5kVA control transformer design

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philly

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Need to supply 120V control power to a 30A disconnect. I have a 480V circuit in the area that I am going to tap off of.

I'm going to purchase a 480V pri / 120V sec 5kVA transformer. On the primary after tapping of 480V circuit, I'm going to put a 15A circuit breaker as the primary protection. (Aprox 125%)

I dont believe I need to put any OCPD on the secondary of the transformer since it is only a 2-wire secondary? Also the 30A fused disconnect (feeding equipment) is located about 5ft away. Would this disconnect serve as an adequate secondary protection for the transformer if it was indded needed? I know it is lower that rated secondary of transformer but if thats all the equipment would ever need than that would be adequate.

Thoughts on this design?

Is there any advantage to using a fuse over a breaker for the primary protection of the transformer?

If the secondary was a 240/120 transformer then would I calc the secondary current using 240V to get a seconday of 20.8A?
 
I may be looking at this incorrectly, but I think you will need secondary OCP. In the 480/120 situation your 30 amp secondary would not meet the "ratio" rule 240.4(F). If you use a 240/120 secondary then you no longer have the two-wire secondary provision.
 
augie47 said:
I may be looking at this incorrectly, but I think you will need secondary OCP. In the 480/120 situation your 30 amp secondary would not meet the "ratio" rule 240.4(F). If you use a 240/120 secondary then you no longer have the two-wire secondary provision.
Click to expand...

Reading 240.4(F) I think I would not need secondary protection however I would need to make sure that my secondary conductors were sized accordingly to meet the multiplier between the primary and secondary.

If I needed secondary overcurrent protection and wanted to use the disconnect 5ft away, is there a rule for how far the secondary protection can be located? What about the cables between the transformer and secondary protection.

For a 5kVA transformer the secondary current at 120V is 41A. If I was using a 120/240 secondary would I use 240V for current calculation arriving at 20A and thus use this 20A for secondary ratings?
 
I definitely stand to be mistaken here, but the way I see it, the only way his primary protection can serve as his secondary protection is if he stays 2 wire pirimary, 2 wire secondary, which your connection would allow.If I read 240.21(C)(1) & 240.4(F) correctly, if he uses secondary conductors rated at 60 amps (15 amp primnary/4:1 ratio) he will satisfy that section of the Code.
I anxiously await more learned input.
 
ignore above,,, I waited too long to edit...

Well, I have confused myslelf now. I started with 240.4(F) which would indicate to me as long as he kept the connectios 2 wire primary+ 2 wire secondary, the seconday condcurtors would need to be 60 amp due to the 1:4 ratio and the 15 amp primary.
But 240.4(F) does not mention length.
His scenerio seems to fall under 240.21(C). iN 240(21)(C)(1) the same ratio rules apply.
Again no length.
Since he has 30 amp secondary protectioon within 10 ft, 240.21(C)(2) would seem to allow a #10 secondary.

I anxiously await input by others.
 
augie47 said:
ignore above,,, I waited too long to edit...

Well, I have confused myslelf now. I started with 240.4(F) which would indicate to me as long as he kept the connectios 2 wire primary+ 2 wire secondary, the seconday condcurtors would need to be 60 amp due to the 1:4 ratio and the 15 amp primary.
But 240.4(F) does not mention length.
His scenerio seems to fall under 240.21(C). iN 240(21)(C)(1) the same ratio rules apply.
Again no length.
Since he has 30 amp secondary protectioon within 10 ft, 240.21(C)(2) would seem to allow a #10 secondary.

I anxiously await input by others.
Click to expand...

In order to meet 240.21(C)(1) I believe I would need to have a secondary conductor rated for 60A as you mentioned.

However since the secondary conductor will be less than 10ft I am now looking at 240.21(C)(2).

If I use a #10 for the secondary conductors I will meet the following.

For 1a. the combined calculated load is less than 30A so I meet this requirement.

For 1b. the overcurrent protection device that the secondary conductors terminate into is a 30A disconnect, so I meet this requirement.

For 1c. The secondary conductors will not be less than 1/10 the rating of the primary overcurrent device multiplied by the ratio. (6A)

For 2, the secondary conductors do not extend beyond the disconnect so I meet this requirement.

For 3, the secondary conductors will be enclosed in rigid conduit from the transformer to the disconnect, so I meet this requirment.

Looking like I meet all these requirements I am going to tap off of the 480V circuit using a #12 and terminate into the primary 15A breaker. From the 15A breaker I will again have #12 from the breaker to the primary of the transformer.

On the secondary of the transformer I will have #10 running between the transformer secondary and 30A disconnect.

Sound right to you guys?
 
090422-1141 EST

philly:

It is quite common in an industrial application to use a plugin disconnect with fuses to connect to a 480 V bus distribution system. A line drops from here to the machine. Another disconnect in the electrical cabinet. More fuses to the various 480 devices which includes a 480 to 120 transformer. On the secondary more fuses or breakers to various items.

Your primary side protection device, fuse or breaker, has to tolerate the worst case primary inrush current of your transformer. For one half cycle this may be 10 or more times the steady state peak full load rating of the transformer. For example: 5 KVA at 480 is 10.4 A. 10.4/0.707 = 14.7 and 10 times this is 147 A. This does not occur every time the transformer is turned on, but can randomly occur.

To tolerate primary inrush you have to size the protection device to have an extremely low probability of tripping on this current pulse, but still protect the transformer. This may be inconsistent with protecting the secondary load device with the primary protection device.

The wires on the secondary side need protection. I suggest it is best to provide this protection at the transformer secondary output even if theoretically the primary protection device could accomplish the desired result. Furthermore it seems appropriate to have a disconnect at the transformer on the secondary side.

Whatever the load device is it should have protection that is appropriate to that load at the load end.

