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

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.

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.

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.

?#1 yes
?#2 load?
?#3 yes yes

Larry

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 60°C rating due they run in underground.
However if he (OP) ran with alum conductors now that will change the rating a bit.

Merci,Marc

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

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).

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

Okie., we now know the load there from first secondary transfomer load I will readjust the numbers here.,

a shy over kilometer ?? dang that pretty long run ok let me get my chart and try to converted for that distance hope it will cover that range.

Yoopersup.,
Keep in your mind I will slect a common OCPD size so that way you will have a " wiggie " room due the load figures will affect at second transfomer.

I will run a voltage drop caluactions basied on 10 amp @ 480 volts 1Ø{ this size is rather semi common fuse or OCPD size }

#4 CU 4.3%VD {20.5V}
#3 CU 3.4%VD {16.3V}
#2 CU 2.7%VD {12.9V}

Keep in your mind I used 60°C rated conductors.

Merci,Marc

60c Wire rateing>???
In Conduit thhn/thwn???
Also protection & Equipment ground wire size???

I think I'd use 75° column and probably THWN since it more than likely is a wet location and the over current devices will likely have 75° terminations.

Another thought would be - Could we use a buck/boost transformer?

Very good question Ernie, I've never had to work a problem like that.

Additional information-

All conductors on the Primary side of the transformer(s) need to be protected at their ampacity. All conductors on the Secondary of the transformer(s) need to either be connected to an over current device or be protected via the over current device on the Primary side of the transformer and factoring in the turns ratio.
It appears to me that the problem Ernie presented using two transformers would need at the very minimum two over current devices to protect the Primaries of the transformers.

NFPA70 2005 Ed.
240.4, 240.21(C)(1), 450.3(B), Table 450.3(B)

Wire size due to VD a #2??
Wire size Equipment Ground #6??

At 208 volts, that would be a L-L connection into a 208Y/120 system. I'm wondering how you would be wiring that. Since neither primary connection is grounded, that rules out an autotransformer arrangement. How would the 480 volt secondary be grounded?

Perhaps an even more important question is how will the 208 volt step-down secondary at the far end be grounded? Grounded at one end? Center tapped? it certainly will not have the 120 degree phase angle relative to ground that 208 volts at the origin would have.

Yoopersup;

Please excuse me jumping in so late, but I have a couple questions to ask, and a few suggestions.

1: The Transformer at the end of this Loop (Circuit): Will the loads connected only be Line-To-Line 208V 1 Phase 2 Wire, or will there be 120V _AND_ 208V loads?

2: Do the connected loads require 208V 1 Phase 2 Wire, or could 240V 1 Phase 2 Wire be used?

3: Any possibility for 3 Phase loads in the future?

* The 208V 1 Phase 2 Wire Primary may be #6 with 55 Amp 2 Pole breaker.

* The Secondary Conductors may be protected by the Primary OCPD, since there is only ONE (1) derived Voltage from the Transformer's output (480V).

* Suggest to size the complete Circuit per the 10 KVA Transformer capacity.
This would include:
a:) The 480V Transmission Feeders (sized for 21 Amps, and adjusted for the Voltage Drop across 3500 feet of Conductor),
b:) The 208V Primary Feeder (55 Amp OCPD, #6 Feeders, #10 EGC),
c:) The Secondary Feeders at the step-down ("Last") Transformer.

* You might consider Grounding one side of the 480V output on the first Transformer.
This is an option, but for a stable Secondary Voltage to Ground, connecting the Transformer to a GES (Grounding Electrode System) is suggested.

* Suggest to Ground the Secondary of the "Last" Transformer to a local GES.
If the System derived from the "Last" Transformer is to be a "Dual Voltage" (120/240), _DEFINITELY_ Ground the Center Tap of the Transformer to a GES.

I can draw schematics for this scenario if needed.

Lastly, is this an actual proposed installation, or a "What-If" scenario?

Looking forward to your reply.

Scott

Scott
Yes this is a Actual Installation.
10 kva 208 volt single phase primary 40 amp. OCP at building Secondary 480 volt single phase secondary. 3500ft. run out to a step dn transformer 10 kva .
Loads at second remote transformer 120 volt. So its 120/240 v. single phase.
I feel main not required at second transformer on primary
225.32 Exception #1.
Remote secondary (40)amp OCP required (240.21).then breakers feeding each circuit.
Secondary of remote transformer grounded per 250.30.
Voltage drop on long run requires #2 thwn, Phase conductors
plus equipment ground.
Equipment ground wire 250.122 .
Well a # 6 do it OR is a #2 equipment ground also required per 250.122(B)
10 kva at 480 volts = 20.8 amps
10 kva at 208 volts = 48 amps
load is 5.5 kva at 480 volts = 11.45 amps
load at secondary remote 22.9 amps
Thats what I come out with yer turn .

Yoopersup

The 208V OCPD can definitely protect the conductors on the 480V side of the transformer, however I do not feel it will be able to be the required 450.3 primary protection for the second transformer.

JBD
According to Transformer section I;d say your probally right,Under Feeder exceptions (No). Thats why I posted it.
Get others opinions .What about the rest of data I posted. Any thoughts there.
Thanks
(Theres a hair line between Pride & stupidy)Right