# 480Vac to 120/208vac TXFR



## ckeely (Dec 6, 2012)

I am installing a new transformer and wondering if I have to bond the XO lug to my ground at the TXFR or if that should be done at my first panel on the 208vac side. 

I have (3) phase conductors and a ground coming from my 480 panel and (3) phases, (1) neutral, and (1) ground leaving going to my 208 panel. 

Any experience with this out there?


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## Pete m. (Nov 19, 2011)

Best advice I can give...

*250.30 Grounding Separately Derived Alternating-Current Systems.* In addition to complying with 250.30(A)
for grounded systems, or as provided in 250.30(B) for un-grounded systems, separately derived systems shall comply
with 250.20, 250.21, 250.22, and 250.26.
Informational Note No. 1: An alternate ac power source,
such as an on-site generator, is not a separately derived
system if the grounded conductor is solidly interconnected
to a service-supplied system grounded conductor. An ex-ample of such a situation is where alternate source transfer
equipment does not include a switching action in the grounded
conductor and allows it to remain solidly connected to the
service-supplied grounded conductor when the alternate
source is operational and supplying the load served.
Informational Note No. 2: See 445.13 for the minimum
size of conductors that carry fault current.
*(A) Grounded Systems.* A separately derived ac system that
is grounded shall comply with 250.30(A)(1) through (A)(8).
Except as otherwise permitted in this article, a grounded con-ductor shall not be connected to normally non–current-carrying metal parts of equipment, be connected to equipment
grounding conductors, or be reconnected to ground on the
load side of the system bonding jumper.
Informational Note: See 250.32 for connections at sepa-rate buildings or structures, and 250.142 for use of the
grounded circuit conductor for grounding equipment.
Exception: Impedance grounded neutral system grounding
connections shall be made as specified in 250.36 or
250.186, as applicable.
(1) System Bonding Jumper. An unspliced system bond-ing jumper shall comply with 250.28(A) through (D). This
connection shall be made at any single point on the sepa-rately derived system from the source to the first system
disconnecting means or overcurrent device, or it shall be
made at the source of a separately derived system that has
no disconnecting means or overcurrent devices, in accor-dance with 250.30(A)(1)(a) or (b). The system bonding
jumper shall remain within the enclosure where it origi-250.26 ARTICLE 250 — GROUNDING AND BONDING
NATIONAL ELECTRICAL CODE 2011 Edition70–106
nates. If the source is located outside the building or struc-ture supplied, a system bonding jumper shall be installed at
the grounding electrode connection in compliance with
250.30(C).
Exception No. 1: For systems installed in accordance with
450.6, a single system bonding jumper connection to the tie
point of the grounded circuit conductors from each power
source shall be permitted.
Exception No. 2: A system bonding jumper at both the
source and the first disconnecting means shall be permitted
if doing so does not establish a parallel path for the
grounded conductor. If a grounded conductor is used in this
manner, it shall not be smaller than the size specified for
the system bonding jumper but shall not be required to be
larger than the ungrounded conductor(s). For the purposes
of this exception, connection through the earth shall not be
considered as providing a parallel path.
Exception No. 3: The size of the system bonding jumper
for a system that supplies a Class 1, Class 2, or Class 3
circuit, and is derived from a transformer rated not more
than 1000 volt-amperes, shall not be smaller than the de-rived ungrounded conductors and shall not be smaller than
14 AWG copper or 12 AWG aluminum.
(a) Installed at the Source. The system bonding jumper
shall connect the grounded conductor to the supply-side bond-ing jumper and the normally non–current-carrying metal en-closure.
(b) Installed at the First Disconnecting Means. The
system bonding jumper shall connect the grounded conductor
to the supply-side bonding jumper, the disconnecting means
enclosure, and the equipment grounding conductor(s).
(2) Supply-Side Bonding Jumper. If the source of a sepa-rately derived system and the first disconnecting means are
located in separate enclosures, a supply-side bonding
