# CEC question... Bonding a step-down transformer



## Navyguy (Mar 15, 2010)

It is not that easy to find that is for sure.

Rule 10-204 covers AC systems. There is a bunch of material in Appendix B.

Basically it states that use the larger load and size from table 16, however there is the issue of what it is being used for and if the primary and secondary neutrals are separate.

Sounds like you are going from a three wire delta (three phase) to a three wire (single phase). Correct? So you don't have to worry about the primary neutral. So I would install my primary wiring and then take my secondary bond at the rated load (40 amps) to the system ground unless it needs to over-sized for mechanical protection.

Cheers


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## Mike in Canada (Jun 27, 2010)

This is strictly 3 phase, no neutral. I'll write down those code numbers so I can check them when I have my book (I left it in the service truck) but the general idea, then, is that you would run your regular bonding in the conduit of a size appropriate for the wire in the conduit, then you'd add a second bonding external to the conduit which covers the largest load? Or would you just have your 3 14ga feed wires with a #8 green bonding conductor in the conduit?

Mike


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## brian john (Mar 11, 2007)

The secondary current will stop at the secondary side, the primary will only see the ratio of the fault current.

If this is a 600 VAC to 200 VAC single phase transformer and max secondary current is 68 amps, you OCP at the secondary is lets say 70 amps. Instantaneous fault current around 700 amps.

Transformer ration 600/220 = 2.72 to 1

Secondary fault of 700 amps = 257 amps primary fault current BUT that fault current will NOT be on the EGC only the primary feeder.


Or at least that is my understanding?

DURN you said 3 phase delta wye 600 VAC to 208/120 VAC.

600/120 = 5 to 1

41 amps secondary 50 amp OCP

500 amps instantaneous fault secondary 83 amps seen on the primary feeder.


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