ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Shout Box
Recent Posts
Changing a Bulb
by gfretwell. 07/14/18 03:12 PM
Tray cable?
by gfretwell. 07/11/18 10:18 PM
Eaton Heavy Duty 30A and 60A Safety Switches
by Admin. 07/11/18 09:49 PM
ORTOVOX Recalls Avalanche Transceivers
by Admin. 07/11/18 09:44 PM
Tingle off the water spigot
by gfretwell. 07/08/18 12:58 PM
New in the Gallery:
Plug terminals
Housebilding DIY wiring
Who's Online Now
0 registered members (), 30 guests, and 11 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 4 1 2 3 4
Transformer Secondary #192570
02/18/10 12:25 AM
02/18/10 12:25 AM
S
SJT  Offline OP
Member
Joined: Jul 2002
Posts: 241
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

Work Gear for Electricians and the Trades
Re: Transformer Secondary [Re: SJT] #192580
02/18/10 12:49 PM
02/18/10 12:49 PM
J
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
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.

Re: Transformer Secondary [Re: JBD] #192583
02/18/10 03:04 PM
02/18/10 03:04 PM
S
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
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.

Re: Transformer Secondary [Re: SteveFehr] #192588
02/18/10 07:57 PM
02/18/10 07:57 PM
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
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.


Few things are harder to put up with than the annoyance of a good example.
Re: Transformer Secondary [Re: Tom] #192589
02/18/10 08:34 PM
02/18/10 08:34 PM
J
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
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.

Re: Transformer Secondary [Re: JBD] #192598
02/19/10 02:03 AM
02/19/10 02:03 AM
T
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
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.


Tesla
Re: Transformer Secondary [Re: Tesla] #192613
02/19/10 06:14 PM
02/19/10 06:14 PM
J
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
Originally Posted by Tesla
45kVA 3-phase XFMRs are suitable to feed ONE 3-phase 100A panel...

According to what NEC article?

Re: Transformer Secondary [Re: JBD] #192615
02/19/10 07:07 PM
02/19/10 07:07 PM
G
Gregtaylor  Offline
Member
Joined: Mar 2005
Posts: 212
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.

Re: Transformer Secondary [Re: Gregtaylor] #192616
02/19/10 10:45 PM
02/19/10 10:45 PM
S
SJT  Offline OP
Member
Joined: Jul 2002
Posts: 241
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

Re: Transformer Secondary [Re: SJT] #192617
02/19/10 11:13 PM
02/19/10 11:13 PM
T
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
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.


Last edited by Tesla; 02/19/10 11:17 PM.

Tesla
Page 1 of 4 1 2 3 4

Featured:

2017 Master Electrician Exam Preparation Combos
2017 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
sbi
sbi
indiana
Posts: 47
Joined: March 2008
Show All Member Profiles 
Top Posters(30 Days)
Admin 5
Popular Topics(Views)
248,134 Are you busy
185,588 Re: Forum
174,923 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.6.1
(Release build 20180101)
Page Time: 0.029s Queries: 16 (0.005s) Memory: 1.0301 MB (Peak: 1.2123 MB) Zlib enabled. Server Time: 2018-07-19 21:19:32 UTC