5kva transformer

[njhvac]

5kva single phase dry transformer 480v primary with 120v secondary for control panel installation.
Shouldn't X2 be bonded to the metal cabinet?

 

[infinity]

Yes, and if I remember correctly a 5kva would require a GEC to a grounding electrode.

 

[petersonra]

Yes, and if I remember correctly a 5kva would require a GEC to a grounding electrode.

You know, I have seen hundreds or maybe thousands of substantially sized control power transformers on machines and never seen a connection to a GE. It is always connected to the EGC.

I take it back. We built some load banks inside some aluminum huts once and I ran a green wire to the metal structure of the hut from the transformer X2. That is the only time I can ever recall a conenction to a GE.

 

[tom baker]

1 KVA and under do not require an GEC and GE. See 250.30 (A)(1) ex 3

 

[infinity]

1 KVA and under do not require an GEC and GE. See 250.30 (A)(1) ex 3

That's the reference I was referring to. At 5 kva he would need a GEC.

 

[petersonra]

1kva is not all that big of a control transformer.

There have to be tens or hundreds of thousands of machine out there with grounded transformers over 1kva that are not connected to a GE, except through the EGC.

Is there any way such a CPT could be wired so it is not an SDS?

 

[redtail_MH]

Two questions:

1) Where does the code state that XFMRs 1kVA and below do not require a grounding electrode? I couldn't find it.

2) What are "class 1, 2, & 3 circuits"?

 

[tom baker]

1) my post gives the code section
2) Class 1, 2 3 are circuits as defined in Art 725. If you are building control panels with PLCs, then the field wiring is probably Class 1 or 2 wiring.

 

[RUWired]

I'm sure if he read the section he would have found it anyway but, 250.30(A)1 ex 3 is for sizing and (A)3 ex 3 is the requirement (not to be technical)
Rick

 

[cripple]

5kva transformer

Section 250.30 has the requirements for the grounding separately derived systems.
In Article 100 separately derived systems is defend as: A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. The 5 Kva control transformer is not servicing a premises wiring system. So with that said does 250.30 still apply? If the control transformer feeds only the control circuit for the control panel why can?t the control transformer be grounded to the control panel enclosure and the equipment grounding conductor that is required for the 120 v control circuit be bonded to the control penal. The control panel and transformer are being fed by a 480 v circuit with I hope a properly sized equipment grounding conductor, which is required to be bonded to the enclosure. Could any one explain the hazards in grounding the control transformer in this manner?

 

[Bayouamps]

In reference to cripple's note. I have connected 3 ea 1.5kVA CP transformers in 2 seperate cabinets at one of our plants. I did use a Green wire and connect it to the green equipment grounding conductor wire brought in on the 480 feed into the cabinets. I too would like clarifacation on the validity of this connection.

 

[njhvac]

In my plant, X2 is typically bonded to the cabinet along with the feeders EGC.
This particular existing control cabinet had the SBJ removed from the cabinet grounding bar by an outside contracor, to be rerouted along with X1, to feed a small UPS to supply a PLC.
I discovered the floating voltage on the X2 neutral 10 months later.
It measured 70VAC

 

[petersonra]

In my plant, X2 is typically bonded to the cabinet along with the feeders EGC.
This particular existing control cabinet had the SBJ removed from the cabinet grounding bar by an outside contracor, to be rerouted along with X1, to feed a small UPS to supply a PLC.
I discovered the floating voltage on the X2 neutral 10 months later.
It measured 70VAC

there is generally no requirement that control circuits be grounded, in fact it is probabl as common to see ungrounded control circuits as grounded.

 

[tom baker]

Section 250.30 has the requirements for the grounding separately derived systems.
In Article 100 separately derived systems is defend as: A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. The 5 Kva control transformer is not servicing a premises wiring system. So with that said does 250.30 still apply? If the control transformer feeds only the control circuit for the control panel why can?t the control transformer be grounded to the control panel enclosure and the equipment grounding conductor that is required for the 120 v control circuit be bonded to the control penal. The control panel and transformer are being fed by a 480 v circuit with I hope a properly sized equipment grounding conductor, which is required to be bonded to the enclosure. Could any one explain the hazards in grounding the control transformer in this manner?

First we need to determine what premises wiring is.
The definition in Article 100:
Premises Wiring (System). That interior and exterior wiring, including power, lighting, control, and signal circuit wiring together with all their associated hardware, fittings, and wiring devices, both permanently and temporarily installed, that extends from the service point or source of power, such as a battery, a solar photovoltaic system, or a generator, transformer, or converter windings, to the outlet(s). Such wiring does not include wiring internal to appliances, luminaires (fixtures), motors, controllers, motor control centers, and similar equipment.

And what does UL 508 state for control transformers? This application may not be under the NEC but rather 508.

 

[Bayouamps]

Tom

I do not have a copy of the UL 508 1.2 - 1.3 specification. I would appreciate a post of this if you have it.

