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Thursday, July 2, 2015
Question:
Good Morning Charlie,
Thank you for daily inputs. A contractor installed a control transformer as shown in the picture. Ground and neutral are bonded in the same point. The manufacturer representative told us this is regular practice.
Please let us know if this is permitted and if there is no discrepancy with NEC 250.142.
Thank you, Ivan Vega
A
Answer:
Hey Ivan thanks for your question and your welcome for the CQD. The picture you sent shows a green wire and a white wire connected to a "saddle type" terminal on the secondary of what appears to be a control power transformer. A black wire is connected to the other secondary terminal but it does not show where the other ends of the conductors terminate. The control power transformer appears to be a separately derived system as defined in Article 100. The green wire appears to be used as a system bonding jumper as defined in Article 100 and Exception No. 3 to 250.30(A)(1) provides sizing information.
The output of the transformer (secondary) is a "new source of power" so connecting the grounded conductor to ground is the only connection to ground and is what grounds the system. This is different than reconnecting a grounded service conductor to ground which is prohibited by 250.24(A)(5) or using a grounded conductor to ground equipment on the load side as prohibited by 250.142(B)