Questions concerning my first transformer and the service feeding it

[mark32]

Hello all,

A friend that does maintenance for a small factory has been tasked to power up a new piece of equipment. The specs for this equipment is written as follows: 3/n/pe, 460v 60hz - rated current = 20a max. This equipment is coming from a sister company that also uses the very same machine (Some kind of fancy heater that fuses conveyor belts together). Their maintenance guy sent my guy a pic of the transformer they use, specs here are: 30kva, Primary 240 Delta, Secondary 480y/277. At their main panel, there's a 250v plate on the Square D I line, and I was told they have 240v coming in from the poco. But I get 210v from phase to phase and 120v from ground to all the phases. So here are my questions, what type of service is this? Is this suppose to be a 240v service or is it a 120/208 service? Furthermore, would the transformer I mentioned be correct for this service? I did commercial work for a few years, but I mainly ran pipe and pulled conductors, I never had the opportunity to wire a transformer, but I'm looking forward to it. Any help is appreciated.

 

[hillbilly1]

Yep, you have a wye connected service. You will probably get close to 460 out of it, the phase to neutral voltage will be less than 277 though.

 

[augie47]

If you are having to purchase the transformer I would go ahead and get the right one: 208 Delta Primay: 480/277 Secondary
Seemingly you could use a 15kva unit,.

 

[mark32]

Thanks guys, I really appreciate it! Augie, I'm coming up with a load just under 16kva (20*460*1.732 = 15,934) Am I looking at this correctly?

 

[Little Bill]

If you are having to purchase the transformer I would go ahead and get the right one: 208 Delta Primay: 480/277 Secondary
Seemingly you could use a 15kva unit,.

I'm not a transformer guy, so I'm trying to figure out how you get a 208V Delta?

 

[augie47]

I'm not a transformer guy, so I'm trying to figure out how you get a 208V Delta?

The transformer winding is a delta... you only connect the phases off your 208Y system.

Thanks guys, I really appreciate it! Augie, I'm coming up with a load just under 16kva (20*460*1.732 = 15,934) Am I looking at this correctly?

15kva transformer should handle that load fine ...if you want to play safe (not know pf, etc) you can go with a 30 but I wouldn't be too worried with a 15.

 

[MD Automation]

Agree with the replies so far. But, just in case you already have the transformer mentioned in the original post (it's unclear if that is coming with the belt welder), you could look to see if it has taps on the primary. If so, you could adjust them for undervoltage and probably come pretty close to 460 on the secondary using your 210 on the primary.

It's likely that, if it's just mostly a heater, the controls will pulse the heaters on and off once it reaches some operating temp. Just like a stove. So a small amount of undervoltage might not be a disaster. Just takes longer to reach the right temp.

Again, the above thoughts are only if you already have that 240 -> 480 transformer. If you have to purchase one, for sure get a 208D -> 480Y. And don't backfeed a 480D -> 208Y step down transformer because there was one for cheap or quickly available. Get the proper step-up.

 

[mark32]

Great stuff guys, thanks again! MD, the unit will not be shipped with a transformer, so I would like to get the correct one since it will have to be purchased. My question now is, this unit is rated for 460v, If I do get the 208 -> 480/277 transformer, I'd like to drop it down, via the taps, if possible, not sure if that's something to be concerned over or not though.

 

[MD Automation]

Mark32, I'm sure that smarter people here than me will weigh in - but I expect the question of 460 VAC and 480 VAC may come down to the difference between Nominal Voltage versus Utilization Voltage ratings.

Search this forum for those keywords and I'm sure there are a few threads describing the "Nominal" voltage of 480 at the Service and the "Utilization" voltage of 460 by the time it gets to the load.

That said, there is nothing to stop you from using Primary side taps to nudge the Secondary output down a tad. That's around 5%, and there are typically taps (up and down) around 2.5% and 5% for sure.

