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corbin801 (Electrical)
15 Mar 10 9:19

I'm working on a 69/13.2 distribution substation upgrade.  On the Low side I'm connecting to two GIFU 15-01, 800:5 C.T.'s with a thermal rating of 1.5. (These C.T.'s are being supplied by the metering guys) The continuous current on the low side is 1093 Amps. These C.T.'s have flat tin plated Nema two hole Pads and I will be connecting to these C.T.'s with double 500 MCM to Nema 4 hole pad terminal connectors. This is a standard design and I'm wondering if there is any thermal concerns going from a 4 hole aluminum connector to a 2 hole connector on the C.T.?

Thanks
waross (Electrical)
15 Mar 10 10:09
If it is possible I would cut two holes off the four hole connector.
Will cutting the connector void the approvals? Probably.
Will using just two of four connecting holes void the approvals? Probably.
Cutting two holes off  the connector will result in a shorter current path and less heat generated in the connector.
I would first look for alternate hardware.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

jghrist (Electrical)
15 Mar 10 12:17

Quote:

Cutting two holes off  the connector will result in a shorter current path and less heat generated in the connector.
I don't see how cutting the holes off shortens the current path.  It does remove some material that would provide some heat dissipation.  I'd use the 4-hole pad without cutting.
 
waross (Electrical)
15 Mar 10 13:23
Hi J,
When a four hole connector is connected to a two hole pad, the unused holes are often between the pad and the cables. The current generally flows past the two unused holes before arriving at bthe connection. Current is flowing trough an area where the design anticipated that it would already be transferring from the pad to the CT connector.
If it is possible to bolt it up close, good workmanship would indicating drilling two more holes and using four bolts.
My comments are directed at a straight connection. For a 90 deg. connection I agree with you completely.

Bill
--------------------
"Why not the best?"
Jimmy Carter

corbin801 (Electrical)
15 Mar 10 15:35
The spacing is such that two more holes cannot be drilled into the two hole pad on the C.T... The holes on the C.T. are aligned vertically so they will mate up to the four hole pad and allow current to flow directly to the double 500 MCM without crossing directly across the two open holes on the four hole pad.

We will be switching to a C.T. with a four hole pad, but we have a few stations currently with the 2 hole pad. My concern is, when dealing with equipment rated at 1200 amps the connection is commonly a 4 hole pad. This would lead one to believe, you need that amount of surface area provided to carry 1200 amps safely. Even though the low side has a maximum of 1093 Amps of rated continuous current, it is possible to overload this transformer. My question would be, is there a guide line or criteria on thermal characteristics through terminal connectors or do I need to start cranking out calculations?

Thanks,
 
Helpful Member!  stevenal (Electrical)
15 Mar 10 16:19
Using both sides of the CT pad will double your surface area. Put a pad on each conductor, and sandwich the CT pad between them.

For an outdoor substation, though, the AL should be above the CU. This last part might be hard to achieve.
racobb (Electrical)
15 Mar 10 17:12
Aside from eliminating another connection that can fail, this is why I always request window type CT's when they get that big.  I don't like putting an 800 amp connection in my 1200 amp bus.

Alan

jghrist (Electrical)
15 Mar 10 17:53
If the CT has a 1.5 thermal factor, that should mean the whole CT, 2-hole pad included, would be OK at 1200A.  A 4-hole pad bolted to the 2-hole terminal will give just as much surface area as a 2-hole pad bolted to the 2-hole terminal.  I admit that I don't know how the standards require the CT to be tested for thermal capacity - does the test include the terminals?
 
racobb (Electrical)
15 Mar 10 19:50
Don't know either, but would think you could not rate a CT for 1200 amps without the terminals being able to handle it as well.

Alan

corbin801 (Electrical)
15 Mar 10 20:31
We are going to the window type C.T.'s for 1200 Amps in the future to put my mind at rest.

The sandwich solution seems to solve the problem at hand(If there actually was a problem?), but if anyone could think of some guide lines for connector construction and ampacity, it would be greatly appreciated.

Thanks
stevenal (Electrical)
17 Mar 10 13:40

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