The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!


2017 NEC and Related
2017 NEC
Now Available!

Recent Posts
Parking lot pole light swap....
by ghost307
Today at 06:25 AM
International Wire Colour Codes
by Tjia1981
Yesterday at 12:08 PM
Son of Sparky
by HotLine1
10/20/16 07:43 PM
Speaking of Plugmold ...
by gfretwell
10/17/16 02:37 PM
Broken battery charger? Check for cobwebs!
by gfretwell
10/17/16 02:30 PM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 13
HotLine1 7
ghost307 7
renosteinke 6
Potseal 4
Who's Online
0 registered (), 262 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#17230 - 11/21/02 08:18 PM transformer ocp ?
mj Offline

Registered: 06/10/02
Posts: 188
Loc: meriidian, ms
this has always been a hard topic for me,9 amp or more at 600v or less sec ocp not required? is it just me ? why sec ocp is not required ?

Work Gear for Electricians and the Trades

Work Gear for Electricians and the Trades
Work Gear for Electricians and the Trades
Arc Flash Clothing, Gloves, KneePads, Tool Belts, Pouches, Tool Carriers, etc. etc....

#17231 - 11/21/02 10:15 PM Re: transformer ocp ?
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
mj, maybe this might help:

If Transformer has only ONE OUTPUT VOLTAGE, the Primary OCPD will be sufficient to protect the Secondary Side Feeders. Remember that we are not protecting the Secondary Windings - only the Feeders on the Secondary Side.

If the Secondary has TWO OUTPUT VOLTAGES, the Secondary Conductors need OCPD.

FYI: Examples of Systems with:

    [*]Secondary having ONE OUTPUT VOLTAGE - 480x240 VAC 1 ph. 2 wire, 600x440 VAC 3 ph. 3 wire.

    [*]Secondary having TWO OUTPUT VOLTAGES -
    480x120/240 VAC 1 ph. 3 wire, 600x208Y/120 VAC 3 ph. 4 wire.

Simplified description: On Secondaries with only ONE output voltage, the Amperes on the Secondary side will correspond to the Amperes on the Primary side (KVA wise), so if the OCPD on the Primary side is rated properly to the maximum KVA for the Primary side, this in a vicarious way will protect the Secondary Conductors (if sized properly).

So much for a simple description!

Scott s.e.t.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals