transformer secondary over current protection
- Thread starter mjc1060
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- Licensed Electrician
210.4(F) is where the answer is. More times than not secondary protection is required.
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- Placerville, CA, USA
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- Retired PV System Designer
I have seen the argument made that an unprotected secondary winding is a tap, and therefore you cannot apply the tap rules to protect the secondary conductors at their far end. Since that would be tapping a tap.
You could only do what you suggest if the conditions for primary-only protection at the transformer were met. And then you would have to apply tap rules to your feeder, limiting the distance.
Primary-only protection only applies to the secondary winding, not the conductors attached to it.
You could only do what you suggest if the conditions for primary-only protection at the transformer were met. And then you would have to apply tap rules to your feeder, limiting the distance.
Primary-only protection only applies to the secondary winding, not the conductors attached to it.
petersonra
Senior Member
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- Northern illinois
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- engineer
how would one square that assertion with 240.4(F) which seems to specifically allow the primary overcurrent protection to protect secondary conductors for delta-delta and single phase 2 wire transformers?
210.4(F) ??
450.3 (for transformer protection) and 240.4(C)(1) (for conductor protection) will give you guidance on the need for OCP or lack thereof.
240.4(C)(2)-(C)(6) will help you decide where the protection is needed.
In a vast majority of situations, you will need OCP within 25 ft conductor length,.
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- Licensed Electrician
I don't know where I went hay wire. 240.4(F), 2011 NEC to be precise, is what I should have said. 240.4(C) is different for me.
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- Illinois
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- retired electrician
There are two totally separate code rules that must be complied with for transformer installations. The protection of the transformer windings is covered by the rules found in Article 450 and the protection of the transformer conductors is found in Article 240. The specific rules for the secondary conductors is found in 240.21(C). In almost all cases if you have properly protected the conductors, you have also properly protected the transformer, but the reverse is not true.
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- Placerville, CA, USA
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- Retired PV System Designer
Touche.
The transformer section refers only to the protection of the windings, while 240 adds specific exceptions for the conductors too. Note that primary protection sized just to protect the transformer may not meet the current ratio test in 240.4, depending on the size of conductors chosen.
Carultch
Senior Member
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- Massachusetts
If it is single phase or 3-wire delta-to 3-wire delta, then the primary OCPD will protect the secondary conductors proportional to its ampacity scaled by the voltage ratio, without any secondary OCPD. For instance, consider a 100A primary OCPD on a 240-to-120 single phase transformer. As long as the secondary conductors are at least rated for 200A, they do not need secondary OCPD.
The idea of the above, is that for simple configurations where faults will line up winding-to-winding, a fault will stay on the same phase across the transformer, and not go undetected because it will not get distributed across multiple primary phases.
This doesn't happen, if you have any transformer with a WYE system or a 4-wire delta system on either side. It will require OCPD on the secondary as well. In these cases, secondary fault currents can distribute across multiple phases of the primary, and go undetected.
240.21(C) gives us the rules for how far transformer secondary conductors can extend, when OCPD is required. And what sizing requirements they have. Indoors, it is limited to 10 ft or 25 ft, depending on what proportion of the total available KVA the conductors will be sized to meet.
kwired
Electron manager
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Read 240.21(C) and all subsections carefully and determine which section most closely applies, or determine what changes you need to make for one of those sections to apply. Only section that would allow for 30 feet of tap is if the tap conductors are outdoors, but there are two situations where they can be up to 25 feet.
Just for clarity, from my viewpoint, that my not be 100% factual, as Carultch pointed out in a similar thread (http://forums.mikeholt.com/showthread.php?t=170337) , Section VIII of Art 240 does contain provisions for greater distances in supervised industrial situations.
Carultch
Senior Member
- Location
- Massachusetts
Or put a fusible disconnect, or enclosed circuit breaker within the distance.
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