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Joined: Mar 2001
Posts: 2,056
Redsy Offline OP
A 3kva buck-boost autotransformer is connected to raise the voltage from 208 to 240 for a 7 1/2 HP motor.
Autotransformer manufacturers tables indicate a 1 kva would be sufficient, but as discussed in my previous post, the 3 kva unit will be OK.
Article 450-4 requires OCPD installed on the input conductors at a maximum of 125% of full load xfmr input.
The original installer fused only the output side.
If I fuse the input according to manufacturers tables (maximum 40 amp), I assume that secondary protection is still required, but I am not sure because of the common connection.
Thanks for the help.

[This message has been edited by Redsy (edited 02-13-2005).]

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Joined: Jan 2004
Posts: 1,507
I have never seen the fusing in the output of an autotransformer, which is what a buck/boost transformer is. Article 450 Section 450.4(A) has us putting overcurrent protection on the input side, sized at 125% of the full load current of the xformer.

George Little
Joined: May 2004
Posts: 162
I agree with Mr. Little but I have done some thinking and I would look towards article 240.4(F). The transformere it self (not the secondary conductors are protected according to 450.3(B) 125% for (under 600V).

You appear to have a 2 wire secondary and these secondary conductors may pe protected by the transformeres pri OCP under the paragraph I listed.


Joined: Mar 2001
Posts: 2,056
Redsy Offline OP
Thanks, guys.

I follow you on the 2-wire secondary.
Although my '02 NEC is in the truck, the '99 Handbook shows an example(figure 210.8) of a buck-boost providing 240 from a 208 system and there is fusing shown on both sides of the autotransformer ?!

Joined: May 2004
Posts: 162

I see your point there are two diagrams in the "02" HB 210.16 and 17.and they clearly show OCP for the secondary conductors. I do not believe they are installed for the transformer secondary.

But if you look at protecting the secondary conductors we would have to go to 240.21 (C)

240.21 (C) Transformer Secondary Conductors. It's opening paragrph follows
Conductors shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (6).
FPN: For overcurrent protection requirements for transformers, see 450.3.

(1) Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary,[text edited out]shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio.

Your motor has a 40A FLC per 430.148. At 125% (430.22) you would need 50 Conductors to the motor.

The pri of a 3kVA trans draws 3/.208 = Approx 15 A.

The sec to pri ratio should be 208/240 = .86

If you multiply the 50 amp wire by .86 the answer is 43 A the 15 A pri fuse looks ok.

If you look at the figure in the 02 Handbook in article 450 page 572 exibit 450.6 it displays a autotransformer with out OCP on the secondary.

I hate it when this stuff happens!!

what do you think??


Joined: Mar 2001
Posts: 2,056
Redsy Offline OP
Thanks guys.
Well, I installed secondary OCP.
It may be overkill, because since there is no isolation, it seems that a fault on the input side is seen by the output side.
The only problem is that a fault current on the output side of the small(low voltage)winding might be limited (choked) due to the reactance of that winding. This may inhibit operation of it's input OCPD.

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