Overcurrent after a transformer?

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dave121

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I'm installing a 240V single phase service, feeding a 240x480 step up trans. The trans. feeds a phase converter 3 feet away. The calculated load on the trans. primary is low enough to easily meet NEC, 450.3B for primary protection only.
But with 240.21.c, I'm not sure, is over current before the phase converter required? The trans secondary will be 3 wire.
Thanks for your help.
 
dave121 said:
I'm installing a 240V single phase service, feeding a 240x480 step up trans. The trans. feeds a phase converter 3 feet away. The calculated load on the trans. primary is low enough to easily meet NEC, 450.3B for primary protection only.
But with 240.21.c, I'm not sure, is over current before the phase converter required? The trans secondary will be 3 wire.
Thanks for your help.
Click to expand...
Is your transformer secondary 240/480 (with a grounded center tap)? If so you will need secondary protection.

If you only use two wire 480 volt on the secondary, even if there is a center tap but you leave it "float", then you effectively have a two wire system. You have to ground one of the outer leads and not the center tap when using it this way. Then your secondary is considered protected by the primary overcurrent device, as the secondary can not draw more then the primary to secondary ratio without also overloading the primary side.
 
Sorry, 1 more thing, we do have experience with the grounding of a phase on the secondary. Question is, is it proper to have 3 secondary conductors from said trans, grounded and white taped L1, ungrounded L2, and a eq. gr. to still qualify for primary protection of these secondary conductors?
 
dave121 said:
Sorry, 1 more thing, we do have experience with the grounding of a phase on the secondary. Question is, is it proper to have 3 secondary conductors from said trans, grounded and white taped L1, ungrounded L2, and a eq. gr. to still qualify for primary protection of these secondary conductors?
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I'm confused. If you are saying L1 on the secondary is your grounded conductor then that would be bonded in the transformer and your EGC would connect there.
 
That is correct. I just wanted to clarify that I can have an egc to leave the trans along with L1 andL2, L1 grounded in the trans, also on the same ground lug is the beginning of the egc to exit the trans, and still qualify for primary oc only.
 
dave121 said:
Yes, 240/480 with grounded center tap.
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240/480 with grounded centrertap in an interesting arrangement.
Which derlta has the center tap, the 240 or 480? It is being left to assume that the 480 delta has the center tap which is not very common but never the less possible to build a transformer that way. I may also assume that it may be more likely that a more commonly available 480v delta pri. with a 240/120 delta secondary with a lighting tap being used as as step-up transformer.
Including a picture of the nameplate of you transformer weouild leave no question..
 
templdl said:
240/480 with grounded centrertap in an interesting arrangement.
Which derlta has the center tap, the 240 or 480? It is being left to assume that the 480 delta has the center tap which is not very common but never the less possible to build a transformer that way. I may also assume that it may be more likely that a more commonly available 480v delta pri. with a 240/120 delta secondary with a lighting tap being used as as step-up transformer.
Including a picture of the nameplate of you transformer weouild leave no question..
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OP has a single phase transformer.
 
kwired said:
OP has a single phase transformer.
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Oops, as you can tell I had 3ph D-D on my mind an well as the mention of a center tap. With no additional info provided then is a common 480-120/240 or 240/120 transformer being used as a step up then?. Otherwise it may be 240- 480/240.
A picture on the nameplate would answer a lot of questions.
What is the KVA as this voltage combination is more common with CPTs and machine tool transformers which could provide a pr to sec ratio issue.
 
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I don''t have a picture of the trans. right now ,but they are being built for us for the purpose of stepping up from 240 to 480, single phase, with the center tap on the secondary for us to be able to ground the secondary. With the 1st piece of eq. after the trans being only 3 ft. away, I'd like not to land on OC. first if possible. ( Cost $.) Just trying to clarify if I make it a GROUNDED PHASE secondary I need only primary protection. Also wanted to make sure with this grounded phase secondary we can still leave the trans. with an EQ GR along with the phase conductors and still only need primary protection.
 
