3 wire or 4 wire transformer connections

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BDBoard

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With a wye secondary on my utility feed transformer it seems prudent to run 4 wire to my MCC so I have a dedicated neutral conductor and a separate, dedicated ground. There are several single phase loads so a balanced system really isn't possible.

One of my branch circuits will feed a step up/step down configuration to get 480V to a secondary MCC 2500' away without dropping a fortune on cable (not to mention voltage drop issues). Seems like I can get away with 3 wire connection through to the step down transformer then connect the wye secondary of that transformer to the other MCC with 4 wire. Being separately derived systems they'll all have their own ground grids bonded together.

Opinions?
 
With a wye secondary on my utility feed transformer it seems prudent to run 4 wire to my MCC so I have a dedicated neutral conductor and a separate, dedicated ground.
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I don't know of any utility that will let you run a grounding conductor from their transformer to your service disconnet. It would also result in a code violation as the grounding conductor would be in parallel with the neutral as a result of the main bonding jumper that must be installed at the service disconnect.
 
it seems prudent to run 4 wire to my MCC so I have a dedicated neutral conductor and a separate, dedicated ground.
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I think it would take 5 wires, 3 phases a neutral and a ground, to get what you described. Also remember that if your MCC is the service disconnect, you must bring a neutral if it is present. If the MCC is the service disconnect, there would not be a ground between the utility xfmr and the disconnect.

Seems like I can get away with 3 wire connection through to the step down transformer
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If your transformer has a delta winding, 3 wires + ground would work. If it is a wye connection, a neutral would likely be necessary, depending on the connection on the other side. What do you plan to step up to?

Jim T
 
When you say MCC, do you mean motor control center ? I wasn't aware that you could get an MCC that was SE rated, but I could be very wrong.

Either way, if this is your service entrance and it's a wye secondary, you have to carry the neutral to the service disconnect. And, as Don said, you can't parallel a grounding conductor with it to the utility.

As far as feeding your step up / step down arrangement, you are correct in thinking you need a 3 wire primary and a 4 wire secondary in your reverse connected transformer.
 
ryan_618 said:
bdarnell said:
I wasn't aware that you could get an MCC that was SE rated, but I could be very wrong.
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I talked with Sqaure D about this once, and they said that they have SE listed MCC's. (or perhaps it was Siemens?)
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I think AB does too. You have to ask for it though, but I seem to recall a more or less standard MCC can be made SE by the addition of a paper sticker.
 
Jim,
I think it would take 5 wires, 3 phases a neutral and a ground, to get what you described.
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Good catch! I read over the number of wires and just assumed 3 hots, neutral and an EGC.
Don
 
It was during my cable selection that I managed to confuse myself. Wish I knew how to post a picture. My metering and main disconnect is by others. I'm running from the main disconnect to a panel board and then to the MCC. If I specify 4 conductor TC it will have 3 hots, a neutral and EGC. Since the main disconnect is by others, they should have already taken care of the main bonding jumper, correct? So I should specify that at no time are the EGC and neutral to be bonded until I get to the load side of the SDS.
 
So I should specify that at no time are the EGC and neutral to be bonded until I get to the load side of the SDS.
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Correct, and then only the neutral from the load side of the SDS will be bonded to the SDS grounding electrode. The supply or primary side neutral will not be run to the SDS. As a result of the required equipment grounding there will be an electrical path between the supply and load side grounded conductors.
Don
 
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