Feeders for transformer where only half of capacity is required

[PhaseShift]

If we have a transformer where we only really need half of the capacity of the transformer can we size the primary feeders and OCPD off of the 1/2 capacity rating that we require? For instance if we had a 50kVA transformer but only required 25kVA of its capacity can we size feeders based upon 25kVA? Is this a code violation?

 

[markstg]

If we have a transformer where we only really need half of the capacity of the transformer can we size the primary feeders and OCPD off of the 1/2 capacity rating that we require? For instance if we had a 50kVA transformer but only required 25kVA of its capacity can we size feeders based upon 25kVA? Is this a code violation?

Yes you can, no it is not.

 

[charlie b]

Agrees. Things only need to be sized on the basis of the calculated load. I presume, of course, that you have calculated the load, and are not just guessing how much of the transformer's capacity you need to use.

 

[jim dungar]

Yes, the feed to the transformer can be smaller than the transformer FLA.

However, be ready to have to upsize your feed (protection and conductors). It is extremely unlikely that a 50% sized primary protective device will be large enough to handle the inrush of the transformer.

 

[Pierre C Belarge]

Yes, the feed to the transformer can be smaller than the transformer FLA.

However, be ready to have to upsize your feed (protection and conductors). It is extremely unlikely that a 50% sized primary protective device will be large enough to handle the inrush of the transformer.

Can the transformer be engergized without tripping the OCPD at the smaller size?

If so, will there be chances of the transformer randoming tripping the OCPD during normal operation or during instances of a large fault on the secondary side?

 

[PhaseShift]

However, be ready to have to upsize your feed (protection and conductors). It is extremely unlikely that a 50% sized primary protective device will be large enough to handle the inrush of the transformer.

Well at this point we have a spare transformer lying around that we want to use to feed misclaneous tools, lights, recpt's etc.. at a fabrication area. Were not really sure what were going to install so we dont have a real load calc. Im thinking however that the 50kVA at 240V is overkill for what we need and to save money on feeders was only thinking about using half.

So if I used 25kVA on the primary of 480A this would be 52A * 1.25= 65A.
I would therefore use a 70A breaker with #4 AWG conductors. Would this be o.k. for this case? Or do you think I would need larger breaker size to handle inrush? How much larger?

And if I increase breaker size then I would need to increase cable size as well to ensure protection of cable?

In your experiences do you think this is worth it, or should it just be sized for full rating although it may be overkill?

 

[jim dungar]

Old transformer had less inrush than the current energy efficient (TP-1 design) ones do.
The inrush current value of a transformer has to almost nothing to do with its connected load, it is primarily a function of the transformer construction and where in the 60Hz cycle the unit is turned on.

I would not expect your 70A breaker to work consistently.
Rarely have I seen breakers <125% FLA applied successfully, although anecdotally I have seen moderately to heavily loaded transformers have slightly less inrush current than lightly loaded ones.

 

[skeshesh]

I'm not sure how old the transformer is, but grab the nameplate and try to find the inrush from the mfg - it'd make it a lot easier than playing guessing games with the numbers.

Also, as you know the needs of your plant best, half load may be a correct assumption for the foreseeable future, but I'd suggest using a large enough conduit to be able to upsize the secondary side feeders should you see a growth in the load sometime down the line.

 

[markstg]

Old transformer had less inrush than the current energy efficient (TP-1 design) ones do.
The inrush current value of a transformer has to almost nothing to do with its connected load, it is primarily a function of the transformer construction and where in the 60Hz cycle the unit is turned on.

I would not expect your 70A breaker to work consistently.
Rarely have I seen breakers <125% FLA applied successfully, although anecdotally I have seen moderately to heavily loaded transformers have slightly less inrush current than lightly loaded ones.

But, can't the CB be sized up to 250% without increasing the feeder size.

 

[david luchini]

But, can't the CB be sized up to 250% without increasing the feeder size.

No, the primary c/b can be increased up to 250% (assuming there is secondary protection per 450.3(b)), but the primary feeder must be protected by the primary c/b, per 240.4.

 

[mxslick]

I'm gonna chime in with this:

It may not be a Code violation, but it is just plain dumb in the sense of false economy.

The added cost of the wire (let's say for sake of discussion that the conduit will be sized for full-load conductors from the beginning, which IMHO is a MUST), at the time of original installation WILL be a LOT less that the costs of wire (remember you're gonna be paying for the half-load wire AND the full-load wire) and labor to re-pull the full-sized conductors later.

