Transformer Feeder

[steve66]

I'm having trouble deciding what is the proper size feeder and what size primary fuse to use for a transformer. Here is my first question:

If I have 25 ft. taps on the secondary of the transformer, does that mean the primary feeder has to be protected at its ampacity? Even if the primary feeder is outside except at the point of termination?

Second question:

What would be the right size fuse for the primary? The full load current is 105 amps. I know article 450 lets me use a 200A fuse (or even higher). But if I reduce it to say 130A, or 150A, will the fuses hold when the transformer is powered up??

Here are some more details:

I am connecting to 4160V with a fusible disconnect on a pole. From the disconnect, I have a cable running underground in conduit to a 750KVA, 4160V to 480V, 3 phase transformer in a vault in a building.

I have two feeders coming off the secondary of the transformer. One set of 350KCM conductors feeds a 300A disconnect, and two sets of 600's feeds a 800A disconnect. Both are indoor taps limited to 25'.

Thanks in Advance:
Steve

 

[websparky]

Steve,

Here is the math:

Secondary load: 480V X 1100A = 528kva
Primary load: 528000VA / 4160V = 127A

Standard Primary fuse size: 125A X 4160V = 520kva
Results in a Secondary: 520000VA / 480V = 1083A

Is this what you were looking for?

 

[websparky]

Steve,

If this transformer is a delta/wye, you are stuck because of 240.21(C)(1).

If this is a delta/delta, you are stuck because of 240.21(C)(6)(2).

Unless there is some details I missed!?

 

[steve66]

Dave:

Thanks for taking the time to read my post, but I guess I'm not making it clear what I'm asking.

On the primary of this transformer, I had 200A fuses and #1 copper XLP (good for 170Amps) specificed. The full load primary current of the transformer is only 105 amps, so I am wondering if the fuse and wire size is overkill.

So should I reduce the wire size down to #2 alumnium, for example, with an ampacity of 115 amps? If I do that, I assume I will have to reduce the primary fuse size. (I can't have 115 amp wire on a 200A fuse, right?)

So how low can I make the primary fuse without worrying about it blowing on transformer inrush?

Steve

Steve

 

[websparky]

The full load primary current of the transformer is only 105 amps, so I am wondering if the fuse and wire size is overkill.

For the primary load, no. At 105A primary the available secondary load at 97% will be 882A.
However, the secondary load could reach as much as 1100A based on the secondary OCDs. This will result in a primary load at 97% which will be 131A.

Therefore, if the primary wire and fuse size is based on the load side potential of only 1050A, the primary conductors can be 1AWG copper and be fused 125A.

With all that said, the "in-rush" current at first start-up will not be a factor. "In-rush" problems usually arise from much smaller transformers with odd set-ups.

You are correct about the primary conductors. They need the OCD sized to protect them.

How'd I do this time?

 

[steve66]

Thanks, that helps more. I guess I keep thinking back to Article 430 which allows you to use a larger breaker for motor starting along with smaller wire. (i.e. the breaker is sized for the starting current, and the wire is sized for the running current). It seems like there should be something similar for transfromers. I think the POCO probably does this all the time. But the NEC doesn't allow it.

It just seems a waste if someone had to size a few thousand feet of primary wire based on the transformer inrush current.

Steve

 

[coulter]

...So how low can I make the primary fuse without worrying about it blowing on transformer inrush?

Steve -
First my disclaimer. I have not done any work designing MV feeders. I have done maintenance on several. (And I don't particularly care for Holiday Inn References are 2002NEC, so please check them against your code.
Table 450.3(A)
240.100, 240.101
215.2(B)(1), (2), (3)

Here is my understanding on sizing primary and secondary OCP and conductors:

1. Supervised location or not. Table 450.3(A), Note 3, also see 215.2(B)(3) on supervised installations

2. Primary Feeder size per 215.2(B)(1)

3. Primary OCP per 240.101(A) and Table 450.3(A)

Unsupervised:
1. Secondary OCP, 125%. Your installation is 1100A, so that's okay.

