ECN Forum Forums Code Related Discussion NEC & other Code issues textile transformer install??????

Ok this should be good.We have 2 37.5 kva xfmers taking 575v 3ph down to 480v 3 ph.The info on the xfmer says kva of both xfmers times 2 times 5 times .86 total= 324 amps primary the secondary 388 amps.My questions are in the wire sizes we were told to use and ocp.They said 500 kcmill.I figured 600 kcmill primary. And 450 ocp on the pri they said 400 Should the secondary condutors be rated for the avalible 388 x 1.25 or the load calculated being used by the machines. The load panel is 400amp panel no ocp inside.Or if someone has a sugestion I`d love to hear the alternatives for this job.

Guys after I read my post I found some details left off.The total kva capacity of the 2 transformers is 37.5 x 2=75 x 5=375 x .86=322.5 kva avalible from the xfmers. Thus kvax1000/575x1.732= 322.8 or 324amps.So I did 324 x 1.25=405a for the conductors.If I use 75 degree col. then I figured the 600kcmil and 450 ocp.But thats the full load cap. They`re saying the load is not actually that much so I`m wrong on my calc.The load panel is 400amp sq. D load center feeding textile mach. total load is actually less 150 amps for the mach. they are hooking up.So do I size to the load or size to capascity of the xfmers.The boss will actually do the calc. tomorrow just trying to see where I stand.TRying to learn all this stuff.When I said size to the load I mean Does my secondary wires need to be for the 400a panel or the 150 amp load or secondary of the xfmer which I figured at 388amps x 1.25=485 for the conductors on the load side.Then go 75 col and 800kcmill. Guess I`d have to 2 sets of something else?

Hey any help would be appreciated.I just moved up from the house wiring crew to this one.Got a lot to learn.

Fun. For the audience, in your souvenir programs turn to page 40 figure H to follow along with the announcer at http://www.federalpacific.com/7700-94.pdf and, for example in this round, we have two open-delta-connected 240x480─120/240 Volt, 37.5kVA drytypes. For each transformer primary coils in series for 480V and secondary coils in parallel for 120V

Start with the two transformer secondaries. For each 120V secondary the current is rated

37500VA/120V=312.5A

Each 480V primary is rated

37500VA/480V=78.1A

root3/2 open-delta multiplier applied


Autotransformer-bank 600V hi-side current is 271 amperes

Autotransformer-bank 480V lo-side current is 338 amperes

These calculations apply to the full autotransformer-bank ratings Article 450 and 240. No concern about separately derived grounding¢®¨£just the plain-vanilla equipment-grounding of the transormer frames,

Overcurrent-device and cable ratings on your own...

Calcs at http://67.115.161.42/dat/ecn.buck120xf2.doc


[This message has been edited by Bjarney (edited 06-14-2002).]

Conductor sizing is based on the calculated load. OCPD and wire selection should always be calculated from the load towards the source. There is nothing wrong with not using all of the capability of the transformer.

Based on your information, I would use the following 75C sizes (please apply any applicable derating):

Calculated load = 150A (use 150 x 1.25) use 3/0 minimum
Transformer to Panel size = 400A panel bussing, use 500Kcmil maximum
Transformer secondary OCPD = 400A maximum (equal to the rating of the panel), actual need dependent on use of Transformer Tap Rules
Transformer primary = 310.69A (use 311 x 1.25) use 389A max = 500kcmil with a 400A OCPD or 600Kcmil with a larger OCPD.

The transformer arrangement you have is "Two Units Connected as Open Delta Auto Transformer for Three Phase Operation", 600V high side and 480V low side. The 3PH KVA = (Total KVA of Both units x 5)x .86, or applying math the simple formula, for identical transformers, is 3PH KVA = 8.6 x 1PH KVA. In this case 8.6 x 37.5 = 322.5 KVA which yileds 310.69A @ 600V and 388.37A @ 480V.



[This message has been edited by JBD (edited 06-14-2002).]

Here`s what happened. We tapped off line side 400amp 575v panel being feed from 500 amp breaker in the substation.We placed a sdD 200 amp ocp/disconn. feeding xfmers. On the pri. side and 3/o rhw cable #4 cu ground. This is what I`m not sure of.on the load side feeding a 400 amp sdD panel.With only 150amp worth of load we ran 4/0 with #4 ground.Shouldn`t we have a disconn.?we don`t.I`m new so I`m asking here.The leadman says since it`s not lighting panel don1t need disconnect on load side .Could some one send me in the right direction in the code please.Give me the art #`s please.I`m sure it wont be changed because the machines are working but at least I`ll know next time.
Thanks in advance

Power panels do not need to have a main OCPD. Conductors must have OCPD where they receive their supply unless they are covered under the "tap rules" in the NEC99 240-21.

Also, According to NEC99 240-3(f) Transformer Secondary Conductors. ... Conductors supplied by the secondary side of a ... three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with Section 450-3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio.

So based on your primary side OCPD of 200A, you could protect secondary conductors rated for 250A (200 x 600/480) minimum or 250kcmil @ 75C. Your secondary conductors of 4/0 would need the primary OCPD to be 184A (230 x 480/600) maximum. Upsizing to next standard rating is not allowed.

Thanks JBD I`ve read the articals you mentioned and I have a better understanding of the ratio and how it works in the calc.So I we should have either decreased the pri. ocp or increased the sec. wire size.Good info.