1 phase 480v transformer and get a neutral?

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I need so help. I have installed some small transformers primarily for control voltage or Lv panels. I understand transformers to a degree but I don't understand the single phase 480v. To set the the scene, I have 3phase 4 wire and need to get 120v w/neutral. I know I can use a 3 phase transformer, that is not the question. What I will use is a 3kva transformer and the client doesn't want the transformer mounted outside of the panel. Also, they have already purchase the 3 panels that they want to use for this job buy copying panels designs that they purchase for their robots because they seen that they could get what they needed from them. 480v in and 120v and 24v out. Then they wanted to take that and add what they wanted and I don't want to fit everything so tight. It is complete mess and is one of the largest companies in the US if not the largest and I could make them buy the correct stuff. So I am wanting to know..... With a 3kva transformer can I use 1Ph of 3Ph of 480v to get 120v with neutral. This would allow me to mount and install this much easier and better for various reasons. The load will be no more than 15-20amp and I don't think I should be concerned with unbalancing the 3ph 480v by using only 1Ph but correct me if I am wrong. This branch of 480v will power some other circuits for drives that will have no more than 5hp motor connected to them. The 120v circuits will be used for auxiliary devices. This is a testing center so it might be used once a day or once a week but none the less the load will not be continuous and the load on the 120v circuit will either be maybe a amp or 2 while the 3ph 480 circuit is being used. I'm came across this before but never found out if I had option here and I haven't seen it in the field. Sorry for the long drawn out scene just trying to explain the mess. I may have missed some need info so let me know If I did.
 
Is there one with a 240/480v primary and a 120/240v secondary?

If so, I would series the primaries and parallel the secondaries.
 
LarryFine said:
Is there one with a 240/480v primary and a 120/240v secondary?

If so, I would series the primaries and parallel the secondaries.
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I believe the correct designation on the primary would be "240 X 480". The difference between the / and the X is both can be used series or parallel but the X is not suitable for 3 wire service.
 

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texie said:
True. I would add that the OP will need to ground/bond one end, have a GES and all proper OCP.
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Isn't there some exception for GES for transformers under a certain size, where you just bond whatever is zero volts on the secondary to the EGC?
 
electrofelon said:
I don't think there is such a thing. There is an exception to grounding SDS's for control circuits, but there are other restrictions. 250.21(A)(3)
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I fount it, only 1kva
2017 250.30(a)5
Exception No. 3: A grounding electrode conductor shall not be requiredfor a system that supplies a Class 1, Class 2, or Class 3 circuit and is derived from a transformer rated not more than 1000 volt-amperes,provided the grounded conductor is bonded to the transformer frame or enclosure by a jumper sized in accordance with 250.30(A)(1), Exception No. 3, and the transformer frame or enclosure is grounded by one of the means specified in 250.134.
 
tortuga said:
This is what I use, however Technically that 120 would not have a 'neutral' it would just be 120.
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True if a two wire output you technically have no "neutral" but you still would ground one conductor most cases, and still have one 120 volt ungrounded and one grounded conductor for utilization purposes.
 
Todd0x1 said:
I fount it, only 1kva
2017 250.30(a)5
Exception No. 3: A grounding electrode conductor shall not be requiredfor a system that supplies a Class 1, Class 2, or Class 3 circuit and is derived from a transformer rated not more than 1000 volt-amperes,provided the grounded conductor is bonded to the transformer frame or enclosure by a jumper sized in accordance with 250.30(A)(1), Exception No. 3, and the transformer frame or enclosure is grounded by one of the means specified in 250.134.
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Good catch. I did misread your post a bit and was thinking of systems not required to be grounded. The systems in 250.30 still need to be grounded, just how that is done is more lax in the exceptions.
 
I think you could get by with a 2kva. Transformers can handle a degree of overload and it sounds like it would be of short duration.

Something like Square D 9070TF2000D1 would do. I'd say fuse the secondary with a 15 amp slow blow fuse. You will need to bond the other side of the secondary to incoming power ground.
 
kwired said:
so would 2mVA :unsure: :)

1 kVA is often considered a high power level when talking about control circuits though.
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Is the intent of the GEC requirement, and the 1kva limit for the exception, so that the return path for ground fault current to the GES or bonded metal has a low enough impedance for an ocpd to operate? I imagine the instant trip curves on a breaker protecting the primary on a 1kva or one on a 2kva transformer to be virtually identical, so what is gained by limiting the exception to 1kva vs 2?
 
Todd0x1 said:
Is the intent of the GEC requirement, and the 1kva limit for the exception, so that the return path for ground fault current to the GES or bonded metal has a low enough impedance for an ocpd to operate? I imagine the instant trip curves on a breaker protecting the primary on a 1kva or one on a 2kva transformer to be virtually identical, so what is gained by limiting the exception to 1kva vs 2?
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I dont think one can find much logic or valid reasoning in many of the SDS grounding requirements and GEC requirements.
 
Todd0x1 said:
Is the intent of the GEC requirement, and the 1kva limit for the exception, so that the return path for ground fault current to the GES or bonded metal has a low enough impedance for an ocpd to operate? I imagine the instant trip curves on a breaker protecting the primary on a 1kva or one on a 2kva transformer to be virtually identical, so what is gained by limiting the exception to 1kva vs 2?
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electrofelon said:
I dont think one can find much logic or valid reasoning in many of the SDS grounding requirements and GEC requirements.
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I agree. GEC has nothing to do with making OCPD operate, that is what EGC's and bonding is about. GEC function is to make a reference to earth. But fault current isn't returning to earth, it is returning to the source, if earth happens to be in the path to the source, with a low enough impedance, then some the fault current will take that path. Otherwise it's purpose is a place to assist with bleeding off transients from external origins
 
kwired said:
I agree. GEC has nothing to do with making OCPD operate, that is what EGC's and bonding is about. GEC function is to make a reference to earth. But fault current isn't returning to earth, it is returning to the source, if earth happens to be in the path to the source, with a low enough impedance, then some the fault current will take that path. Otherwise it's purpose is a place to assist with bleeding off transients from external origins
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I understand what the GEC does. I was thinking in having one on a SDS could shorten the fault current path for a fault on the secondary, depending on where the fault occurs. Is that incorrect?
 
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