X0 for 75KVA Delta-Wye ???

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kusneric

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Can someone please advise whether or not to connect X0 to the frame for a Siemens 75 KVA delta-wye?

Primary is 3 phase 480 with no neutral and secondary is 240-120 3 phase with neutral to 42 CB panelboard. Service is feeding an office area with separate, insulated ground bar in panel for computer outlets. Transformer primary is fed with 480 conducters using 2 awg wire from Siemens/ITE bus plug and an 8 awg ground - no neutral to delta. Secondary is 4/0 3 phase and neutral to 250 amp panelboard with 6 awg ground.

Panelboard has separate 6 awg ground back to fusible bus plug for isolated ground bar in panelboard. Isolated ground ran through its own 3/4 inch conduit, separate from the 4/0 feed ground wire.

I hooked up a bonding wire for X0 to the xmfr frame using 30 inches of left-over 4/0 cable (NEC table 250.66 said to use 2 awg or larger).

I also bonded the panelboard netural to the panelboard frame, using the included green bond screw.

Are these measures correct? Thanks for helping out an instrument and automation guy on his first commercial electrical job! Every time I read the NEC I get a different meaning and just need an expert to verify my thinking.


Regards,

Andy

P.S. Is it also okay to mount the insulated ground bar on the bottom of the CB panelboard, as opposed to the back plane? I wish to have the insulated ground bar centered in the panel; to facilitate connecting insulated computer outlet grounds from either side of the breaker rows.
 
Re: X0 for 75KVA Delta-Wye ???

Can someone please advise whether or not to connect X0 to the frame for a Siemens 75 KVA delta-wye?
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This is required.

Primary is 3 phase 480 with no neutral and secondary is 240-120 3 phase with neutral to 42 CB panelboard.
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Your secondary voltage is 120/208 not 240 if you have a wye secondary.

Secondary is 4/0 3 phase and neutral to 250 amp panelboard with 6 awg ground.
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I'd say that your panelboard would need a 225 amp main or a disconnect fused at 225 amps to protect the #4/0. #4/0 is rated for 230 amps not 250.

Your #6 should have been a #2. This conductor is sized according to 250.66.

I hooked up a bonding wire for X0 to the xmfr frame using 30 inches of left-over 4/0 cable (NEC table 250.66 said to use 2 awg or larger).
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This is fine.

I also bonded the panelboard netural to the panelboard frame, using the included green bond screw.
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You need to remove the bonding screw. You've created a violation by providing a parallel path for the neutral current to travel on the #6 conductor.

Is it also okay to mount the insulated ground bar on the bottom of the CB panelboard, as opposed to the back plane?
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Yes this is fine. Mount it where ever you want.

Did you have an engineer design this installation or did you design it yourself?
 
Re: X0 for 75KVA Delta-Wye ???

I agree with Trevor (Infinity).

I will ask how you are grounding the isolated grounding bar?

That isolated grounding bar should also be connected to the the transformers XO with a conductor no smaller than any single branch circuit grounding conductor terminated on it.
 
Re: X0 for 75KVA Delta-Wye ???

Panelboard has separate 6 awg ground back to fusible bus plug for isolated ground bar in panelboard. Isolated ground ran through its own 3/4 inch conduit, separate from the 4/0 feed ground wire.
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NO. You can't do that!!! The isolated ground must originate at the system bonding jumper for the secondary of the transformer. It cannot extend back to the building or primary grounding electrode.
Don
 
Re: X0 for 75KVA Delta-Wye ???

Infinity,

Thank you for taking the time to reply. Sorry for misquoting the secondary voltages - I posted very late after a long day and was a little punchy.

To answer your question, an engineer designed the majority of the installation and sized most wires, but left open the method for terminating the isolated ground for computer receptacles.

However, the engineer did specify running the separate, "isolated" ground bar wire back to the 100 amp fusible bus plugs, as opposed to Bob's recommendation to land the isolated ground wire in the transformer feeding the panelboard. Based on Bob's comments, I am now concerned that the isolated ground is incorrectly installed.

The 6 awg wire for the isolated panelboard ground is in a separate 3/4 EMT conduit between the panelboard and the fusible bus plug; terminated on the bus plug ground screw post, where the orginal 8 awg ground for the transformer feeders is landed.

Infinity, I also misquoted the mains rating late last night - it is 225 amps. However, I think the engineer selected the 6 awg ground accompanying the 4/0 based on equipment grounding found in table 250.122 for 200 amps, not table 250.66 - Please let me know your thoughts on this, as now I am really concerned with a potential undersized equipment ground.

I must also mention, aside from 3/4 EMT used for the separate, isolated 6 awg ground wire, all conduit used was 1.5 inch (bus plugs to transformer) or 2.5 inch (transformer to panelboard) greenfield (FMC). A large enough conduit bender was not available, so the more cumbersome and expensive flex steel was used as an option to EMT, offered by the project engineer. Two panelboards had roughly 6-7 foot runs of 2.5 inch FMC and one had 81 feet via two pull boxes. The 1.5 inch FMC runs vary in lenght from about 36 feet upwards to 68 feet with requisite numbers of pull boxes when reaching four 90 degree bends.

The 8 awg ground wire for the 2 awg primary feed is grounded at each bus plug, pull box and subsequently the transformer frame. The 6 awg (and incorrect sized ground per Infinity) is grounded to the transformer frame, pull boxes for the 4/0 wire and subsequently to the frame mounted ground bar in the panelboards.

When pulling the isolated 6 awg ground between the panelboards and bus plugs via 3/4 EMT, the isolated ground was not landed in the pull boxes, so as to not defeat the purpose of the isolated ground.

Thanks Infinity and Bob for your informative posts! I am removing the green bonding screws between the neutral and backplane in the panelboards. Based on your comments, I am very concerned the 4/0 secondary feeders do not have an adequate ground wire and the isolated ground is landed in the wrong location (bus plugs).

Thanks again for the comments and look forward to your replies to this post.

Regards,

Andy
 
Re: X0 for 75KVA Delta-Wye ???

Don,

Thank you for your post and Bob's similar comments. I will re-route the isolated ground bar wire back to the transformer.

1) Can this be in the same conduit as the transformer secondary feeders or is a separate conduit warranted?

2) I assume the isolated ground wire would not land at each pull box as does the equipment grounding wire?

Thanks,

Andy
 
Re: X0 for 75KVA Delta-Wye ???

The ground wire is part of the circuit and needs to be run with the rest of the circuit conductors.

Correct, it does not get bonded at each pull box.

This is seems to be another case of terminology (isolated versus dedicated) getting in the way of a proper installation.

All dedicated grounding conductors must only be connected to their load on one end and their power source on the other end (in your case this is the X0 terminal) at which point they are also bonded to the system ground. Jumping from one load device to another (i.e. daisy-chaining) can negate some of the advantages of dedicated gounding, the preferred method is to have seperate runs (star connection) from a common point.

An isolated ground terminal (in a panelboard or on a receptacle)is used for terminating dedicated grounding conductors
 
Re: X0 for 75KVA Delta-Wye ???

However, I think the engineer selected the 6 awg ground accompanying the 4/0 based on equipment grounding found in table 250.122 for 200 amps, not table 250.66 - Please let me know your thoughts on this, as now I am really concerned with a potential undersized equipment ground.
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This conductor should be sized according to 250.66 not 250.122. For the #4/0 secondary conductors that would require a #2 cu conductor not a #6. Take a look at 250.30(A)(2) for this requirement.


The 6 awg wire for the isolated panelboard ground is in a separate 3/4 EMT conduit between the panelboard and the fusible bus plug; terminated on the bus plug ground screw post, where the orginal 8 awg ground for the transformer feeders is landed.
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Typically when we did Isolated Grounds (we don't install them anymore because they are basically useless), we brought the IG conductor from the IG grounding bar, directly to the transformer XO routed with the secondary conductors. I've seen all sorts of IG's over the years, some connected to building steel, some to the XO and the best one, a conductor connected to a piece of rebar chopped out of the concrete slab. No kidding.
 
Re: X0 for 75KVA Delta-Wye ???

Kusneric,

Did you run to a grounding electrode (building steel) from the XO/grounding connections?
 
Re: X0 for 75KVA Delta-Wye ???

Ben,

No, I did not. The engineer designing the project only had an 8 awg ground wire from a Siemens fusible bus plug feeding a ground to the frame of the transformer - along with the 3each 2 awg 480volt feeders.

X0 for the new installation i am doing is currently bonded to the frame of the transformer. The only path to ground is via the aforementioned 8 awg ground wire that accompanies the 480 volt feeders.

I have reservations about the lack of a electrode conductor to the building steel frame.

My interpretation of NEC 250 is the transformer acts as a separate derived service, and thus should be grounded to the building steel.

I have observed that a previously installed transformer for another building system is not grounded to the building steel, nor does it have a ground wire to the transformer from the main switchgear for the building (aside from the grounding offered by the conduit).

I am not anxious to challenge the engineer's design and drawings produced - just want to install a safe system that will pass UCC inspection. Please feel free to offer your comments.
 
Re: X0 for 75KVA Delta-Wye ???

Please feel free to offer your comments
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The installation as described has multiple violations of the NEC.

As the electrician it is your job to point the problems out.

Engineers are not necessarily familiar with the NEC requirements.

250.30(A)(2) requires this separately derived system to have a GEC sized per 250.66.

250.30(A)(4) requires running that GEC above to a grounding electrode which is usually building steel.

