I'm installing a multi building system ( large shop / home ) The primary
feed is going to the shop 600 amp @ 480 4 wire. I plan to run a sub feed to
the house with 175 amp 480 3 wire ( 4-0 AL @ 300') to a 150 Amp breaker /
disconnect box. Then to a 112.5 kva 3 phase transformer ( 480 delta to
208/120 Y) At the house I'm going to run a ufer ground and also the
required 2, 8' ground rods. I assume that I will need to bond the derived
grounded connector from the transformer to the equipment safety ground in
the 208 V panel?? or should it be in the service disconnect ( normal place
to bond, but seems like a strange place, as there is no neutral in that
If this is just a wild eyed hypothetical then you got me.
If you are tackling a 480 delta by 208/120 volt wye transformer
installation without the help of a well trained and experienced
electrician you are "sailing into danger".
There is no requirement for using ground rods in the US NEC. The code
only regulates how they are installed when they are used. The decision
whether or not to use them is a design issue. What is required is that
you must construct a "Grounding Electrode System" from all the
electrodes that are available. The reason that driven rods are so often
installed as grounding electrodes is that they are easily installed
after the opportunity to install a Ufer or ground ring has been missed
because the builder did not involve an electrician sufficiently early in
the construction process. Once you have a Concrete Encased Electrode
(Ufer) you do not need to use ground rods.
You appear to be confusing the required "building disconnecting means"
with the "service disconnecting means" of the derived system. The
building disconnecting means provides the means to disconnect the
ungrounded conductors of the feeder that will supply the home from their
source of supply as required by 250.32. The service disconnecting means
of the derived service that will supply the home's lighting and
appliance panel board will be the main breaker of that panel.
The Grounding Electrode Conductor (GEC) should be terminated at the Xo
of the transformer. From that same multibarreled lug you will run
separate EGC and Grounded Conductor (neutral) to separate buss bars in
the lighting and appliance panel board for the dwelling. The code
allows you to make the connection of the GEC to the Grounded Conductor
at either the transformer or the service disconnecting means of the
derived system but best practice is to connect at the Xo of the
You also should run an Equipment Grounding Conductor with the conductors
of the feeder that will supply the home and use it to bond the building
disconnecting means enclosure back to the main bonding jumper of the
shop service. That will provide a low impedance fault current path back
to the Xo of the utility service transformer. That EGC will continue on
the the frame of the transformer in the home.
You might want to check about running 480 to a residence with your local
inspector. I am probably wrong, but for some reason my feeble brain says
that it is a no-no...I am not even sure that you can have a 3 phase service
to a single family dwelling, regardless of voltage.
You may have to put the transformer in your shop and feed the lower voltage
to your home.
Do you have to meter your home separately so that you don't run into tax
problems with the IRS?
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