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Joined: Mar 2003
Posts: 849
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Single phase 208 volt primary. 480 volt single phase secondary. 10 kva transformer.(stepping up)
40 amp breaker on Primary.
Question 1 : secondary protection required??
Question 2 : Distance 1000 feet on secondary with 2 # 2 & 1 # 6 Ground. OK??
Question 3 : Step back down at end of 1000 ft to 208 volt Protection required Primary or secondary or both???
I got my ideas want yours .
Yoopersup

2017 / 2014 NEC & Related Books and Study Guides
Joined: Feb 2003
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Originally Posted by Yoopersup
Single phase 208 volt primary. 480 volt single phase secondary. 10 kva transformer.(stepping up)
40 amp breaker on Primary.
Question 1 : secondary protection required??
Question 2 : Distance 1000 feet on secondary with 2 # 2 & 1 # 6 Ground. OK??
Question 3 : Step back down at end of 1000 ft to 208 volt Protection required Primary or secondary or both???
I got my ideas want yours .
Yoopersup


I have few question realted to the 10KVA transfomer size are you sized by 80% of rated transfomer ?

Next step here for rest of the question I will post the answer for each one

1] Yes you will need secondary protection either CB or Fuse { you will see more detail when I get the latter part}

2] #2 conductor is not a issue with that kind of load however you will have issue with EGC conductor I did check the voltage drop with #2's 1.5% VD {7.4V} #4's 2.4% {11.7V } #6's 3.9% {18.7V} #8's 6.2% {29.6V} keep in your mind I figures with 20 A @ 480V any larger loads than what I listed here the bet is off.

3]get nonfused primary disconnect switch that will be used as isolation switch useage and yes you will need OCPD on secondary side to finetuned to that load.

I will let other members here to see if they come up with answer if they are the same page or not.

Merci,Marc



Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

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I would step up to 600V if possible.

Because your transformer is a single voltage output, you could use the primary OCPD as protection for the secondary conductors. But, because you have two transformers in series you will, need an OCPD somewhere between them.

Joined: Dec 2002
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Marc breings up some good points.

Dry type transformers are not rated for overload like the liquid filled "pigs" on poles. Keep the loading at 80%.

Next be aware that not all transformers may have their primaries and secondaries swapped. Don't do it for transformers under 1kva, they typically have uneven primary to secondary turns ratios to make up for their small wire and voltage drop. Check with the manufacturer before using ANY transformer in this manner to be certain.

Otherwise, in doing voltage drop calculations, you need to consider how balanced the load is, the type of tranformer connection (wye-wye, wye delta, etc), even the type of load (harmonics and power factor). Without checking Marc's numbers they are a good place to start.

Stepping up to 600 volts can sometimes increase the overall cost. Cost it out both ways, realizing you also need to redo the voltage drop calcs for the 600 volt and proportionately smaller current.


Joined: Jan 2004
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Can someone tell me how you came up withe the voltage drop calculation when you don't know what the load is?

I'm not an engineer but I think we need more details before we can answer Yoopersup's question.


George Little
Joined: Sep 2007
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?#1 yes
?#2 load?
?#3 yes yes

Larry

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Originally Posted by George Little
Can someone tell me how you came up withe the voltage drop calculation when you don't know what the load is?

I'm not an engineer but I think we need more details before we can answer Yoopersup's question.


George.,

Let you know I did the voltage drop calucation based on max load of 20 amp @480 volts due the transfomer rating with copper conductors with 60C rating due they run in underground.
However if he (OP) ran with alum conductors now that will change the rating a bit.

Merci,Marc



Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

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I would not be one that would say the size of the transformer equals the load. Another item missing is the size of the conductors on the primary of the first transformer.

As for the derating of the transformer to 80%, I can't find nay evidence that transformers are to be derated.

What's this about Wye/Delta Wye/Wye. It's a single phase transformer not a 3 phase.

Okay guys/gals, lay it on me smile

Last edited by George Little; 02/13/09 11:00 AM.

George Little
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Transformers are designed and intended for 100% loading.

Transformers are basically only affected by heat. You can overload any transformer as long as you give it sufficient time to cool down (i.e. 2 hours of 120% may then require 8 hrs at 60%). This is how the utility pole mount transformers are sized.
Other than possible reduced life, an overloaded transformer may have reduced voltage regulation, and the conductor losses go up by the 'square' of the loading (110% load = 121% losses).

Joined: Mar 2003
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Revisions on Question.
#1 Load is 7.29 amps on secondary of ist 10 KVA transformer.
# 2 Distance is 3500 ft not 1000 ft as stated before.
Is this enough info ?? also state code sections for answers .
Yoopersup

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