480 to 208 Transformer OCPD

[peter d]

Got a job where we need to proved 3-phase power to a machine. We have 480 3-ph. available and will provide a transfomer at the machine, sized for this machine only. Current on the 208 volt machine is 3 amps, so on the 480 volt side it will be almost nil. Does this transformer need secondary OCPD?

 

[mdshunk]

Not if the primary OCPD is sized right.

 

[peter d]

Not if the primary OCPD is sized right.

Do tell....

 

[mdshunk]

For primary current less than 9 amps, you're allowed to skip secondary protection if you use primary fuse protection up to 125% or a breaker up to 167%. This is twisting my brain a little bit from my factory electrician time, but double check it in the 450's someplace.

 

[peter d]

Thanks Marc, you're the best electrician in the whole wide world.

 

[jim dungar]

Not if the primary OCPD is sized right.

Mark is correct.

But actually if the primary protection is sized correctly, <= 125%, no transformer (600V or less) ever needs secondary protection, table 450.3(b).

But, the secondary conductors may need protection, see 240.4.

 

[mdshunk]

Cool. Run your circuit to a primary disco, pop some 4 amp fuses in there, and call it good. What sort of machine is it, anyhow? For that low of a draw, I wonder if it wouldn't be cheaper to change the motor, control transformer, or whatever instead?

 

[peter d]

Cool. Run your circuit to a primary disco, pop some 4 amp fuses in there, and call it good. What sort of machine is it, anyhow? For that low of a draw, I wonder if it wouldn't be cheaper to change the motor, control transformer, or whatever instead?

I don't know the exact name, but it's a "spooler" for putting cuts of wire rope from large master reels onto smaller ones for sale to the end user/customer.

We already looked into changing the motor and quickly decided against that because we didn't want to mess with internals of the machine (it's old and not in the best shape) and it has an ancient Allen-Bradley reversing starter on it, and finding coils and heaters was ruled too time consuming to bother. Not only that the machine needs to be completely rewired, and the company needs to get it up and running since they have a big order coming up that they need to process (allegedly.)

 

[peter d]

Update:

The installation is complete. BUT...the transformer we got is a 480 delta to 240 delta. The primary is 480 wye.

The machine works fine but the confusion is about the secondary grounding. At the moment we have it connected as an ungrounded delta on the secondary. Do we need to corner ground this? Or install ground indicator lights?

 

[mdshunk]

Update:

The installation is complete. BUT...the transformer we got is a 480 delta to 240 delta. The primary is 480 wye.

Uhhh.... so what? On the primary side, you don't need the neutral anyhow, so it doesn't matter but for a hill of beans. You're fine. Make sure you have an EGC to the case, and never look back.

 

[peter d]

Uhhh.... so what?

Uhhh...having an ungrounded secondary to feed the machine is fine?? The machine does have an EGC, but like any ungrounded system it will require two faults to operate the EGC. I don't know if this is ok or not.

 

[acrwc10]

I think you may need to ground the system based on 250.20(D) and as I read 250.21 doesn't cover you application.
I don't see how it is a bad idea (required or not) to ground the system.
Is your transformer a Delta to Delta or does it have an XO terminal on it ?

 

[peter d]

I
Is your transformer a Delta to Delta or does it have an XO terminal on it ?

Delta-Delta. No Xo terminal.

The machine was originally in a building with a 208 volt wye service. The motor in the machine is a tyipical 3-phase dual voltage 240/480 motor. I don't know how well it worked on 208 but it did.

 

[acrwc10]

Delta-Delta. No Xo terminal.
The machine was originally in a building with a 208 volt wye service. The motor in the machine is a tyipical 3-phase dual voltage 240/480 motor. I don't know how well it worked on 208 but it did.

sounds like you need to corner ground it then.

 

[peter d]

sounds like you need to corner ground it then.

Yeah, that's what I'm thinking as well.

 

[mdshunk]

Uhhh...having an ungrounded secondary to feed the machine is fine??

Hey, you never said you didn't have a ground. You only said you didn't have a neutral. What are you talking about, exactly?

 

[acrwc10]

Hey, you never said you didn't have a ground. You only said you didn't have a neutral. What are you talking about, exactly?

Update:

The installation is complete. BUT...the transformer we got is a 480 delta to 240 delta. The primary is 480 wye.

The machine works fine but the confusion is about the secondary grounding. At the moment we have it connected as an ungrounded delta on the secondary. Do we need to corner ground this? Or install ground indicator lights?

I think if you reread post #9 you might see he did say it had no ground.

 

[peter d]

I apologize if I didn't explain this well. The machine is fed from an ungrounded delta via the transformer we installed. The machine is grounded via an EGC. Obviously because it's an ungrounded delta, when you check continuity at the machine there is no continuity between L1, L2, L3 and the frame.

So, my question is this: Is this permitted by the NEC? Or should a connection be made to corner ground the delta? Right now I'm thinking that the NEC requires this setup to be grounded.

 

[LarryFine]

So, my question is this: Is this permitted by the NEC? Or should a connection be made to corner ground the delta? Right now I'm thinking that the NEC requires this setup to be grounded.

This is allowed, but generally not recommended unless necessary. An example of necessary would be where unplanned shut-downs can be costly or detrimental.

In such cases, a ground-fault detector is necessary specifically because such a fault will not open a fuse or breaker, meaning that nthe first fault will go unnoticed.

Otherwise, grounding a phase is not really different than grounding a neutral. The same rules apply, such as coloring, except for including it in multi-pole breakers.

 

[templdl]

This is a small transformer application but I don't see that the NEC differentiates transformer KVA ratings in what OCP must be provided.
One can not be limited in considering a simple L-L-L load on a 208Y. One must consider that the X0 is in fact grounded and ground faults WILL BE an L-N load.
As such I don't believe that the neutral (X0) being required isn't the issue at all because if there is L-G fault it will not show up on the primary the same an a L-N load. The pri OCPD is only looking at L-L-L.
Addressing questions is often like shooting at shadows, which compete information, is often omitted. I get that all of the time in my position and have to ask many question until I can get a grasp of the actual application.
One would assume that a 3ph transformer with a 480v primary and a 208v secondary would be a delta-wye, the primary a 480D and th secondary being 208Y/120. It is a 4w secondary regardless of the load being only 3ph3w. As such NEC at 240.4 (F) would apply.
If the transformer was a D-D then it wouldn't apply.
Also, I often get calls for requirement for a 240 3ph secondary transformer to feed a simple 240v 3ph3w load. The choice is to provide a 240d or a 240Y/139 secondary.
Now 139v appears to be strange and in fact it is. It's basically a useless voltage. But you have the choice of going with the 240v delta and corner grounding it or with the wye and grounding the X0.
With the corner grounded delta a ground fault would result in 240v to ground. Thus, a common winding to ground motor failure would be a 240v arcing fault.
When using a 240Y that same fault would result is a 139v line to ground arcing fault. In either case the motor is failing but which system would cause less damage?
Now, the OCPD. Yes a corner grounded 240D would only require 2 fused and a 240Y 3 fuses. Either would use a 3p breaker. As such the OCPD is a mute point.
But, if one must apply an existing transformer there isn't much choice but to us what you have.