Do it right the first time and avoid the extra costs and downtime in the future.

Look at it this way, you have already saved a bundle by having the transformer on hand, so even with full-load rated feeders you're still way ahead on the money. :grin:

 

[LarryFine]

It may not be a Code violation, but it is just plain dumb in the sense of false economy.

Look at it this way, you have already saved a bundle by having the transformer on hand, so even with full-load rated feeders you're still way ahead on the money. :grin:

Now, there's a sensible and logical argument.


Stop doing that! grin

 

[mxslick]

Sorry, couldn't help myself.. :grin:

 

[PhaseShift]

I've decided to supply capacity for full 50kVA of transformer. Here are a few other decisions I've made that I wanted to hear others opinions on.

Since the transformer primary current is 104A taking 125% of this gives me 130A required for primary OCPD. Next size up is 150A breaker so I would use this with #1AWG.

However this feeder circuit is coming from an MCC and I want to have some spare capacity for 3-phase equipment that may be added in the future. So I've decided to put a 175A breaker in the MCC and run a 2/0 feeder out to the location to feed the transforemr and have about 45A spare capacity for any future 3-phase equipment.

Now my question is what to do with a disconnect out at the remote location. My initial thought was to put a 150A disconnect with 150A fuses after the feeder cable to feed the primary of the transformer and if we wanted to add a 3-phase load later we could just tap off of the primary of this disconnect. This way disconnect would provide transformer primary protection. (Transforemr secondary protection is also provided) However I know that the 175A upstream breaker in the MCC can provide the primary protection for the transformer (up to 250%) so I wanted to see if others thought it would be necessary to put the fused disconnect out at the transformer keeping in mind the possible addition of future loads. I want to put some sort of disconnect out there to be able to turn off power at the remote location but my question is weather or not to put a fused disconnect to protect the transformer at 150A or put a non-fused disconnect and allow upstream 175A breaker to protect transformer primary?

Any thoughts?

 

[PhaseShift]

Also with what I listed above and the possible future loads would you initially install a 3 pole or 2 pole disconnect out at the transformer location?

 

[DetroitEE]

So this 50 kva transformer...is this single phase? I see you are doing your calculations using the value of 480 to find the primary current; I'm just a little confused, you're taking a single phase feed out of the MCC?

 

[DetroitEE]

I've decided to supply capacity for full 50kVA of transformer. Here are a few other decisions I've made that I wanted to hear others opinions on.

Since the transformer primary current is 104A taking 125% of this gives me 130A required for primary OCPD. Next size up is 150A breaker so I would use this with #1AWG.

However this feeder circuit is coming from an MCC and I want to have some spare capacity for 3-phase equipment that may be added in the future. So I've decided to put a 175A breaker in the MCC and run a 2/0 feeder out to the location to feed the transforemr and have about 45A spare capacity for any future 3-phase equipment.

Now my question is what to do with a disconnect out at the remote location. My initial thought was to put a 150A disconnect with 150A fuses after the feeder cable to feed the primary of the transformer and if we wanted to add a 3-phase load later we could just tap off of the primary of this disconnect. This way disconnect would provide transformer primary protection. (Transforemr secondary protection is also provided) However I know that the 175A upstream breaker in the MCC can provide the primary protection for the transformer (up to 250%) so I wanted to see if others thought it would be necessary to put the fused disconnect out at the transformer keeping in mind the possible addition of future loads. I want to put some sort of disconnect out there to be able to turn off power at the remote location but my question is weather or not to put a fused disconnect to protect the transformer at 150A or put a non-fused disconnect and allow upstream 175A breaker to protect transformer primary?

Any thoughts?

Sorry, looks like I didn't read this post close enough.

If I understand you correctly...the transformer is single phase, but you want to take a three phase feeder out to the transformer so that they have 3-phase power available at that location for future use?

 

[PhaseShift]

Sorry, looks like I didn't read this post close enough.

If I understand you correctly...the transformer is single phase, but you want to take a three phase feeder out to the transformer so that they have 3-phase power available at that location for future use?

Yes this is correct.

 

[PhaseShift]

Do you guys agree with this design? What about the disconnects?

 

[PhaseShift]

I have decided to go ahead and use a 3-pole disconnect fused at 150A for protecing the transformer now only using 2 of the poles. Then I will have this 3-pole disconnect and can use it to serve 3 phase loads in the future and buy a seperate 2pole disconnect for the transformer at that time.

What do you guys think?