It is useful to have indicators at these various locations so that it is easy to troubleshoot any problems.

.
 
gar said:
090422-1141 EST

Your primary side protection device, fuse or breaker, has to tolerate the worst case primary inrush current of your transformer. For one half cycle this may be 10 or more times the steady state peak full load rating of the transformer. For example: 5 KVA at 480 is 10.4 A. 10.4/0.707 = 14.7 and 10 times this is 147 A. This does not occur every time the transformer is turned on, but can randomly occur.
Click to expand...

I will use a thermal type breaker here which will allow for this inush. I am thinking about using a Siemens ED4 molded case circuit breaker rated for 15A.

gar said:
090422-1141 EST

The wires on the secondary side need protection. I suggest it is best to provide this protection at the transformer secondary output even if theoretically the primary protection device could accomplish the desired result. Furthermore it seems appropriate to have a disconnect at the transformer on the secondary side.

.
Click to expand...

You are saying that even if I meet all the reuirements of 240.21(C)(2) and am covered by code, and the load disconnect is only 5ft away from the transformer that I should install a secondary breaker on the transformer?

I dont see why this would be necessary if this transformer is dedicated for this circuit only and can shut off the main if need to work on any part of the service, and the fact that the disconnect will act to limit any overloads on the secondary of this transformer.
 
090422-1326 EST

philly:

I did not want to imply that you needed to do all the items I mentioned, rather I was trying to point out the theory of the elements. You pick and choose from these elements what is required by code and your needs.

Certainly you can put large enough wires on the transformer output that the input protector could can provide their protection.

If the load on the secondary is small in relation to the primary protection as reflected to the primary, then the primary protection may be insufficient to protect the load.

.
 
gar said:
090422-1326 EST

philly:

I did not want to imply that you needed to do all the items I mentioned, rather I was trying to point out the theory of the elements. You pick and choose from these elements what is required by code and your needs.

Certainly you can put large enough wires on the transformer output that the input protector could can provide their protection.

If the load on the secondary is small in relation to the primary protection as reflected to the primary, then the primary protection may be insufficient to protect the load.

.
Click to expand...

I guess my question is now, since I satisfy the requirments of 240.21(C)(2) do I need to also meet the requirements of 240.21(C)(1)?

In other words, since I meet the requirements of 240.21(C)(2) can I keep my secondary conductor as a #10 although this would not meet 240.21(C)(1)?
 
philly said:
I guess my question is now, since I satisfy the requirments of 240.21(C)(2) do I need to also meet the requirements of 240.21(C)(1)?

In other words, since I meet the requirements of 240.21(C)(2) can I keep my secondary conductor as a #10 although this would not meet 240.21(C)(1)?
Click to expand...
No. 240(C)(1) through (6) are alternate methods of protecting the secondary conductors. You only have to comply with one of them.
 
Don, I think I started down the wrong path originally on this question as I got hung up on 240.4(F) before consultiig 240.24(C).
This particular application seesm to meet 240.24(C), but I'm left with a question.
In a 2 wire-->2 wire application where you meet the ratio rule in selecting the ampacity of your secondary conductors, is there any limit as to secondary length or OCP ?
 
philly said:
From the 15A breaker I will again have #12 from the breaker to the primary of the transformer.

On the secondary of the transformer I will have #10 running between the transformer secondary and 30A disconnect.

Sound right to you guys?
Click to expand...

Philly, the primary conductors and primary protection sounds right, but the secondary conductors need to be rated for at least 60 amps without secondary protection and unlimited length using 240.21(C)1. If the 30 amp rated disconnect is fused at 30 amps and with in 10' of the secondary of the transformer, the conductors can be 10 awg.

Rick
 
RUWired said:
Philly, the primary conductors and primary protection sounds right, but the secondary conductors need to be rated for at least 60 amps without secondary protection and unlimited length using 240.21(C)1. If the 30 amp rated disconnect is fused at 30 amps and with in 10' of the secondary of the transformer, the conductors can be 10 awg.

Rick
Click to expand...

Yes the 30A rated disconnect on the secondary is within 10ft and will actually be fused at 15A, so I believe as you stated the conductors can still be #10.

It turns out the transformer that I will use is actually a 3-wire 120/240 transformer. I dont think that changes anything as far as being able to use #10 because I will still be satisfying 240.21(C)(2). I will wire the secondary coils in parallel so that the output is only 120V. However if I did leave the secondary as a 120/240 output, then would my secondary change?

I have seen most transformers have fuses on the primary and CB's on the secondary. Is there any advantage to using fuses on the primary of the transformer?
 
With a 240/120 transformer secondary my rated secondary current would be 20A wouldn't it? I would then have to size this secondary off of 20A as opposed to the 41A with just a 120V transformer secondary.
 
augie47 said:
Don, I think I started down the wrong path originally on this question as I got hung up on 240.4(F) before consultiig 240.24(C).
This particular application seesm to meet 240.24(C), but I'm left with a question.
In a 2 wire-->2 wire application where you meet the ratio rule in selecting the ampacity of your secondary conductors, is there any limit as to secondary length or OCP ?
Click to expand...
Gus,
There is no limit on the length of the secondary conductors in that case and no requirement for additional protection. They are protected by the primary OCPD.
 
OK, before I go and finalize this design I just had the lingering question in regards to primary fuses vs a primary breaker. Are there any advantages to using a primary fuse as opposed to a circuit breaker.

Some of the reasons to use a fuse that I can think of is that a disconnect and fuse may be cheaper than a breaker and that a fuse size can be changed more easly and less expensively than a breaker.

These reasons that I have are mainly economic reasons, but is there any reasons from an electrical integrity standpoint to use a fuse over a breaker for the primary protection of the transformer?
 
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