jumper shall be installed with the circuit conductors from
the source enclosure to the first disconnecting means. A
supply-side bonding jumper shall not be required to be
larger than the derived ungrounded conductors. The supply-side bonding jumper shall be permitted to be of nonflexible
metal raceway type or of the wire or bus type as follows:
(a) A supply-side bonding jumper of the wire type
shall comply with 250.102(C), based on the size of the
derived ungrounded conductors.
(b) A supply-side bonding jumper of the bus type shall
have a cross-sectional area not smaller than a supply-side
bonding jumper of the wire type as determined in 250.102(C).
(3) Grounded Conductor. If a grounded conductor is in-stalled and the system bonding jumper connection is not
located at the source, 250.30(A)(3)(a) through (A)(3)(d)
shall apply.
(a) Sizing for a Single Raceway. The grounded con-ductor shall not be smaller than the required grounding
electrode conductor specified in Table 250.66 but shall not
be required to be larger than the largest derived ungrounded
conductor(s). In addition, for sets of derived ungrounded
conductors larger than 1100 kcmil copper or 1750 kcmil
aluminum, the grounded conductor shall not be smaller
than 121⁄2 percent of the circular mil area of the largest set
of derived ungrounded conductors.
(b) Parallel Conductors in Two or More Raceways. If
the ungrounded conductors are installed in parallel in two
or more raceways, the grounded conductor shall also be
installed in parallel. The size of the grounded conductor in
each raceway shall be based on the total circular mil area of
the parallel derived ungrounded conductors in the raceway as
indicated in 250.30(A)(3)(a), but not smaller than 1/0 AWG.
Informational Note: See 310.10(H) for grounded conduc-tors connected in parallel.
(c) Delta-Connected System. The grounded conductor
of a 3-phase, 3-wire delta system shall have an ampacity
not less than that of the ungrounded conductors.
(d) Impedance Grounded System. The grounded con-ductor of an impedance grounded neutral system shall be in-stalled in accordance with 250.36 or 250.186, as applicable.
(4) Grounding Electrode. The grounding electrode shall
be as near as practicable to, and preferably in the same area
as, the grounding electrode conductor connection to the
system. The grounding electrode shall be the nearest of one
of the following:
(1) Metal water pipe grounding electrode as specified in
250.52(A)(1)
(2) Structural metal grounding electrode as specified in
250.52(A)(2)
Exception No. 1: Any of the other electrodes identified in
250.52(A) shall be used if the electrodes specified by
250.30(A)(4) are not available.
Exception No. 2 to (1) and (2): If a separately derived
system originates in listed equipment suitable for use as
service equipment, the grounding electrode used for the
service or feeder equipment shall be permitted as the
grounding electrode for the separately derived system.
Informational Note No. 1: See 250.104(D) for bonding
requirements for interior metal water piping in the area
served by separately derived systems.
Informational Note No. 2: See 250.50 and 250.58 for re-quirements for bonding all electrodes together if located at
the same building or structure.
(5) Grounding Electrode Conductor, Single Separately
Derived System. A grounding electrode conductor for a
single separately derived system shall be sized in accor-dance with 250.66 for the derived ungrounded conductors.
ARTICLE 250 — GROUNDING AND BONDING 250.30
2011 Edition NATIONAL ELECTRICAL CODE 70–107
It shall be used to connect the grounded conductor of the
derived system to the grounding electrode as specified in
250.30(A)(4). This connection shall be made at the same
point on the separately derived system where the system
bonding jumper is connected.
Exception No. 1: If the system bonding jumper specified in
250.30(A)(1) is a wire or busbar, it shall be permitted to
connect the grounding electrode conductor to the equip-ment grounding terminal, bar, or bus, provided the equip-ment grounding terminal, bar, or bus is of suffıcient size for
the separately derived system.
Exception No. 2: If a separately derived system originates
in listed equipment suitable as service equipment, the
grounding electrode conductor from the service or feeder
equipment to the grounding electrode shall be permitted as
the grounding electrode conductor for the separately de-rived system, provided the grounding electrode conductor is