That said, your post above from NEC 100 does seem to indicate that the CP xfmr is supplying a premises wiring system. I content that the CP xfmr must be bonded to the EGC brought along with the 480 volt ac supply from the distribution panels in the main power room/enclosure. The only other way would be to pull the 3 480 ungrounded conductors, the EGC for the 480v supply and the EGC for the cabinet. Then you would have to mount all of the CP xfmr EGCs to an isolated connector strip with just the CP xfmr ungrounded wiring conduits being connected to it. This would be just wrong. What if the 120 V shorted to one of the conduit walls. Yes, it might pull enough current to trip the protective device but I would not bet on it. More likely, it would just sit there and leave a 60-80 v ac touch voltage on the conduit wall. Why, because of the length of the CP xfmr EGC. We would pull it sized for the rating of the CP xfmr. A 5 kva CP xfmr produces right at 21 amps 240 volts. So what size would this EGC be? I read it to be a 12 solid. Now let us use the control panel that I had to contend with. It was located 140' from the main power room. This is a pretty far piece. The resistance of this wire would be: 0.22246 ohms, keeping in mind that this is perfect wire. Allowing a minimum shorting current of 500 amps the voltage drop on the EGC would be .22246 * 500 = 111.23 volts ac. Yikes, did I do something wrong? No, because the X2 was not connected to the CP xfmr EGC in the panel. So am I wrong in how I see this? I say connect all the EGC together to the 480V EGC brought from the power distribution panels in the main power room/enclosure. This is how I did it. I am eager to hear from you guys.

 

[petersonra]

16.2 A transformer secondary that is required to be grounded in accordance with 16.1, shall have a main bonding jumper factory connected to the transformer secondary and to the enclosure and grounding electrode conductor terminal (or only to the enclosure when a grounding electrode conductor terminal is not provided). The size of the main bonding jumper shall be as specified in 75.1.4, based on the transformer secondary rating. A grounding electrode conductor terminal sized to retain the required grounding electrode conductor in accordance with 75.1.4, based on the transformer secondary rating, shall be provided in the enclosure containing the transformer and a marking as specified in 54.10 shall be provided.
Exception: When the transformer is rated not more than 1000 volt-amperes and supplies only control circuits, the grounding electrode conductor terminal is able to be omitted and the main bonding jumper shall not be smaller than a No. 14 AWG (2.1 mm2) copper conductor. The jumper is not otherwise required to be larger than the phase conductors connected to the transformer secondary.

It would appear UL508a only requires a terminal be provided for the purpose of connecting to a GEC. Since virtually every panel has some kind of ground terminal provided, it is an installation issue if a green wire is not run to the closest GE.

 

[cripple]

5kva transformer

UL-508 does require a grounding electrode terminal stats the following:
16 Transformer and Power Supply Secondary Grounding
16.1 A secondary circuit that contains field wiring terminals and is supplied from a power transformer, control transformer, or power supply shall have the secondary grounded under any of the following conditions:
a) When the secondary voltage is less than 50 volts; and
1) The supply to the primary is over 150 volts to ground; or
2) The supply to the primary at any voltage is ungrounded;
b) When the secondary voltage is 50 volts or greater and the secondary circuit is able to be
grounded so that the maximum voltage to ground on the ungrounded conductors does not
exceed 150 volts;
c) When the secondary is a 3-phase, 4-wire, wye connected in which the center point of the
wye is used as a circuit conductor; or
d) When the secondary is a 3-phase, 4-wire, delta connected in which the midpoint of one
phase winding is used as a circuit conductor.
Revised 16.1 effective March 1, 2007
16.2 A transformer or power supply secondary that is required to be grounded in accordance with 16.1, shall have a main bonding jumper factory connected to the secondary and to the enclosure and grounding electrode conductor terminal (or only to the enclosure when a grounding electrode conductor terminal is not provided). The size of the main bonding jumper shall be as specified in 75.1.4, based on the secondary rating. A grounding electrode conductor terminal sized to retain the required grounding electrode conductor in accordance with 75.1.4, based on the secondary rating, shall be provided in the enclosure containing the transformer or power supply and a marking as specified in 54.10 shall be provided.
Exception: When the transformer is rated not more than 1000 volt-amperes and supplies only control circuits, the grounding electrode conductor terminal is able to be omitted and the main bonding jumper shall not be smaller than a No. 14 AWG (2.1 mm2) copper conductor. The jumper is not otherwise required to be larger than the phase conductors connected to the transformer secondary.
Revised 16.2 effective March 1, 2007
16.3 When components marked with a slash voltage rating, such as 120/240V, 480Y/277V, or
600Y/347V, are provided on the secondary side of a power transformer or power supply, the secondary shall be grounded as in 16.1(b), 16.1(c), or at the center point of the wye for a 3 phase, 3 wire secondary circuit, and 16.2.
Added 16.3 effective March 1, 2007
So now if you have a 4 story concrete building with a 480 volt feeder to a power panel located in a equipment room in penthouse on the roof and the only transformer a 5kva 480v to 120v control voltage transformer for a control panel, would apply 250.30(A)(3) exception (3)? It seem to me it would be more effective to have every thing bonded to the secondary side of the control voltage transformer.

 

[tom baker]

This is most likely a UL508 Industrial Control Panel application. I don't have a copy of 508 at home.
However, under either the NEC or 508, you must have a system bonding jumper.