Control Panel CPT Grounding

[mls1]

I have a control panel in which the UL508 panel builder grounded the CPT secondary neutral via a screw on the back panel. The back panel is grounded. It seems to me to be a bad practice considering there is a ground bus within the enclosure and the builder just got lazy but I don't know if it is a violation. Is there a requirement that the main bonding jumper of a CPT be grounded to a grounding bus or can the enclosure/back panel serve as part of the bonding jumper? Also, would it be a better practice to not come straight off the X2 terminal but rather apply the bonding jumper at the neutral bus? Thanks!

 

[DRWeig]

CPT = Control Power Transformer, I assume?

16.2 For a transformer or power supply that is required to be grounded in accordance with 16.1, the secondary circuit shall be factory connected by a system bonding jumper to the enclosure and the grounding electrode conductor terminal. The size of the system bonding jumper shall be as specified in 75.1.4, based on the secondary rating. A grounding electrode conductor terminal sized to retain the required grounding electrode conductor in accordance with 75.1.4, based on the secondary rating, shall be provided in the enclosure containing the transformer or power supply and marked as specified in 54.10.

Exception No. 1: When the transformer is rated not more than 1000 volt-amperes and supplies only control circuits, the grounding electrode conductor terminal is able to be omitted and the system bonding jumper shall not be smaller than a 14 AWG (2.1 mm2) copper conductor. The jumper is not otherwise required to be larger than the phase conductors connected to the transformer secondary.

Exception No. 2: Where multiple separately derived systems are provided within the same industrial control panel enclosure, a single grounding electrode conductor terminal suitable for a 3/0 AWG conductor
is able to be supplied in the industrial control panel as the field wiring connection for the common grounding electrode conductor.

Best to you,

Goober Dave

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[DRWeig]

Forgot to mention: to comment further, I'd need to see pictures or a wiring diagram. In general, the screw to the back panel is fine for the enclosure ground when it's less than 1000 VA.

Best to you,

Goober Dave

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[mls1]

Thanks, DRWeig. Yes, by CPT I meant control power transformer. It is exactly 1000 VA single phase with a 480V primary and 120V secondary. The 16.2 language confuses me a bit but if I understand the statement "the secondary circuit shall be factory connected by a system bonding jumper to the enclosure and the grounding electrode conductor terminal." correctly the bonding jumper can be to the enclosure and does not need to go to the ground bus directly and since it is "not greater than 1000 VA" an external ground rod connection is not required, correct?

 

[DRWeig]

Yes, I think you've got it - in my opinion. We've been running a 508a panel shop for many years, and no inspector has ever taken issue with it.

Best to you,

Goober Dave

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