of suffıcient size for the separately derived system. If the
equipment grounding bus internal to the equipment is not
smaller than the required grounding electrode conductor
for the separately derived system, the grounding electrode
connection for the separately derived system shall be per-mitted to be made to the bus.
Exception No. 3: A grounding electrode conductor shall
not be required for a system that supplies a Class 1, Class
2, or Class 3 circuit and is derived from a transformer
rated not more than 1000 volt-amperes, provided the
grounded conductor is bonded to the transformer frame or
enclosure by a jumper sized in accordance with 250.30(A)(1),
Exception No. 3, and the transformer frame or enclosure is
grounded by one of the means specified in 250.134.
(6) Grounding Electrode Conductor, Multiple Sepa-rately Derived Systems. A common grounding electrode
conductor for multiple separately derived systems shall be
permitted. If installed, the common grounding electrode
conductor shall be used to connect the grounded conductor
of the separately derived systems to the grounding elec-trode as specified in 250.30(A)(4). A grounding electrode
conductor tap shall then be installed from each separately
derived system to the common grounding electrode con-ductor. Each tap conductor shall connect the grounded con-ductor of the separately derived system to the common
grounding electrode conductor. This connection shall be
made at the same point on the separately derived system
where the system bonding jumper is connected.
Exception No. 1: If the system bonding jumper specified in
250.30(A)(1) is a wire or busbar, it shall be permitted to
connect the grounding electrode conductor tap to the
equipment grounding terminal, bar, or bus, provided the
equipment grounding terminal, bar, or bus is of suffıcient
size for the separately derived system.
Exception No. 2: A grounding electrode conductor shall not
be required for a system that supplies a Class 1, Class 2, or
Class 3 circuit and is derived from a transformer rated not
more than 1000 volt-amperes, provided the system grounded
conductor is bonded to the transformer frame or enclosure by
a jumper sized in accordance with 250.30(A)(1), Exception
No. 3, and the transformer frame or enclosure is grounded by
one of the means specified in 250.134.
(a) Common Grounding Electrode Conductor. The
common grounding electrode conductor shall be permitted
to be one of the following:
(1) A conductor of the wire type not smaller than 3/0 AWG
copper or 250 kcmil aluminum
(2) The metal frame of the building or structure that com-plies with 250.52(A)(2) or is connected to the ground-ing electrode system by a conductor that shall not be
smaller than 3/0 AWG copper or 250 kcmil aluminum
(b) Tap Conductor Size. Each tap conductor shall be
sized in accordance with 250.66 based on the derived un-grounded conductors of the separately derived system it
serves.
Exception: If a separately derived system originates in
listed equipment suitable as service equipment, the ground-ing electrode conductor from the service or feeder equip-ment to the grounding electrode shall be permitted as the
grounding electrode conductor for the separately derived
system, provided the grounding electrode conductor is of
suffıcient size for the separately derived system. If the
equipment ground bus internal to the equipment is not
smaller than the required grounding electrode conductor
for the separately derived system, the grounding electrode
connection for the separately derived system shall be per-mitted to be made to the bus.
(c) Connections. All tap connections to the common
grounding electrode conductor shall be made at an acces-sible location by one of the following methods:
(1) A connector listed as grounding and bonding equipment.
(2) Listed connections to aluminum or copper busbars not
smaller than 6 mm × 50 mm (1⁄4 in. × 2 in.). If alumi-num busbars are used, the installation shall comply
with 250.64(A).
(3) The exothermic welding process.
Tap conductors shall be connected to the common ground-ing electrode conductor in such a manner that the common
grounding electrode conductor remains without a splice or
joint.
(7) Installation. The installation of all grounding electrode
conductors shall comply with 250.64(A), (B), (C), and (E).
(8) Bonding. Structural steel and metal piping shall be
connected to the grounded conductor of a separately de-rived system in accordance with 250.104(D)

