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#192570 - 02/17/10 08:25 PM Transformer Secondary
SJT Offline
Member

Registered: 07/24/02
Posts: 242
Loc: PATCHOGUE, N.Y.
When wiring a 45KVA transformer, the secondary conductors must terminate in a single disconnect, right? Or to a 100 Amp Main Breaker Panel. There is protection on the 480V primary side. I looked at one job, and they have a three phase 480V to 208V 45 KVA transformer. The secondary conductors go to a Wireway, and they spliced feeders to two- 100 amp panels. This would be a violation, right? Thanks

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#192580 - 02/18/10 08:49 AM Re: Transformer Secondary [Re: SJT]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
The conductors on the secondary of a transformer need to follow the 'tap' rules in 240.21(C).

Depending on the distances and cable sizes involved, it is possible that this is a valid installation.

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#192583 - 02/18/10 11:04 AM Re: Transformer Secondary [Re: JBD]
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
If the protection on the primary is less than 125% of the transformer rating, per NEC Table 450.3(B), no secondary protection is required. If the primary OCP is larger than 125%, the secondary protection cannot be more than 125%, and two 100A breakers is in violation.

So... if that transformer is fed from a 60A breaker, I think you're OK. At least from a transformer protection standpoint; ampacity, etc, is a different issue.

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#192588 - 02/18/10 03:57 PM Re: Transformer Secondary [Re: SteveFehr]
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
Though the transformer may be protected by the primary side circuit breaker, all the conductors involved still need to be protected. It is not permissible to protect the secondary conductors of this installation with a primary side breaker only, see the last sentence of 240.21(C)(1)

A transformer can be loaded to 100% of its capacity but you;ll have to size the conductors and OCPD at 125% of the secondary full load current in order to take advantage of this. Note 2 to table 450.3(B) says you can have multiple circuit breakers on the secondary side, but they cannot total more than what a single breaker would be.

IMO, the two 100 amp panels described by SJT are not in compliance.
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#192589 - 02/18/10 04:34 PM Re: Transformer Secondary [Re: Tom]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Originally Posted By: Tom
A transformer can be loaded to 100% of its capacity but you;ll have to size the conductors and OCPD at 125% of the secondary full load current in order to take advantage of this. Note 2 to table 450.3(B) says you can have multiple circuit breakers on the secondary side, but they cannot total more than what a single breaker would be.

You are mis-reading 450.3.

A transformer is required to have a primary protective device rated at not more than 125% unlessthere is secondary protection. So, if the 45kVA primary side is protected at not more than 67A, all secondary protection may be ignored.

But, the secondary conductors need to be protected under 240.21(C). I can envision this particular installation as being acceptable under 240.21(C)2, 3, or 6.

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#192598 - 02/18/10 10:03 PM Re: Transformer Secondary [Re: JBD]
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
45kVA 3-phase XFMRs are suitable to feed ONE 3-phase 100A panel...

OP did not mention whether or not the 100A twins were 3-phase or single phase...

It is also not clear whether these panels have internal OCPD -- that is a back-fed 100 dual-pole breaker...

OP does not provide enough information to determine Code compliance.
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#192613 - 02/19/10 02:14 PM Re: Transformer Secondary [Re: Tesla]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Originally Posted By: Tesla
45kVA 3-phase XFMRs are suitable to feed ONE 3-phase 100A panel...

According to what NEC article?

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#192615 - 02/19/10 03:07 PM Re: Transformer Secondary [Re: JBD]
Gregtaylor Offline
Member

Registered: 03/02/05
Posts: 212
Loc: Boise, Idaho, USA
Tom, I'm with JBD on this one. The main OCPD's in 2-100A panels fed by a single 45KVA XFMR aren't added together that way any more than you would count the 20A CB's in a panel to determine it's load. If the conductors to each are sized properly and the primary OCPD is less than 125% of the XFMR's rating, multiple panels could be fed depending on computed load. And I would also echo the comment to Tesla. It's the most common set-up but that doesn't make it the only code-compliant arrangement.

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#192616 - 02/19/10 06:45 PM Re: Transformer Secondary [Re: Gregtaylor]
SJT Offline
Member

Registered: 07/24/02
Posts: 242
Loc: PATCHOGUE, N.Y.
OK, what I'll check are the size of the Fuses in the Disconnect on the Primary side, and if they are within the 125% of the XFMR's rating, I should accept the 2 - 100AMP Panels then. Sounds like if there was an Overload, the Primary fuses would go, to protect the Transformer. There are other Electric utility rooms in the same Building, with similar situations, but the key would be those fuses in the Primary side. Code wise, but not practical, there could be 4- 100AMP Main Breaker Panels on the secondary, as long as the Primary fuses are correct? Good Weekend

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#192617 - 02/19/10 07:13 PM Re: Transformer Secondary [Re: SJT]
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
You may NOT use only primary side protection for a 3-phase set-up.

Primary side only is permitted only by the exceptions which focus on buck-boost and single phase set-ups.

Further study will reveal that UNDER CERTAIN CONDITIONS the collapsing field of the secondary can generate staggering voltages which the OCPD is there to drain. Otherwise you get the exact same type of physics that causes your spark-plug to fire off at 35,000 Volts even though the automobile system voltage is only 12 to 14 VDC.

IF THE WIRE SIZE meets the tap rules you might be looking at a transformer set up for isolated power for Point Of Sale or the back-office. In such situations it may occur to the EC to use TWIN single phase panels working off of alternate windings of a WYE secondary to produce plenty of cheap 1-phase slots in an otherwise 3-phase installation.

FYI, when you thumb through Ugly's -- or any manufacturer's XFMR line you will note that a 40kVA or 50kVA is NOT ON OFFER.

The industry standard sizes are listed in Ugly's:
3
6
9 Code provision cut-offs at this size...
15
30
45 sized for 100A/125A situations ... special Code provision permits this popular size to hang above grid ceilings...

75 suitable for 200A/225 situations ... the largest commonly occurring for just that reason -- sometimes feeds mother-daughter panels when the isolated circuits are many but draw low power... this size is also a favorite with Poco's all over for small establishments...

100
150 More Code provision cut-offs inre wire-type/insulation
225
300
500
750 The largest my Poco will permit in a vault -- their vault that is... enough to supply a substantial hotel, etc... any loads bigger than this must use twined XFMRs.

So as to why 45kVA = 100A or 125A set-ups ... talk to the NEMA panel.



Edited by Tesla (02/19/10 07:17 PM)
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