Hey there everybody, I have a question regarding tapping a transformer secondary. I currently have a 50kva single phase transformer feeding a temporary office trailer that has a 125 amp main breaker panel and is located about 100' away from the xfrmr. The reason for the xfrmr is that the customer didn't have any single phase near the location of trailer. All they had was 230 3 phase. Used existing 3 phase disconnect, using A & C phases to xfrmr, to aquire 230 single phase and pick up a neutral. Now customer wants to add a second temp trailer next to first one. Okay, finally the question! Can I tap the secondary of existing transformer, as long as each set of conductors has its own overcurrent protection? I located 240.21(c), but that refers to secondary conductors that don't need overcurrent protection. This supply has dual voltage (230/110), so this would not apply, correct? If this is NEC compliant, where is it referenced?
Transformer secondary taps
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augie47- thanks for responding. I'm still a little confused though. Can't seem to find the answer in (c)(2). Doesn't this explain secondary conductors that are not over 10 ft long? My secondary sets are about 100 ft + going out to the temp trailer. Maybe I'm misunderstanding you. Thanks again.
Thanks again, it makes some sense-still a little confused. Doesn't 240.21(C)(4) relate to secondary conductors without overcurrent protection at the secondary as stated in 240.21(C)? Since this is not a single voltage situation(this application is 230/110), I would need overcurrent protection at the secondary. Would I need separate disconnects as stated in 240.21(C)(4)(2) for each group of conductors, as well as secondary overcurrent protection rated at 125%
rixer said:
If the transformer is in the building or a building that supplies it then you probably need OCP with in 10 or 25 feet of the transformer assuming you have that length of secondary conductor before leaving the building to go to the trailer. Of course I assume the trailer is outside, (apparently this is not the case) and a stand alone unit, on second thought you probably have not given enough information to get an intelligent response so why don't you clarify your installation and be a bit more specific regarding the layout.
I'm sure some one can help you!
Any wire connected to the secondary of a transformer is a tap conductor. It is a tap conductor until it reaches the first overcurrent protection device.
There is a rule that prevents "tapping a tap". So you can not splice more wires into the existing wires to feed another disconnect. However, you can connect two sets of wires directly to the transformer secondary, and run each set of wires to its own OCP. So basically, you have two separate sets of tap conductors.
With that said, then you have to refer to the tap rules to determine what size wire you need, and how far you can go before hitting the first overcurrent device.
Steve
There is a rule that prevents "tapping a tap". So you can not splice more wires into the existing wires to feed another disconnect. However, you can connect two sets of wires directly to the transformer secondary, and run each set of wires to its own OCP. So basically, you have two separate sets of tap conductors.
With that said, then you have to refer to the tap rules to determine what size wire you need, and how far you can go before hitting the first overcurrent device.
Steve
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