Basic Questions on a Separately Derived System

[KWH]

I have a 277/480 volt panel and need to install a stepdown transformer to 120/208. I am a little confused on the wiring between the 277/480 panel and the transformer, concerning the grounded conductor do I need to install this between the 2 or just ABC and a equipment ground then the way the xfrmer is tapped it will create the neutral. The building is metal but doesn't qualify as a grounding electrode so I would bond this and wire from ground rod at transformer. There is no water lines or Ufer. The wiring from load side of xfmer would be ABC,neutral and system bonding jumper.

 

[david luchini]

For a D-Y stepdown transformer, you only need ABC and EG on the transformer primary feeder.

 

[roger]

If the transformer is a delta/wye (most likely is) then all you need are the phase conductors and an EGC. See 250.30 for the grounding and Grounding Electrode information.

Roger

 

[ggunn]

I have a 277/480 volt panel and need to install a stepdown transformer to 120/208. I am a little confused on the wiring between the 277/480 panel and the transformer, concerning the grounded conductor do I need to install this between the 2 or just ABC and a equipment ground then the way the xfrmer is tapped it will create the neutral. The building is metal but doesn't qualify as a grounding electrode so I would bond this and wire from ground rod at transformer. There is no water lines or Ufer.

Are you speaking of the ECG or the neutral? I am new to this myself, but my understanding is that you want a 480 delta to 208 wye transformer and you will bond neutral to ground on the secondary. CAVEAT: I reiterate - I am a newbie to this type of system. I'm sure more experienced hands will weigh in and correct me if I am mistaken.

 

[infinity]

I have a 277/480 volt panel and need to install a stepdown transformer to 120/208. I am a little confused on the wiring between the 277/480 panel and the transformer, concerning the grounded conductor do I need to install this between the 2 or just ABC and a equipment ground then the way the xfrmer is tapped it will create the neutral. The building is metal but doesn't qualify as a grounding electrode so I would bond this and wire from ground rod at transformer. There is no water lines or Ufer. The wiring from load side of xfmer would be ABC,neutral and system bonding jumper.

Do you have a handle on sizing the supply EGC, the system bonding jumper, the GEC and the supply side bonding jumper?

 

[Smart $]

... then the way the xfrmer is tapped it will create the neutral.

Not the way it is tapped... this will be an isolation transformer, not an autotransformer. The way the 3 secondary windings are configured (wye) will establish the secondary neutral.

The building is metal but doesn't qualify as a grounding electrode so I would bond this and wire from ground rod at transformer (???). There is no water lines or Ufer. The wiring from load side of xfmer would be ABC,neutral and system bonding jumper.

Building should already be bonded to the 480/277 service. I'm assuming the 480/277 service is at the same building. Advise if otherwise.

With no underground metal water pipe or structural steel electrodes, run your GEC to the service grounding electrode system (GES). Need more detail info on GES to advise further...

 

[KWH]

I will be installing the 480v service and with no water or building steel that qualifies there will be 2 ground rods, so the gec from the xfmr would just terminate in the 480v panel then.

 

[Smart $]

I will be installing the 480v service and with no water or building steel that qualifies there will be 2 ground rods, so the gec from the xfmr would just terminate in the 480v panel then.

Afraid not. The GEC must be continuous from the xfmr (or secondary disconnect) to electrode(s). For starters, see 250.64(C).

If all the equipment is in the same localized area, and because the rods are required to be at least 6' apart, I'd suggest running one GEC, from 480 service disconnect to and through each rod clamp and back to xfmr (or secondary disconnect), sized to the larger required for the service or SDS.

 

[cadpoint]

Think of it like this: your transformance of power can be an establishment of new service thus the grounding requirements.


IE In speaking of your title "Seperately Devried system" - thus your transference.

 

[KWH]

Thanks for the explanation, your saying it is like starting over at the xfmr so coming out of the 480 panel for my gec is incorrect need to go to the gec system (ground rods ) and since the building steel is bonded from 480 panel there is no need to bond again from xfmr.

 

[cadpoint]

ok yes and no...

If the existing 480 enters the structure, there seems to be an establishment of service, and a grounding point.

Again it's a transformation of a service that requires things of the Code is what I'm implying, one address the requirement of that type of service, from this point on... IE a transformer!

 

[Smart $]

Thanks for the explanation, your saying it is like starting over at the xfmr so coming out of the 480 panel for my gec is incorrect need to go to the gec system (ground rods ) and since the building steel is bonded from 480 panel there is no need to bond again from xfmr.

Essentially correct... but for clarification, there is only one grounding electrode system (GES). The two systems—service and SDS—share this one GES. There are several ways to connect each system to the GES. But running a jumper from the bus of one system's GEC connection to the other is not a permitted method. The Code essentially wants the only reversible connections at the electrodes and each system's connection point and nowhere in between.

 

[ActionDave]

Thanks for the explanation, your saying it is like starting over at the xfmr so coming out of the 480 panel for my gec is incorrect need to go to the gec system (ground rods ) and since the building steel is bonded from 480 panel there is no need to bond again from xfmr.

I will be installing the 480v service and with no water or building steel that qualifies there will be 2 ground rods, so the gec from the xfmr would just terminate in the 480v panel then.

These two statements look to be in conflict with each oher. If the building steel is bonded why is it not part of the grounding electrode system?

 

[augie47]

The terminology may have changed, but the principal is as shown in this picture, except with no steel or water present, your grounding electrode will be ground rods.

 

[Smart $]

These two statements look to be in conflict with each oher. If the building steel is bonded why is it not part of the grounding electrode system?

I don't know the particulars of his situation, but the 2011 NEC revised the qualifying conditions for a "Metal Frame of the Building or Structure" electrode.

(2) Metal Frame of the Building or Structure. The metal
frame of the building or structure that is connected to the
earth by any of the following methods:

(1) 3.0 m (10 ft) or more of a single structural metal member
in direct contact with the earth or encased in concrete
that is in direct contact with the earth

(2) Connecting the structural metal frame to the reinforcing
bars of a concrete-encased electrode as provided in
250.52(A)(3) or ground ring as provided in 250.52(A)(4)

(3) Bonding the structural metal frame to one or more of
the grounding electrodes as defined in 250.52(A)(5) or
(A)(7) that comply with 250.56

(4) Other approved means of establishing a connection to
earth

(2) Metal Frame of the Building or Structure. The metal
frame of the building or structure that is connected to the
earth by one or more of the following methods:

(1) At least one structural metal member that is in direct
contact with the earth for 3.0 m (10 ft) or more, with or
without concrete encasement.

(2) Hold-down bolts securing the structural steel column
that are connected to a concrete-encased electrode that
complies with 250.52(A)(3) and is located in the support
footing or foundation. The hold-down bolts shall
be connected to the concrete-encased electrode by
welding, exothermic welding, the usual steel tie wires,
or other approved means.

 

[kwired]

These two statements look to be in conflict with each oher. If the building steel is bonded why is it not part of the grounding electrode system?

Not all building steel is a qualifying electrode - see sections that Smart$ posted.