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mininum T-lead spacing in terminal box for 13.2kv motor

mininum T-lead spacing in terminal box for 13.2kv motor

mininum T-lead spacing in terminal box for 13.2kv motor

T-leads are of course insulated but unshielded. We often have 6 in a terminal box (3 phase leads and three nuetral). Sometimes the routing gets tricky and it's difficult to separate the conductors to prevent them from coming too close along their length (I'm not talking abou the terminations). I usually tell our guys to get as much spacing as reasonably achievable and a half inch if possible, but when it gets tight, I get asked what is the real minimum that we could live with if we had to (because correcting it can involve a whole lot of work).

To my thinking, it's the same as minimum clearances we expect to see phase-to-phase in endwinding crossover region. Arguably it should be a bit higher in the T-leads because they are more susceptible to movement over time, but the endwindings are still my benchmark because there are similar insulation dielectric constants and in both cases we expect to see most of the voltage to appear across the air gap distance between the external surfaces of the insulation.

Of course if you place the T-leads too close, that's not an immediate failure, just a location for partial discharge to start working.

So my thought is around 0.25" minimum spacing phase to phase. Maybe a bit lower phase to neutral. And if the situation demanded (because the work involved in creating more space was prohibitive) I might even go lower. We do inspect the motors terminal boxes every 12-18 months. I've seen a lot of small spacing and direct contact (with resulting range of small gap distances adjacent to the contact on the round profile) and I've seen resulting white powder on surfaces but never any failures from that (maybe just lucky?).

What's your thoughts? Do you have any references?

(2B)+(2B)' ?

RE: mininum T-lead spacing in terminal box for 13.2kv motor

Aside from the general question above, I do have a specific example in front of me (attached). Here is the way the terminal box was found. I'm pretty sure it's been that way for years (except the neutral tie wraps that were just added). Phases 1 and 2 are definitely in direct contact at the location I tried to show in slide 2 (it's really hard to convey in photos). Everywhere I poked I didn't even see white powder on this one. I've asked them to work on improving the spacing but I'm not sure how much they'll be able to do. We probably can't pull any slack out form the motor. I think at least we might swap the two conductors attached on the bus on the middle phase.

(2B)+(2B)' ?

RE: mininum T-lead spacing in terminal box for 13.2kv motor

eletricpete As an OEM, I would be shooting for 0.5 inch minimum, 1.0 inch preferred. However, based on conductivity of air, if it's clean and dry it can hold up under 3 MV/m which works out to be about 0.19 inch at 13.8 kV. Looks from the photo like your OEM in this case was happy with roughly 0.25 inch.

Note - I have seen corona / PD at voltages as low as 2300 V, but that was at relatively extreme elevation (5500 m).

Converting energy to motion for more than half a century

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