ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
What does your work place look like?
by Lostazhell - 12/01/20 03:44 AM
Where is Everyone?
by gfretwell - 11/30/20 04:12 PM
Happy Thanksgiving
by gfretwell - 11/30/20 04:04 PM
From an outsider- How does tipping work?
by gfretwell - 11/30/20 04:00 PM
Ground fault remover :-)
by dsk - 11/30/20 03:51 AM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
0 registered members (), 18 guests, and 12 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Confused about transformers #48314 02/09/05 10:18 AM
Joined: Feb 2005
Posts: 1
S
swampy Offline OP
Junior Member
I 'm studying for my Journeyman's. One question is ; A 10 kva trans. is 240/120v. step down. secondary ocp is 125%. What is max. cb required for primary if eff. is 100%?

Can anyone give me some pointers? Thank you!

Work Gear for Electricians and the Trades
Re: Confused about transformers #48315 02/09/05 11:20 AM
Joined: May 2004
Posts: 162
C
cpal Offline
Member
The formula for pri current is kVA/E pri times the phase multiplier. In this case a single-phase multiplier is 1. If it were 3 phase the multiplier would be 1.73.

It would be a good idea to read section 450.3(B) and study the table. You should understand that in general a transformer operating at 600V or less is considered (in general) when the pri OCPD is set at 125% of the pri FLC) The table will go on to indicate that in cases where there is OCP at the secondary of the transformer which is set at 125% of the secondary FLC, the pri OCPD may be increased to 250% of pri FLC.

Your transformer should have a 41A pri and 2.5 X 41 yield 102. (see notes to tables ) I'd call it 100A.

Re: Confused about transformers #48316 02/09/05 11:33 PM
Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
Cpal said "Your transformer should have a 41A pri and 2.5 X 41 yield 102. (see notes to tables ) I'd call it 100A."
That is correct. Note that the table allows you to go up to 250% but not exceed that figure. If the 250% does not yeild a standard
fuse or breaker, you may not round up.


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)
Trumpy 17
Popular Topics(Views)
272,147 Are you busy
206,030 Re: Forum
194,051 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3