2 Primary feeder has to be at least sized for the xfmr nameplate, 104A.

3. Primary OCP can be 300%, say 300A in this case.

What I've seen installed:
Primary conductors from the OCP to the transformer are as small as is allowed. VD is rarely a problem.

Primary OCP is as large as allowed - generally for coordination/inrush reasons. Although the fuse curve needs to be inside of the transformer damage point. This would come into play if there were damage to the xfmr secondary conductors resulting in a secondary short circuit.

I would question why would you want to set the primary OCP down? What would you be protecting?

Question for Dave:

...the "in-rush" current at first start-up will not be a factor. "In-rush" problems usually arise from much smaller transformers with odd set-ups...

Could you give me some references on this. I would do some reading. My experience is that inrush is a problem, the bigger the transformer and the lower the impedance, the bigger the problem

 

[steve66]

Carl:

Am I reading this right? 240.101(A):

The continuous ampere rating of a fuse shall not exceed 3 times the ampacity of the conductors.

So does this waive or modify the requirement to protect a conductor at its ampacity (240.4)?

So I can protect 100 Amp wire at 300 Amps, if its over 600 volts?

Thanks:
Steve

 

[dlhoule]

Good luck

Good luck

So I can protect 100 Amp wire at 300 Amps, if its over 600 volts?

If you can prove that it is protected, go ahead. Good luck!:smile:

 

[jim dungar]

240.4 refers to 310.15. 310.15 is for conductors rated to 2000V.
240.101 refers to conductors over 600V.

So, up to 2000V 240.4 applies, from 2001 to 35000V 240.101 is the governing article.

 

[websparky]

Steve,

I made a mistake in my calculations above. It occurred to me today when I looked up a reference for 3 phase motors that I had use a single phase formula. Sorry!!

Here are the correct values for a 3 phase transformer.

720000va Primary: 4160V - 100A
720000va Secondary 480V - 866A

It appears that a 100A fuse on the primary will limit the secondary to 866A.
The load on the secondary appears to be too much for the primary fuse.

However, to solve your situation I would suggest finding the manufacturer's rating info and see if it lets you increase the primary to 150A which will give you 1300A available on the secondary. If so, you will need to check the primary conductor's amp rating to see if will carry the 150A load.

Dave

 

[steve66]

Just when I thought I knew something about the NEC, something like this comes along

So since 240.4 refers to 310.15, and 310.15 only applies to 2000V, 240.4 doesn't apply over 2000V.

That's a bassackwards way to write a rule.

So I can protect a wire at up to 3 times its ampacity at over 2000V, right? Does that mean I am supposed to do the studies in 240.100 to verify the wire is protected

I don't really want to use a fuse at 3x the wire size, but it would be nice to use a 200A fuse on this transformer primary with 105A wire.

Steve

Edit: I wrote "right a rule" instead of "write a rule". Big difference

 

[steve66]

Dave;

I noticed you used 1 phase, but I didn't say anything. Its not really the math I was asking about. It was more how much above the primary current the primary fuse should be rated. And from there, what size wire is necessary.

After Carl's post, I now think I can keep the 200A primary fuse, and use 105A wire.

Don't get too tied up on whats tied to the secondary. The load calculation is under 750KVA (therefore, the 750KVA transformer), but the secondary conductors are upsized to the next standard size breakers. I mostly just posted the secondary stuff because I one could use the transformer tap rules on the primary.

Steve

 

[coulter]

...So I can protect 100 Amp wire at 300 Amps...

In this case, yes

...If you can prove that it is protected, go ahead.

Well, what are we protecting?

Overload, of xfmr, secondary conductors, or primary conductors: All of these are protected by the secondary OCP

Short Circuit downstream of the secondary OCP: Xfmr, pri conductors, sec conductors are protected by the secondary OCP

Short circuit on xfmr sec conductors (backhoe attack): Protection of the secondary conductors is not required - they are dead - we just want to get the fire put out. Xfmr and pri conductors need protecting. Xfmr SSC is about 900/.05 = 18,000A (for a 750kVA, 5% impedance) - neglecting the primary distribution impedance, which is likely an okay assumption for this calc. Primary current is on the order of 18,000 X 480 / 4160 = 2000A. That trips a 300A fuse pretty quick. Wait, what about arcing ground faults and the like? Well, if you don't want to wait until it goes phase to phase, you gotta pay for better protection than just fuses.

Xfmr fault: No xfmr to protect, just primary conductors and get the xfmr fire out. Primary SSC is limited by distribution impedance and somewhat by the transformer impedance (depending on where the fault occured inside of the xfmr). So we are looking at 2000A+ SSC. Again a 300A fuse should pop pretty quick

Primary Feeder fault (crane attack - cat attack): I'll leave this one to you.

So, I would ask, what is not protected?

 

[websparky]

Steve,

After Carl's post, I now think I can keep the 200A primary fuse, and use 105A wire.

As far as the NEC, table 450.3(B) does not allow oversizing of the conductors at the same OCD rating. It does allow up to 250% for the primary of the xfr but you need to up size the conductor accordingly.

Maybe one of the power guys will jump in and answer about the NESC and it's OCD for 4160V.

Could you give me some references on this. I would do some reading. My experience is that inrush is a problem, the bigger the transformer and the lower the impedance, the bigger the problem

Coulter,

Excitation current does not increase proportionally with the size of the coils. Typically, the higher the voltage the lower the amperage just like Ohm's Law!

 

[kevinware]

What Happened???

What Happened???

I can't wait till I can rattle this stuff off the top of my head like most of you guys can. Can someone clear up the math with me?? On the primary I am doing this: (NEC 2005)

Table 450.3(A)

750kva / (4160 x 1.7321) = 104A

I am saying the location will be considered a supervised one so:
104A x 2.50 = 260A OCP (note1 T450.3(A)), 240.6 allows me to up-size to 300A fuse. Is that just wrong? or would 300A be considered the MAXIMUM size fuse you would want to ever use?

Thanks for your understanding,
Kevin

 

[coulter]

...NEC, table 450.3(B) does not allow oversizing of the conductors at the same OCD rating. It does allow up to 250% for the primary of the xfr but you need to up size the conductor accordingly....

Dave -

I think the applicable section is 450.3(A), Primary over 600V. That coupled with 240.101 does allow upsizing the fuse over the conductor ampacity.

I don't think the NESC necessarily applies just because the pri is 4160V.

 

[coulter]

...On the primary I am doing this: (NEC 2005),Table 450.3(A)

750kva / (4160 x 1.7321) = 104A
...

Looks good to me.

... location will be considered a supervised one so:
104A x 2.50 = 260A OCP (note1 T450.3(A)), 240.6 allows me to up-size to 300A fuse. ...

Supervised, fuses, no secondry protection. Still Good

...Is that just wrong? or would 300A be considered the MAXIMUM size fuse you would want to ever use?...

No. Yes.

For the conditions you listed, the NEC would say 300A is the max. Supervised, pri CB, secondary protection, one can go to 600% pri FLA. and if I have a winding temp trip, see note 5 to table 450.3(A), I may not even need secondry protection. That sounds really loose to me.

But, the NEC is not a design guide. (oh-oh I can feel a few members reaching to wash my mouth out (wash my fingers out?))

In this case, I would be much more concerned with coordination and fault protection than I would be about hitting the NEC minimums for a cheap job. Customers with this kind of stuff really want it to last.

Leaves one hoping the designers/installers know what they are doing.

 

[websparky]

Education is the answer!

Education is the answer!

Carl,

Ahhh....... Thanks for the correction! I now see the error of my ways. I failed to look at the >600A table and thanks for the reminder of 240.101.

I agree with Carl,

In this case, I would be much more concerned with coordination and fault protection than I would be about hitting the NEC minimums for a cheap job. Customers with this kind of stuff really want it to last.

and add I would feel better if the primary fuses were only 200A.


Kevin,
You are on the right track. Now figure the secondary maximum load based on the overcurrent protection and you should come up with 913,000va. Then use the formula to figure the primary amp draw. Then add up to 3X for the fuses.

 

[dlhoule]

I can't wait till I can rattle this stuff off the top of my head like most of you guys can.

Kevin, I think it is only a select few can rattle this stuff off the top of their heads. After studying for a few more years I hope to join them.