Main Breaker on Delta/Wye Transformer Secondary?

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mike abbott

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Is a main breaker (or main fuse) required on a typical subpanel (with more then 6 handles) that is fed from a delta/wye transformer?

[ January 26, 2004, 11:28 AM: Message edited by: mike abbott ]
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

Please add some details (more information), so this question may be answered correctly.

btw: Would the Secondary be dual voltage? (i.e. 208Y/120V 3? 4W).

Scott35
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

Mike,
More information is still needed. What is the kva rating of the transformer? What is the rating of the primary OCPD. What is the calculated secondary load? How long are the secondary conductors? What size are the breakers in the secondary panel?
Don
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

The issue is with the panel and feeder not the transformer.

In general, Appliance and Branch Circuit panelboards require a single main OCPD. There is an exception for split-bus panels, but the "6-disconnect rule" definitly does not apply.

Power panels do not need any main OCPD. However, the feeders to power panels must be protected.

Secondary conductors of wye connected transformers may need to be protected by a single OCPD.

In my opinion, the 6-disconnect rule is only applicable to service entrance equipment.
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

This is a typical application:

Primary: #8 CU, 480V, 3-phase feeder protected with 50A fuses.

30-KVA delta//wye transformer

Secondary: #4 CU, 120/208V, 3-phase, 4-wire feeding a 100A MLO, 24-circuit subpanel. Wire length is aprox 15'.
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

Mike,
Jim is correct that in general an OCPD on the line side or within the panel will be required to protect the panel. I asked all of those questions because there are cases where neither the secondary conductors nor the panel will require protection. It appears that the panel in question is a lighting and appliance branch circuit panel and that a main or line side protection will be required. You need to be looking at 3 code sections for the answers to this type of question. 450.3 for transformer protection, 408.16 for panel protection and 240.21(C) for transformer secondary conductor protection.
Don
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

Mike,

Following the code sections that Don cited.

30KVA @ 480V delta to 208Y/120
36A primary
83A secondary

First the panelboard;
Because this system has a neutral, the panel can not be considered a power panel if more than 10% of the circuits are single pole breakers at 30A max.
In your example the panelboard requires a 100A OCPD if more than 2 single pole breakers are installed. The OCPD can be located anywhere upstream of the panel

Now for the secondary conductors;
This a wye connected transformer, so the secondary conductors require a single OCPD. Because the cable length is less than 25', the secondary OCPD can be located at the end of the run.
In your example the #4 conductors need an OCPD of not more than 110A if the cable insulation is at least 75C.

Finally the transformer;
Primary protection must not exceed 125% of the primary current. The primary OCPD can be increased in size to 250% if a single secondary OCPD of not more than 125% of the secondary current is also provided.
In your example the primary OCPD is 50A which is greater than 125% (45A) so a single secondary OCPD of not more than 100A must be provided.
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

Jim,
Does the code require you to "round" the primary current? The current is 36.127 amps and 125% is 45.159 amps. Note 1 to Table 450.3 permits rounding up to the next standard size of OCPD which in this case is 50 amps. Is this permitted in this case?
Don
 
Re: Main Breaker on Delta/Wye Transformer Secondary?

Don,
I used the quick Amperes per KVA charts available in manufacture's catalogs, they typically round down their numbers.

There have always been discussions over the ability to upsize transformer protection to the next standard size. My opinion has always been - not allowed. The FPN #1 to 450.3 seems to imply it is allowed, but old habits die hard.

[ January 26, 2004, 05:49 PM: Message edited by: jim dungar ]
 
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