ECN Forum Forums Code Related Discussion NEC & other Code issues Transformer Installations

Welcome to the forum Mike.

Roger

Thank you Roger

My welcomes as well, Mike.


Thats the problem, there is no OCPD protecting the IG and phase conductors on the secondary side, other than the primary OCPD. I wonder if the proposal should use 250.122 based on the primary feeder, or 250.66 based on the derived conductors. I would hate to have to go to 250.122 for the IG and 250.66 for the EGC on the secondary side. Just one more rule to try to remember.

I have two questions on this installation. Question#1 is the transformers usually have a line above which no field wiring is allowed, but are not those primary conductors coming in from above?. Question #2 is I cannot make out what is in the conduit on the bottom left side. It appears to be the gec. If so where is it terminated and would the bonding jumper sent to the bushing on that conduit entry need to be the same size as the gec?

I have installed many transformers just like the one shown. I used TA lugs just like that. My question is, when you perform PM and you can not shut down power, your going over for tight and the lug rotates......does it scare the heck out of you? It does me!
To answer myself kinda, I wish the Mfg's of Xfmr's supplied TA lugs with shoulders and Dividers between termination ponits.

Rob

So back to the original discussion.
You've decided that it is a 75 KVA transformer 480D-208Y/120
On the primary side we have a full load current of 90 amps
On the secondary side we have a full load current of 208 amps.

If we are to provide primary and secondary protection, the primary protection can't
exceed 250% [450.3(B)]. I usually choose about 125%. So I pick a 125A trip breaker for primary protection.

Primary conductors are chosen 125% of full load, so if terminations are 75 degree,
3-#2awg copper + 1-#6awg copper Gnd. I like a separate EGC even though you would be installing it in EMT or RGS,

Secondary protection (within 10' of secondary windings) is (calculated at 260A) I choose 300A trip.

I start by choosing secondary conductors as 4-350kCMIL copper + 1-4awg copper Gnd.
Since there is 4 current carrying conductors in the secondary conduit, I check that 80% of 350A (ampacity of 350kCMIL @ 90 deg) is 280A, so since it is not 300A to match the secondary protection, I will change the secondary wire size to 4-400kCMIL copper + 1-4awg copper Gnd. Again 80% of 380A is ok with the OCPD, so it's ok.

From 250.66, I choose a 1/0 awg copper for the n-g bonding jumper and grounding electrode.

Hopefully this gets the conversation going again.

Now, in my toils as a knuckledragger, I believe reading that the tranformer that comprises an SDS must be grounded to building steel and the water pipe.

If I wasn't headed to bed, I'd look it up. Off the top of my head, is that right?

I believe it's found in 250.30(Z)(zz)...

Now that Ron has pointed out the section that covers overload protection of transformers, I see that I was far too conservative in my breaker ratings. Good job, Ron! (Although, I don't know it those are the right answers, either. But at least they're reasoned from applicable provisions in the code.)

I note that, unless there are high harmonic currents, the neutral on the secondary doesn't count as a "current-carrying conductor", so there are only three CCCs in that conduit.

I'm used to designing feeders for systems that contain lots of non-linear loads, so I just did it from habit. I couldn't help myself

Ryan,
The IG conductor is in addition to the required EGC. So I'm not counting on the IG conductor to clear a fault, just its mythical job of reducing noise.
When I'm asked to provide an IG circuit, I provide an IG panel with an IG sized to match the EGC.
Since an IG is a design issue/choice, I don't think the NEC should get involved in the size.