Or bh

Or bh

dave121 said:
I don''t have a picture of the trans. right now ,but they are being built for us for the purpose of stepping up from 240 to 480, single phase, with the center tap on the secondary for us to be able to ground the secondary. With the 1st piece of eq. after the trans being only 3 ft. away, I'd like not to land on OC. first if possible. ( Cost $.) Just trying to clarify if I make it a GROUNDED PHASE secondary I need only primary protection. Also wanted to make sure with this grounded phase secondary we can still leave the trans. with an EQ GR along with the phase conductors and still only need primary protection.[/QUOT

Thanks for the reply. From what you are saying is that you are having a custom transformer built with a delta sec that has a center tap of which you intend to use the centertap as your ground. The one gray area that I can see is if you were to use the center tap to serve a 1ph load which I know that you are not going to but let's say that you just for grins then you would have a 3ph4w sec which requires a sec OCPD. They do this because sec currents are not seen in proportion on the primary as they would and a basic 3,ph3w.
When the centertap is used for grounding will the pri OCPD respond to an arcing or bolted ground fault accordingly? I would confirm that it would. I don"t think there would be any issues with overload protection. But I would confirm that the inst short circuit protection would respond to ground faults since you may be looking at really a 3ph4w because of your grounding the centertap where tthe pri OCPD may not respond to a ground fault.
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The info on the trans. is they are wired series/ parallel. 3 wire, 240 v, 1 ph. input.. / 480 1 ph. out. Will only use 480 volts on the output. Although there is a center tap on the secondary, will leave float. ( or get trans. without center tap )The grounded phase and eq. gr. land on same ground lug inside trans., Leaving the trans, 1 intentionally grounded current carrying conductor( L1 )- 1 ungrounded cond.( L2 )- and 1 Eq GR.. Hoping this qualifies for primary protection only.
Thanks
 
templdl said:
dave121 said:
I don''t have a picture of the trans. right now ,but they are being built for us for the purpose of stepping up from 240 to 480, single phase, with the center tap on the secondary for us to be able to ground the secondary. With the 1st piece of eq. after the trans being only 3 ft. away, I'd like not to land on OC. first if possible. ( Cost $.) Just trying to clarify if I make it a GROUNDED PHASE secondary I need only primary protection. Also wanted to make sure with this grounded phase secondary we can still leave the trans. with an EQ GR along with the phase conductors and still only need primary protection.[/QUOT

Thanks for the reply. From what you are saying is that you are having a custom transformer built with a delta sec that has a center tap of which you intend to use the centertap as your ground. The one gray area that I can see is if you were to use the center tap to serve a 1ph load which I know that you are not going to but let's say that you just for grins then you would have a 3ph4w sec which requires a sec OCPD. They do this because sec currents are not seen in proportion on the primary as they would and a basic 3,ph3w.
When the centertap is used for grounding will the pri OCPD respond to an arcing or bolted ground fault accordingly? I would confirm that it would. I don"t think there would be any issues with overload protection. But I would confirm that the inst short circuit protection would respond to ground faults since you may be looking at really a 3ph4w because of your grounding the centertap where tthe pri OCPD may not respond to a ground fault.
Click to expand...
Click to expand...
 
Damn!! Did it again!! I am stuck on 3ph!! I apologize.
With a 480/240 1ph secondary you would face the same issue with detecting a L-G fault with a pri only OCPD. Even though you don't have any L-N loads you essentially still have to consider that grounded center tap as 1ph 3w. I don't think there would be any issue with overload as there are no unbalanced loads. But a L-G fault would be unbalanced which would not be seen proportionally by a 3p pri OCPD I would think.
Your thoughts?
 
dave121 said:
The info on the trans. is they are wired series/ parallel. 3 wire, 240 v, 1 ph. input.. / 480 1 ph. out. Will only use 480 volts on the output. Although there is a center tap on the secondary, will leave float. ( or get trans. without center tap )The grounded phase and eq. gr. land on same ground lug inside trans., Leaving the trans, 1 intentionally grounded current carrying conductor( L1 )- 1 ungrounded cond.( L2 )- and 1 Eq GR.. Hoping this qualifies for primary protection only.
Thanks
Click to expand...
Yes.
 
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