 

[mark32]

Okay, here are a couple of questions that I forgot to ask. I am coming up with a primary current of 83 amp (30,000/(208*1.732) Now to size the breaker, I'm looking at 450.3(B), which states the max rating of the ocpd is 125% (For currents 9 amps or more). 125% of 83a brings me to 103a. Does this mean I have to go up to the next size breaker, or can I just use a 100 amp breaker? As for over current protection for the secondary, I am unsure if it is necessary or not. The load is 175' from the transformer and it is feeing a single piece of equipment. I'm guessing because the secondary is Y connected, it is needed, and if so, where must this ocpd be located?

 

[mark32]

Also, a hypothetical question. Lets say I were to use a 30kva transformer and fuse the secondary conductors at 20 amp. Could I also downsize the primary breaker and primary conductors?

 

[don_resqcapt19]

Also, a hypothetical question. Lets say I were to use a 30kva transformer and fuse the secondary conductors at 20 amp. Could I also downsize the primary breaker and primary conductors?

The issue with a smaller primary OCPD is that when you get too small, it may open when you turn the transformer on.

 

[LarryFine]

The load is 175' from the transformer and it is feeing a single piece of equipment.

Install the step-up as close to the source as you can, so most of the 175' run is 480v and not 208v.

 

[hillbilly1]

Install the step-up as close to the source as you can, so most of the 175' run is 480v and not 208v.

And smaller wire and conduit can be used.

 

[mark32]

Thank you Don, Larry, and Hillbilly! Since this is only a 20amp load, I'm going to ask my maintenance friend to contact the transformer company he is in talks with and give them the specs for load to see if we can get away with a 15kva transformer. Otherwise, I will feed this 30kva transformer with #3 cu and protect them with a 100 amp breaker. I would then install a fusible disconnect (30000(460*1.732)*1.25 = 47amps, so 50 amp fuses) within a few feet of the transformer for the secondary conductors, then run 175' to the equipment. I apologize, but I have three questions now: Is this secondary ocpd required? Two, if I provide this secondary protection, am I now required to up the primary protection 250%? And third, can I fuse the secondary at 20 amps, as that's what this cord and plug connected equipment is rated for? Thanks again in advance~

 

[don_resqcapt19]

A secondary OCPD is required for the transformer secondary, any time that the primary OCPD exceeds 125% for the primary full load current. Note there are two sets of independent rules at work here. The rules in Article 450 only cover the protection of the transformer windings. You also have to comply with the rules in 240.21(C) for the protection of the secondary conductors. Where Article 450 requires transformer secondary protection, the conductor protection almost always meets the requirements for the protection of the transformer and the conductors.
There is never a requirement to increase the size of the primary OCPD to 250% no matter what you have done on the secondary.
There is no reason to size the secondary conductor protection any higher than what is required by the load.

 

[Strathead]

125% of 83a brings me to 103a. Does this mean I have to go up to the next size breaker, or can I just use a 100 amp breaker?

It means you can go up to a 110A breaker, not that you have to. As long as the calculated load is lower than the ampacity of the circuit, you are fine.

 

[mark32]

Thank you Don! The more I read into this installation, the more confused I became but your reply has cleared things up, thanks once more.

Strathead, thank you for stopping by as well! I'd like to use #4 cu and put it on a 90 amp breaker, but I don't want to push the issue of it tripping due to the inrush current, not sure if my concern here is warranted or not.

 

[mark32]

Just an update, I called tech support for Acme transformer (That's who my supply house deals with) and their guy favored the 30kva transformer over the 15kva for my application, and he also recommended a 110 amp breaker for the primary as opposed to a 100 amp breaker that I had intended on using. While online, I came across Maddox transformers, and their price was much better than the Acme. Anyone familiar with Maddox?

 

[Strathead]

Just an update, I called tech support for Acme transformer (That's who my supply house deals with) and their guy favored the 30kva transformer over the 15kva for my application, and he also recommended a 110 amp breaker for the primary as opposed to a 100 amp breaker that I had intended on using. While online, I came across Maddox transformers, and their price was much better than the Acme. Anyone familiar with Maddox?

Did he say why?