ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Shout Box
Recent Posts
Positive Ground on 125 VDC Station Batteries
by The Watt Doctor. 05/23/19 08:55 AM
2020 is at the printers.
by gfretwell. 05/22/19 10:11 AM
Chinese wire symbols?
by Texas_Ranger. 05/19/19 03:09 PM
Pool - Daisy chained GFCI
by gfretwell. 05/18/19 01:30 PM
110.26 working clearances
by HotLine1. 05/14/19 02:08 PM
New in the Gallery:
What is this for?
Plug terminals
Who's Online Now
0 registered members (), 5 guests, and 9 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Transformer secondaries...again #95759
10/04/05 07:18 AM
10/04/05 07:18 AM
Redsy  Offline OP
Joined: Mar 2001
Posts: 2,056
Bucks County PA

We know that Table 450.3(B) indicates that primary OCPDs alone, are adequate to protect the transformer. However, you always have secondary conductors.
We also know that 240.21(C) also permits primary OCPDs alone, to protect secondary conductors under certain conditions.
The question is, are we talking about 2 different issues (xfmr secondary protection vs. secondary conductor protection)?
I believe that Table 450.3(B) values for primary only protection are actually used only if & when you are taking advantage of the 4 exceptions in 240.21(C).
Otherwise, secondary protection is always required.
Anyone else?

[This message has been edited by Redsy (edited 10-04-2005).]

2017 / 2014 NEC & Related Books and Study Guides
Re: Transformer secondaries...again #95760
10/04/05 07:25 AM
10/04/05 07:25 AM
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
The rules in 450 have nothing to do with the rules in 240. 450 covers the transformer and only the transformer. 240 applies to the conductors and not to the transformer.

Re: Transformer secondaries...again #95761
10/04/05 07:58 AM
10/04/05 07:58 AM
Redsy  Offline OP
Joined: Mar 2001
Posts: 2,056
Bucks County PA
That's the source of my confusion, Don.
Under what conditions would you be protecting the transformer secondary, but not the conductors?

What application would you need to refer to Table 450.3(B) and not 240.21(C)?

Re: Transformer secondaries...again #95762
10/04/05 09:06 AM
10/04/05 09:06 AM
JBD  Offline
Joined: Jul 2001
Posts: 599
(1) Start with primary protection only, 450.3(B).

(2) Then use 240.21 for the secondary conductors.

(3) Now go back and look at 450.3 and see if the OCPD chosen using 240.21 will allow (but not force) you to change the transformer primary device.

Don't forget that a main device in a panelboard can also be used to meet the 240.21 reuirements.

Re: Transformer secondaries...again #95763
10/04/05 09:36 PM
10/04/05 09:36 PM
Dave T  Offline
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
I have always looked at it this way:
When the transformer is sized for the load as well as the secondary conductors size correctly and secondary protection is properly sized to protect the conductors then the transformer is protected from overload.
Should you size the pri. OCPD to also protect the transformer from overload it is redundant OL protection. If the pri. OCPD is viewed as short circuit protection because the transformer already has OL protection then the pri. OCPD is viewed as taking the transformers off line should the transformer short circuit internally and fail.
I believe that we agree that transformer overload protection of a delta-delta transformer can be done from the primary only as the secondary doesn't have a neutral. Currents seen at the secondary are seen proportionately on the primary.
But with a Delta-wye that has a neutral the L-L currents seen on the primary don't necessarily reflect those L-N currents on the secondary so a pri. only OCPD can not adequately protect the transformer from OL.
And, as such, pri. and sec. OCPD are provided and one could opt for the 250% pri. OCPD but you must pay attention to that 25' tap rule.
In summary the sec. OCPD protects the transformer from OL and the pri. OCPD protects the distribution system from a transformer failure.
Dave T

Re: Transformer secondaries...again #95764
10/05/05 07:05 AM
10/05/05 07:05 AM
Redsy  Offline OP
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Thanks for the input, guys.


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


Member Spotlight
L.I. New York
Posts: 99
Joined: August 2003
Show All Member Profiles 
Top Posters(30 Days)
Popular Topics(Views)
255,260 Are you busy
191,296 Re: Forum
181,087 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.6.1
(Release build 20180101)
Page Time: 0.022s Queries: 14 (0.004s) Memory: 0.9770 MB (Peak: 1.1222 MB) Zlib enabled. Server Time: 2019-05-26 23:26:06 UTC