Pete


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## tates1882 (Sep 3, 2010)

ckeely said:


> I am installing a new transformer and wondering if I have to bond the XO lug to my ground at the TXFR or if that should be done at my first panel on the 208vac side.
> 
> I have (3) phase conductors and a ground coming from my 480 panel and (3) phases, (1) neutral, and (1) ground leaving going to my 208 panel.
> 
> Any experience with this out there?


Ditch the egc from xform to main on the secondary side. Bond in the panel at the main. It can be done either way but bonding in the xformer allows for a second path for neutral current to flow if the grounded conductor ever fails.


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## glen1971 (Oct 10, 2012)

tates1882 said:


> Ditch the egc from xform to main on the secondary side. Bond in the panel at the main. It can be done either way but bonding in the xformer allows for a second path for neutral current to flow if the grounded conductor ever fails.


What I have done in the past to help reduce the chances of that happening are:
1) C-tap off your ground wire inside the transformer and take one to the X0 and one to the case ground.
2) Run the ground conductor through the X0 lug and then to the Xfmr case.

Either way there are only 2 spots for failure - the bolt holding the X0 lug on or a break in the ground wire. I think you can only build it factoring in so many "what ifs"..


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## BBQ (Nov 16, 2010)

Pete m. said:


> Best advice I can give...


Holy cut and paste Harry. :jester:


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## Pete m. (Nov 19, 2011)

BBQ said:


> Holy cut and paste Harry. :jester:


:laughing:... Just figured he needed the whole code section.

Pete


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## ponyboy (Nov 18, 2012)

One of these threads should be made sticky because it comes up so much. I thought it was taught during apprenticeship...
The bonding takes place either in the stormer or in the first disconnecting means I.e. your 208 panel, not both just pick one. Wherever you do decide to bond is where you provide the grounding electrode for the new system. Also wherever the bond occurs is the last place in the system where grounded and grounding conductors will be bonded. If you do it in the xformer every panel downstream is now treated as a sub and all grounds and mistrals will be seperated from each other.


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## tates1882 (Sep 3, 2010)

glen1971 said:


> What I have done in the past to help reduce the chances of that happening are:
> 1) C-tap off your ground wire inside the transformer and take one to the X0 and one to the case ground.
> 2) Run the ground conductor through the X0 lug and then to the Xfmr case.
> 
> Either way there are only 2 spots for failure - the bolt holding the X0 lug on or a break in the ground wire. I think you can only build it factoring in so many "what ifs"..


I like to separate the primary grounding conductor and the secondary grounding conductor. I never tie x0 to the case unless I'm using primary over current protection only. A transformer should be treated just like a service drop minus kw meter. New grounding electrode, bonded at the first main disconnect.


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## don_resqcapt19 (Jul 18, 2010)

tates1882 said:


> I like to separate the primary grounding conductor and the secondary grounding conductor. I never tie x0 to the case unless I'm using primary over current protection only. A transformer should be treated just like a service drop minus kw meter. New grounding electrode, bonded at the first main disconnect.


There has to be a path from XO to the transformer case. This can be via a the supply side bonding jumper when the system bonding jumper is installed at the first disconnect.


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## glen1971 (Oct 10, 2012)

tates1882 said:


> I like to separate the primary grounding conductor and the secondary grounding conductor. I never tie x0 to the case unless I'm using primary over current protection only. A transformer should be treated just like a service drop minus kw meter. New grounding electrode, bonded at the first main disconnect.


 
All the utility transformers I've done have 2 taps to the grid around them, and connected to the ground bus in the transformer.. The grounds on the primary side are done to the same buss.. The secondary cable grounds are also connected to it.. Metering ground to the same buss..


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