect is why I’m using the 2/0 Al , and with the 450 feet run is where I get the 2.91% voltage drop the scoreboard itself however will not be pulling a full 120 A obviously it will only be pulling 50 A at best at times upwards of 60 A
 
zbang said:
I assume that's 45KVA; not a small scoreboard there (or some lighting is included).
Click to expand...
david luchini said:
I would think 2/0 aluminum would give you a much smaller voltage drop than 2.91%.

The size of the conductor on the load side of the transformer will depend on the Overcurrent Protective Device that it is terminated at.
Click to expand...
I’m actually glad that I have 480 V supplying the transformer because if I only had 208 V leaving the building the calculations call for a minimum of 250 MCM aluminum to supply the 120 amp three phase needed for the scoreboard
 
What is the 480 volt load on the transformer primary? Too bad they didn't make the scoreboard in 480 volts.
 
Roscoe said:
I’m actually glad that I have 480 V supplying the transformer because if I only had 208 V leaving the building the calculations call for a minimum of 250 MCM aluminum to supply the 120 amp three phase needed for the scoreboard
Click to expand...
I'm still not seeing a 2.91% drop on 2/0Al, 480V, three phase, for 450'.

Are you using 120A load current instead of 52A?
 
infinity said:
What is the 480 volt load on the transformer primary? Too bad they didn't make the scoreboard in 480 volts.
Click to expand...
I know it everything on that scoreboard is 120 V single phase except for the video playback. It’s 2083 phase other than that all the different lighting backers are all single pole 20 amp circuits. The maximum amp draw of the whole scoreboard is 120 A, which is what we have to figure our load out obviously but it will most likely only pull maybe half of that at best. I’ve had some people tell me that the 45KVA transformer might be too small and that it may be running at full capacity But I just don’t see that happening with 50 to 60 A being the ultimate load, some have recommended a 75KVA transformer but when I spoke with the engineers at the company that manufactures the transformers, they assured me that the 45KVA should be more than sufficient
 
What do you mean ' The maximum amp draw of the whole scoreboard is 120 A' and 'But I just don’t see that happening with 50 to 60 A being the ultimate load'. Where do these numbers come from?

How many 20A 120V circuits are there, and what is the amp rating of the 208/3 scoreboard?

If I assume that you have 120A of three phase load at 208V, then the 45 kVA transformer is sufficient. It has a current rating of 125A.

If you assume 120A at 208V, then the 480V load is about 52A. Using 52A you only need 2AWG Al conductors.
 
winnie said:
What do you mean ' The maximum amp draw of the whole scoreboard is 120 A' and 'But I just don’t see that happening with 50 to 60 A being the ultimate load'. Where do these numbers come from?

How many 20A 120V circuits are there, and what is the amp rating of the 208/3 scoreboard?

If I assume that you have 120A of three phase load at 208V, then the 45 kVA transformer is sufficient. It has a current rating of 125A.

If you assume 120A at 208V, then the 480V load is about 52A. Using 52A you only need 2AWG Al conductors.
Click to expand...
Yes, the primary side of the transformer will be 480 V. The secondary will be 208 V three phase but they’re talking about adding some lighted billboard signs along the fence. I did forget to add that into the equation but I had it in the back of my head when sizing the transformer, the LED billboard signs use very little they are 120 V single phase require a 20 amp breaker but draw only 5 A
 
This is good to know that’s why I hopped on here. I always appreciate everyone’s insight. This will be my first transformer job so I just wanna make sure everything goes well. It does not take place until August. I’m just trying to get all my ducks in a row and get my transformer ordered as well as my service cable trying to get it all arranged now so I have a material total for the GC
 
45Kva at 480v is less than half of the current as 45Kva at 208v. That's why we prefer the higher voltages.

To simplify (hopefully), the current in one line at 277v will be lower by the same ratio as one line at 120v.
 
Roscoe said:
This is good to know that’s why I hopped on here. I always appreciate everyone’s insight. This will be my first transformer job so I just wanna make sure everything goes well. It does not take place until August. I’m just trying to get all my ducks in a row and get my transformer ordered as well as my service cable trying to get it all arranged now so I have a material total for the GC
Click to expand...
We're here to help and may even end up saving you some $. :giggle:

Your voltage drop calculations for the primary can use the primary current as David stated that's around 52 amps. See what you come up with when using 52 amps @ 480 volts.
 
infinity said:
We're here to help and may even end up saving you some $. :giggle:

Your voltage drop calculations for the primary can use the primary current as David stated that's around 52 amps. See what you come up with when using 52 amps @ 480 volts.
Click to expand...
Yes, I definitely appreciate that. I had a feeling that might be the case when I saw what the difference in what-my gauge of wire would’ve been for that 450 feet when we originally thought we had 208 3 ph straight to the panel board on the back of the scoreboard after the disconnect. It called for 250 MCM aluminum as soon as I went to 480 V three phase the gauge of the wire required went down significantly.
 
Roscoe said:
It called for 250 MCM aluminum as soon as I went to 480 V three phase the gauge of the wire required went down significantly.
Click to expand...
As long as you're using power figures, not current, that's exactly what should happen.

That's why the POCOs transfer at such high voltages. Insulation is cheaper than conductor.
 
You must log in or register to reply here.
Top