ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
Lock-down Thread
by Bill Addiss - 02/27/21 01:16 PM
Northern Tool Recalls Powerhorse Generators
by Admin - 02/25/21 09:49 PM
You will never guess
by gfretwell - 02/25/21 07:48 PM
New tool
by SMOKEYBOB - 02/15/21 04:59 PM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
0 registered members (), 13 guests, and 16 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
XFMR Secondary Protection #167980 08/24/07 02:35 PM
Joined: Dec 2006
Posts: 43
W
windmiller Offline OP
Junior Member
What is the article that allows the secondary of a transformer to be protected by the primary?

Thank you,

Windmiller


We all live under one King
2017 / 2014 NEC & Related Books and Study Guides
Re: XFMR Secondary Protection [Re: windmiller] #167981 08/24/07 02:42 PM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline
Member
try 450.3(B)

Re: XFMR Secondary Protection [Re: windmiller] #167984 08/24/07 04:38 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Originally Posted by windmiller
What is the article that allows the secondary of a transformer to be protected by the primary?

Thank you,

Windmiller


Well that depends on the transformer, many transformer can not use the primary to protect the secondary.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: XFMR Secondary Protection [Re: iwire] #167986 08/24/07 05:49 PM
Joined: Nov 2000
Posts: 2,148
R
resqcapt19 Offline
Member
And even if the secondary of the transformer is protected by the primary OCPD, you still have to look to 240.21(C) for the protection of the secondary conductors.
Don


Don(resqcapt19)
Re: XFMR Secondary Protection [Re: resqcapt19] #168140 08/27/07 07:08 PM
Joined: Jan 2004
Posts: 1,507
G
George Little Offline
Member
I agree with Don and Bob- It's possible to proptect the xformer secondary and the secondary conductors with the overcurrent protection on the primary but we need more details about the xformer and the associated conductors on both sides of the xformer.


George Little
Re: XFMR Secondary Protection [Re: George Little] #168155 08/28/07 09:02 AM
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
Don't forget 240.4(F), this and 240.21(C)(1) are the only places that specifically allows secondary conductors to be protected by the primary.

450.3 doesn't care about conductors it is only concerned with the transformer.

The rest of 240.21(C) requires over current protection on the secondary but it is allowed to be at the end of the conductorsa instead of at the source.

Re: XFMR Secondary Protection [Re: JBD] #168203 08/28/07 09:36 PM
Joined: Jan 2005
Posts: 155
D
Dave T Offline
Member
Look at it this way, A 1PH transformer with either a 240/120 or a 120/240 secondary providing power to a 1ph3w load requires secondary protection.
A 3ph transformer that supplies power to a 3ph4w load must have secondary protection.
A 1ph transformer that simply feeds a 1ph2w-load need only to have primary protection is the pri. OCPD is sized correctly according to art 450.
It is the same for a 3ph transformer, which simply feeds a 3ph3w load.
In both of these examples the secondary current is proportional to the primary current so a pri. OCPD can protect the secondary.
Not so with a 1ph-3w or a 3ph-4w secondary. Think of it this way, if it is possible for the secondary to provide power where there are unbalanced L-N unbalanced loads if primary only over current protection was provided it would not see the unbalanced load and would not provide adequate transformer overload protection.

Re: XFMR Secondary Protection [Re: Dave T] #168214 08/29/07 12:20 PM
Joined: Dec 2006
Posts: 43
W
windmiller Offline OP
Junior Member
Hi,
So if I come from a 480 to 120/208 xfmr to a 3P/4W panel i need to go to a fuse or breaker ahead of the panel and not to a MLO panel?

This is getting interesting.

Thanks

Windmiller


We all live under one King
Re: XFMR Secondary Protection [Re: windmiller] #168224 08/29/07 04:33 PM
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
Originally Posted by windmiller
Hi,
So if I come from a 480 to 120/208 xfmr to a 3P/4W panel i need to go to a fuse or breaker ahead of the panel and not to a MLO panel?

This is getting interesting.


This is why it is dangerous to give "how is it done" advice on the internet.

By bringing a panel into the discussion, you have just added a new requirement to look at 408.36(D).

Last edited by JBD; 08/29/07 04:34 PM.
Re: XFMR Secondary Protection [Re: JBD] #168243 08/29/07 10:21 PM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline
Member
Then again it depends on the Classification of the Panel board . 408.30(A) Then 408.36.
450.3(B)
240.21
408.30(A)
408.36
all must be reviewed to get the total picture.
2005 NEC


Featured:

2020 National Electrical Code
2020 National Electrical
Code (NEC)

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
HCE727
HCE727
Delaware County, PA, USA
Posts: 186
Joined: November 2005
Show All Member Profiles 
Top Posters(30 Days)
Popular Topics(Views)
275,535 Are you busy
209,427 Re: Forum
196,827 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3