Grounding Motor and Instrumentation.

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TDN

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Dear Mr. Holt,

Would you please help me with the following questions:

1) A 200HP motor starter is mounted on 13.8KV, 50MVA, 3000A generator skid.
There are 2 ground wires on this motor, one is connected to the base of
generator skid (the generator skid is connected to the ground grid), the
other (equipment ground) is connected to the motor control center located in
the Control house. Could you please let me know how to size the ground
conductor that connected to the generator skid, and the equipment ground
conductor that connected to the motor control center. Does NEC require to
have the ground conductor that connected to generator skid in addition with
the equipment ground conductor? If yes, what is the function and what do we
call this ground conductor?

2) CSA inspector required us to ground all 24VDC pressure transmitters
located on the gen. skid, all the metallic armor cables that connect to
these transmitters also being grounded. What size of ground conductor should
we use to ground the 24VDC instrumentation and what is the purpose of
grounding these 24VDC sensors?

3) My existing ranger draw about 23A and has 10AWG conductor & 30A breaker,
is it OK if I replace it by a ranger that draws only 15A without changing
the breaker?
 
Is this an NEC or CEC question?
In the NEC, the size of the motor EGC is based on the overload.
I see no reason to ground the 24 VDC instruments, its typically not required in the NEC.
Whats a ranger?
Our local Community Colleges nickname is the Rangers.
 
don't you have a set of engineered drwgs for this installation ? or at least manufacturers drwgs ? isn't their a site engineer available to answer these questions ?
 
Grounding Motor and Instrumentation

Grounding Motor and Instrumentation

The application is to meet both NEC & IEC. I used NEC table 250.122 to size EGC for the motor (480VAC) and table 250.66 to size the ground conductor that connects the motor frame to the base of the 13.8 KV generator skid.
The reason I think the table 250.66 is correct to use because I think the function of this conductor is to limit the lighting or higher voltage accidentally strike on the motor, and I think it could be consider as grounding electrode conductor since it directs the current to earth (in this case it goes to the skid and the skid is grounded to the ground grid). I am not sure if using 250.66 for this application is correct and I am not sure if this ground conductor is required by NEC. I would like to have the advices from NEC expert.

If using table 250.66 is correct answer, then it looks a little odd when I use it for 1HP motor installation, the ground conductor will be much bigger the phase conductor since the smallest size on table 250.66 is 8AWG.

I meant the range, I misspelled.
Thanks for your advices.
 
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