OCPD on Secondary side of transformer?

Status
Not open for further replies.
E

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Hi,
I will be installing a 240v delta to 208v wye transformer and was wondering if I need a main breaker for the panel I am feeding from the secondary side of the transformer? I have a fused disconnect on the primary side. The transformer, fused disconnect on the primary side and the panel I will be feeding from the secondary side are all within 10ft of each other, if that matters. Could you give a code reference.
Thanks!
 
Don's mentioned code references mean that nearly all situations like yours will require a main breaker or overcurrent protection ahead of the panel.

About the only times you can protect with primary side protection only is with a single phase two wire secondary, or a three phase three wire secondary. Otherwise you can easily develop situation where you haven't exceeded the input current of the transformer but still have unbalanced the output side enough that you can overload a portion of the secondary windings. With a two wire single phase secondary current on the secondary will be directly proportional to the primary current, with a three wire secondary you may double the current of half the secondary on one side and still hold the primary overcurrent device.
 
kwired said:
Don's mentioned code references mean that nearly all situations like yours will require a main breaker or overcurrent protection ahead of the panel.

About the only times you can protect with primary side protection only is with a single phase two wire secondary, or a three phase three wire secondary. Otherwise you can easily develop situation where you haven't exceeded the input current of the transformer but still have unbalanced the output side enough that you can overload a portion of the secondary windings. With a two wire single phase secondary current on the secondary will be directly proportional to the primary current, with a three wire secondary you may double the current of half the secondary on one side and still hold the primary overcurrent device.
Click to expand...


This is confusing to read,,,, can you clarify?

JAP>
 
jap said:
This is confusing to read,,,, can you clarify?

JAP>
Click to expand...
Let's say you have a 480--120/240 120kVA transformer. Rated primary current 250A, secondary 500A. If you had only primary protection on this transformer, you could put a 120V 1000A load (or fault) on the secondary and not trip the primary OCPD (that is not until secondary fries, smokes, etc. and compromises the integrity of the primary)
 
Smart $ said:
Let's say you have a 480--120/240 120kVA transformer. Rated primary current 250A, secondary 500A. If you had only primary protection on this transformer, you could put a 120V 1000A load (or fault) on the secondary and not trip the primary OCPD (that is not until secondary fries, smokes, etc. and compromises the integrity of the primary)
Click to expand...

I realize that but it read as though you could protect the secondary with the Primary overcurrent on a 3 phase 3 wire secondary in the first sentence in red.

but

You could not in the last sentence highlited in red.

thats where my misunderstanding is.

JAP?
 
On a single phase transformer the secondary can only be considered as protected by the primary when the secondary is two wire and then only under the conditions specified.
On a single phase transf. a 3 wire secondary such as 240/120 can not be protected by the primary.
On the phase, similar rules apply.. 3 phase 3 wire might be protected, 3 phase 4 wire secondary is not.
 
jap said:
I realize that but it read as though you could protect the secondary with the Primary overcurrent on a 3 phase 3 wire secondary in the first sentence in red.

but

You could not in the last sentence highlited in red.

thats where my misunderstanding is.

JAP?
Click to expand...
I think he's referring to a single phase, 3-wire secondary in the last sentence... but I now see where you got confused. Sentence structure not the greatest... ;)
 
How about "Only the normal full rated current will be drawn in the primary when twice the full secondary current is drawn from one half of the center tapped secondary.

Tapatalk!
 
I'm starting to see the light now,,,,

Sometimes it takes me longer than others.:)

JAP>
 
jap said:
I realize that but it read as though you could protect the secondary with the Primary overcurrent on a 3 phase 3 wire secondary in the first sentence in red.

but

You could not in the last sentence highlited in red.

thats where my misunderstanding is.

JAP?
Click to expand...

With a three phase three wire secondary it is possible to unbalance enough to overload it. This type of system is not uncommon but not nearly as common as four wire secondary either. I can't tell you exactly why NEC does allow protecting that one by primary only, but the majority of the time you do see it it is connected to a three phase load that will be fairly balanced. You could easily supply a single phase transformer from it and unbalance it though. Many times a three phase three wire secondary is supplying a fairly limited load though.

But even if you went with full transformer kVA rating to a single phase load, line to line is not going to be as much of an overload (on only a portion of the windings) as if you tried to place full kVA load from one line to neutral.
 
Status
Not open for further replies.
Top