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#102152 04/08/05 07:55 AM
Joined: Oct 2003
Posts: 172
G
Member
just got a notice of deficiency for fusing a 225 kva trans at 250amps, the inspector says he wants 225amps.is this right, am i not allowed to fuse within 125%?

#102153 04/08/05 11:57 AM
Joined: Jan 2003
Posts: 174
B
Member
Need a little more info here such as:single or three phase, Voltage , Dry or Oil, both primary and secondary protection?


[This message has been edited by bigrockk (edited 04-08-2005).]

#102154 04/08/05 12:22 PM
Joined: Oct 2003
Posts: 172
G
Member
6oo/208-120 3phase dry type,fusing is on primary

#102155 04/08/05 03:22 PM
Joined: May 2001
Posts: 144
T
Moderator
Get Electric,

If the secondary device is a panelboard, and the mains are rated at 800-amps, then you should not have received a defect.

It should have been accepted with 250-amp fuses on the primary, when the secondary equipment has the ability to withstand the transformer inrush current.

The Ontario Electrical Safety Code allows protection of a panelboard using overcurrent protection in the primary of a transformer provided:

when the panelboard rating, adjusted by the turns ratio of the transformer, equals or exceeds the primary overcurrent device rating,

the primary overcurrent device rating is adequate to withstand the transformer inrush current, (Rule 2-034
page 16) and


there is permanent marking on the primary disconnect switch to indicate the maximum fuse size and type to be used for continued protection (Rule 2-100(3) page 17)

Minimum conductor sizes and minimum panelboard ratings can be based on the rating of the overcurrent device.

The Appendix B note to Rule 26-256 (page 464) provides a "rule of thumb" for determining the adequacy of the overcurrent device to withstand the transformer inrush current for dry type transformers. Specifically:


The current which causes the overcurrent device to operate at 0.1 seconds must exceed 12 times the transformer rated primary current, and


The current which will cause the overcurrent device to operate at 0.01 seconds must exceed 25 times the
transformer primary rated current.

Where there is a question concerning a particular installation, the contractor or consultant should:

1-consult the transformer manufacturer about the inrush currents,

2-the overcurrent device supplier concerning the operating times for the overcurrent devices, and

3-be prepared to show evidence the overcurrent device can carry the inrush currents.

FULL LOAD CURRENT AMPERES (208 - 600V)
kVA 208V 240V 480V 600V
3 8.3 7.2 3.6 2.9
4.5 12.5 10.8 5.4 4.3
6 16.6 14.4 7.2 5.8
9 25 21.6 10.8 8.6
15 41 36 18 14.4
22 62 54 27 21.6
30 83 72 36 28
45 124 108 54 43
75 208 180 90 72
112.5 312 270 135 108
150 416 360 180 144
225 624 541 270 216
300 832 721 360 288
500 1387 1202 601 481
750 2084 1806 903 723


Tony Moscioni
Electrical Inspector
Electrical Safety Authority

#102156 04/08/05 04:12 PM
Joined: Jan 2003
Posts: 174
B
Member
Well according to my calcs based on 26-256 & subrules 1,2,& 3 of the CEC, 250 amps should be no problem at all.

I Pri x 1.25 = 270.6
Subrule 3 allows you to go up to the next stanard rating of overcurrent device!

Did the inspector say why he wanted 225 amp fuse?

#102157 04/13/05 08:15 AM
Joined: Oct 2003
Posts: 172
G
Member
I believe his reason was something to do with the secondary feeders being parallel 350mcm,and by the way this is feeding a meter center.


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