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#85090 - 05/28/03 08:56 PM x-fmr feeds equipment
Steve T Offline
Member

Registered: 02/14/01
Posts: 312
Loc: Oak Park, IL, USA
A building has a 120/240 high leg three phase service.

The contractor wants to install a delta-wye 240/480 volt x-fmr that will directly feed a piece of equipment (industrial lathe, in a commercial building).

I am not sure how far away the equipment will be from the x-fmr, but lets say more than 25 feet for this question.

First off, can this be done?

Looking at 240-21 (c)(2) and (3), over 25 feet would not be allowed.

But even if it is not over 10 feet 240-21(c)(2)(c) says the conductors must terminate at an enclosed switchboard, panelboard, or control device, or back of an open switch board. Is a fused disconnect a control device as used in this section?

If so, does the neutral need to go to the disconnect? To the machinery?

Does the secondary transformer winding need to be grounded to all available grounding electrodes on site? What size does it have to be.? (15KVA x-fmr)

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#85091 - 05/29/03 06:17 AM Re: x-fmr feeds equipment
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
For a 15kVA 3-wire [no neutral] 240V-delta 480V-delta 3-wire [ungrounded] transformer, per 99NEC240-3(f), one could use the primary-side overcurrent protection to also protect the secondary side. Note that a delta-Y transformer will require 240V- and 480V-winding overcurrent protection and tap-rule application.

Per 99NEC450-3(b) suggest 45A 250V dual-element fuses for primary {with no secondary} overcurrent protection. 1.25∙15kVA/240V/sqrt3 = 45 amperes

{Small aside: Instead of a 150°C-rise drytype, suggest a 115 or 80°C-rise unit be obtained. Under load, expect the higher-temp unit to run very hot.}




[This message has been edited by Bjarney (edited 05-29-2003).]

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#85092 - 05/31/03 05:44 PM Re: x-fmr feeds equipment
WebSparky Offline
Member

Registered: 02/21/03
Posts: 144
Loc: Cleveland, Ohio
Hi Steve,
 Quote:
But even if it is not over 10 feet 240-21(c)(2)(c) says the conductors must terminate at an enclosed switchboard, panelboard, or control device, or back of an open switch board. Is a fused disconnect a control device as used in this section?


Yes

 Quote:
If so, does the neutral need to go to the disconnect? To the machinery?

In the application you describe, there is only a 3-phase load of 480V, so there is no need for a neutral.

 Quote:
Does the secondary transformer winding need to be grounded to all available grounding electrodes on site? What size does it have to be.? (15KVA x-fmr)

Yes. The "XO" terminal at the xfmr needs to be bonded to the enclosure of the xfmr, and to the load side disconnect as a grounding conductor. The xfmr also needs to be bonded to the equipement grounding of the supply side (240V system).
The size of the bonding conductor to the load side is based on 250.66 and the equipement grounding conductor on the supply side needs to be sized according to 250.122.

If I did the math correctly, in both cases an 8AWG conductor will be adequate for both.

Hope this helps!
Dave
_________________________
Dave

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#85093 - 06/01/03 08:56 AM Re: x-fmr feeds equipment
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
Dave,
 Quote:
Yes. The "XO" terminal at the xfmr needs to be bonded to the enclosure of the xfmr, ...

The 480 volt secondary of this system is not required by the code to be a grounded system. You can leave XO floating. If you choose to have a grounded 480 volt system, then you must tie XO to a grounding electrode system.
Don
_________________________
Don(resqcapt19)

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#85094 - 06/01/03 10:40 AM Re: x-fmr feeds equipment
WebSparky Offline
Member

Registered: 02/21/03
Posts: 144
Loc: Cleveland, Ohio
Hi Don,

You are correct. Thanks!
IMO
It seems safer to me to bond the secondary for possible ground fault currents.
_________________________
Dave

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#85095 - 06/02/03 08:43 AM Re: x-fmr feeds equipment
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
No personal offense intended, but there’s a slight terminology issue here. By ANSI definition, a lower-voltage winding is designated as having “X” terminals and the higher-voltage winding is identified by “H” terminals. For the described transformer, there would be no X0 terminal—a 480V wye-point connection would be labeled H0.

Connecting the 240V “neutral” to a winding midpoint would serve no purpose. [Small aside: based on IEEE C57.12.70, the 240V delta midcoil tap would be labeled X4 or X6.]

If there are no 277V loads to be served, then a 480Y/277V winding is unnecessary, and may have a longer leadtime and higher price. In this case, the 480V system is so small that system grounding gains very little. A delta-delta transformer may be more easily obtained.

An economical approach could be http://www.jeffersonelectric.com/pdf/3-phaseencap.pdf page 7 for a 240-to-480V autotransformer configuration, using two 5kVA 1ø transformers to serve up to a 17.3kVA 480V 3ø 3-wire load.




[This message has been edited by Bjarney (edited 06-02-2003).]

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#85096 - 06/02/03 06:55 PM Re: x-fmr feeds equipment
WebSparky Offline
Member

Registered: 02/21/03
Posts: 144
Loc: Cleveland, Ohio
Bjarney;

No offence taken, ever!
You are correct. I am too used to the step down that I forgot there is a difference!
Seems like the best solution is to go with a delta/delta.
Thanks for reminding us
_________________________
Dave

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