250.30(A)(5) requires the bonding jumper (The EGC between the transformer and the panel) to be sized per 250.66 as Infinity pointed out.

250.104(A)(4) Requires the metal water piping in the vicinity to also be bonded to XO.

250.146(D) requires the isolated grounding conductor to terminate at an equipment grounding conductor terminal of the applicable derived system or service.

Which means it must not go further than the transformer.

I am not anxious to challenge the engineer's design and drawings produced
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Than you should not take on this kind of work.


As this job has an engineer and prints I bet somewhere you have some paperwork that states all work you perform will comply with all local and national codes.

I would not install it as your plans describe

It is not properly grounded or bonded which means it IMO it is hazardous.

[ January 22, 2006, 07:16 AM: Message edited by: iwire ]
 
Re: X0 for 75KVA Delta-Wye ???

Keep in mind that most engineers are very willing to learn something new if you handle us with "kid gloves" :)
If possible, approach him/her separately and discuss your concerns, not dictate. Than he/she will gladly agree with you, especially if you have a code book handy to point out violations. Tell the engineer you want to learn, and could he/she better explain.
 
Re: X0 for 75KVA Delta-Wye ???

Ron brings up a great point that applies to all of us.

We take pride in our work and if someone walks up and says "Hey your work sucks" we instantly close our ears and go on the defensive.

Approach the engineer as you would like to be approached, with respect. :)

As I understand it engineers are not required to know the NEC as part of their training, many end up learning it as they go. Don't forget that they are required to know a lot that we do not.

Perhaps this engineer has little experience with SDS, he/she might be grateful to learn the NEC rules regarding SDS.
 
Re: X0 for 75KVA Delta-Wye ???

Thank you very much for all comments. The foggy verbiage of SDS has been lifted.

I would never contemplate approaching someone with ridicule or contempt to point something out. Especially someone benefiting - like myself from asking questions to advance my understanding. The caliber of people on this forum is top shelf and I appreciate all the feedback and also appreciate your time in replying.

Results of forum feedback and job status:

1) I have already begun correcting the IG by pulling a green 12 awg wire between the panel IG bar and X0 of the transformer. (Branch conductors to office areas are Okonite MC 12-3 and 12-4 with 12 awg conductors)

2) Working on replacing the 6 awg ground between the panels and transformers with a 2 awg - based on Table 250.66

3) Next is grounding X0 to the building steel and bonding to a water pipe. Any recommendations or suggestions on physical connections would be most appreciated. Only problem here is I will have to bust through block walls to hit pipes and or building steel from the electrical and mechanical closets.

X0 currently has a bonding wire from transformer frame, a connection from the panel neutral, the ground wire to the panel and the IG ground landed on X0.

There are no more taps on X0 unless it is okay to double terminals on a hole. With taps used up, is it okay to tie the building ground to the frame of the transformer which is currently bonded to X0?

Also, how do you tie X0 into the building steel? Must the ground go via insulated wire conduit or can a bare conductor be used? Is a clamp to the steel acceptable?

Thanks to all!
 
Re: X0 for 75KVA Delta-Wye ???

Originally posted by kusneric:
3) Next is grounding X0 to the building steel and bonding to a water pipe. Any recommendations or suggestions on physical connections would be most appreciated. Only problem here is I will have to bust through block walls to hit pipes and or building steel from the electrical and mechanical closets.
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Usually we cad weld them to building steel, however you can drill the beam and bolt on a foot lug.

There are also some products out there that work like a beam clamp but are rated for grounding.

There are no more taps on X0 unless it is okay to double terminals on a hole.
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I would not stack the lugs.

Usually we buy lugs for multiple wires.

You will need a terminal at XO that will handle 4 conductors.

In your installation I see the following on XO.

The IG

The Building Steel

The Neutral

The Main Bonding jumper from the frame

The lug on the transformer should also handle 3 conductors.

The Main Bonding Jumper from XO

The incoming EGC

The outgoing bonding jumper to the panel.

It could also have the water connection.

However per the exception in 250.104(A)(4) you can place a bonding jumper from building steel to the water line.

[ January 22, 2006, 11:14 AM: Message edited by: iwire ]
 
Re: X0 for 75KVA Delta-Wye ???

Thank you very much "iwire".

This is the largest job I have done to date, and your advice as well as others posting has certainly helped a lot.

Being an automation/instrument person now delving into electrical construction, you certainly have helped clarify the NEC twists in the jargon; which is really hard for guys like me that have dyslexia.
 
Re: X0 for 75KVA Delta-Wye ???

Add to the list of violations the long run of 2.5 inch to one of your panels if there is no overcurrent protection ahead of it(on the 208 volt side of the transformer). NEC 240.21 (C) limits the length of the unprotected conductors originating at the secondary of the transformer. From what you have described, you would be limited to 25' of wire length before your 225 amp main breaker.

Fred
 
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