Thread: What size bond conductor for Transformer - Neutral to Case

Greetings,

I am hoping for a cut and dry answer here.

I have a 75KVA transformer. 480V Delta Primary and 208/120V Wye Secondary.

In order to avoid having a floating neutral, I need to bond the neutral on the secondary to ground.

What size conductor do I need for this?

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Shawn

I deleted the previous post because I misunderstood what your asking. I thought you were inspecting but sounds like you are actually doing the work.

If so you need to go to a professional electricians forum like this below. You need to get the size of your secondary conductors, ...you will need that to get to what you want to know. Your going to be asking what your MJB (main bonding jumper) size should be.

Electrician Talk - Professional Electrical Contractors Forum

Hi Roger,

Thank you for the link. I've been looking for a site like this for a while.

Your welcome and be careful ....

For all others out there reading this who may still want the info, the wire size for this is #2 AWG according to the NEC and section 250.30(A)(1).

Originally Posted by Shawn Willy

according to the NEC and section 250.30(A)(1).

Are you sure?

250.30 Grounding Separately Derived Alternating-Current Systems.

Separately Derived System. 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.

You are correct - a transformer is a separately Derived System.
You need to follow the ruls of article 250.30 for the grounding of transformers.
Equipment Bonding jumpers for transformers are covered in article 250.102(C)



To answer your question with out giving the actual answer:

You size it according to the size of the derived ( secondary) conductors using table 250.66.

Originally Posted by ken horak

You are correct - a transformer is a separately Derived System.
You need to follow the rules of article 250.30 for the grounding of transformers.
Equipment Bonding jumpers for transformers are covered in article 250.102(C)



To answer your question with out giving the actual answer:

You size it according to the size of the derived ( secondary) conductors using table 250.66.

I suppose it won't hurt to explore this. But first I'm real rusty at this stuff and admit to blowing some of the dust off a few books to help my memory......

Second there is no way the question is cut and dry without more clarification..

So I'm going to set up the installation....

After mounting the 75 KVA transformer (dry type) to the building concrete floor I'm going to feed a main breaker panelboard surface mounted on a wall 9 feet away from the transformer. This panelboard supplies lighting loads for the warehouse. All conductors are thhn in emt conduit.

To add:


Not all transformers are a separately derived systems (SDS). The key is if the grounded neutral conductor is carried/connected by both the primary and secondary. If it is it is not separately derived. If it isn't then it is a separately derived system.

A delta primary/wye secondary is an SDS. Grounded conductor is not solidly connected between primary and secondary.

The transformer in Shawns question is an SDS by description. Its 75 KVA. The system bonding jumper is sized according to your secondary derived conductor size using table 250.66.

Amperage of transformer primary is 75,000 VA / (480 x 1.72) = 90A
Amperage of transformer secondary is 75,000VA / (208 x 1.72) = 210 amps
You must protect the primary at 125% of its rating...NEC table 450.3(B) ,,,1.25 x 90 = 113 amps next size up ocpd is 125 amps

The transformer is supplying continuous lighting loads in a warehouse. So my primary conductors are 1.25 x 90 amps = 113 amps ... so primary conductors are # 2 copper thhn 75C rated 115 amps. Deration not calculated.

Now secondary conductors using the 10 foot tap rule do not need ocpd at the termination end but we have a main breaker panelboard so that protection must be sized to protect the secondary conductors and those conductors must not be less than the ocpd in ampacity.

So OCPD in panelboard must be 1.25 x 209 amps = 261 amps next size up 300amps.

This equates to a 350 KCMIL thhn conductor rated 310 amps.
The secondary conductors are in conduit and there are 4 current carrying conductors so we must derate those conductors by 20% so using 90 c ... 350 amps x .80 = 280 amps so the conductor is too small we need to go to a 400 Kcmil conductor which gives us 380 amps x .80 = 304 amp ...conductor is now not less than 300amps.

Now to size the system bonding jumper /main bonding jumper use table 250.66 in accordance with the size of the derived secondary ungrounded conductor.

400 Kcmil = 1/0 copper or 3/0 aluminum for system/main bonding jumper

So my calculation shows that # 2 awg (assuming copper here) is too small for the system bonding jumper for my example

Now it is important to note that this is not necessarily correct but is my opinion as to the correct method to reach the maximum required SBJ for a 75 KVA 480delta/wye 208/120 transformer based on the amperage of the secondary using thhn conductors in conduit. Also this is only one of many possible situations. The tap rule your using and secondary conductor size changes will change the SBJ size for a particular transformer installation.

I also think it would be advantageous to calculate the maximum SBJ and install that in all situations. This would eliminate any changes to the transformer secondary load causing an undersizing of the SBJ to occur.

Just to add the calculation for the SBJ is just one of the calculations to get this installation correct. This isn't childs play and should be taken very seriously. You should be a qualified person with a bunch of experience with transformers before even thinking about installing and connecting this SDS.

So Shawn ... if this isn't a test question and you are actually doing the work........ If your not positive about what you are doing .. do not energize that transformer till you have a qualified electrician check your work and get an inspection.

Roger
You are claimiung to use the 10' tap inyour example - You can not use that as you stated the panel is mounted 9' from the transformer.

When using the tap rules the length of the tap ( 10' 25' ) is the length of the entire conductor from termination point to termination point.

Question: What reference book did you copy that information from?

You are claimiung to use the 10' tap inyour example - You can not use that as you stated the panel is mounted 9' from the transformer.

I'm feeding the panelboard with secondary conductors 9 feet long, I'm actually not tapping those secondary conductors. I may have incorrectly called this the 10 foot tap rule.Which was probably a mistake since the only thing I'm tapping is the secondary of the transformer not a secondary conductor.
What I should have said was I am running secondary conductors that are 9 feet in length not that the panel is 9 feet away from the transformer. Thanks for the correction.

When using the tap rules the length of the tap ( 10' 25' ) is the length of the entire conductor from termination point to termination point.

My understanding of the 10 foot tap rule is ...Taps not over 10 feet in length.

Question: What reference book did you copy that information from?

I applied the information from several sources to the transformer Shawn described. Primarily Ec&M Magazine and Tom Henrys Grounding and Bonding of Transformers...plus a few books by mike holt.

The EC&M source is a file I have. I may be able to find the internet article and provide the link I copied the article from...do you want me to do that..?

As said it has ' been a while ' since I have connected/installed a transformer like above or made the calculations necessary for a transformer installation so it might be very likely that I am incorrect. Just gave it my best shot as it looked like it may not be correct that a # 2 copper was the SBJ for this transformer.

I tried to give an example that I remember as being very common in commercial settings. We usually set a dry type and ran the secondary conductors directly to a main breaker panelboard. In my experience (which is limited in this area) we never feeder tapped the secondary conductors before they reached a panelboard.

I should mention that my example is on a non-inspected vessel so we are using 4/0 TEW in free air (no derating).

I am a qualified electrician (not too much experience with TXs coming off of generators on a vessel) and #2 is sufficient. I called the transport authorities (TC) and they confirmed it.

My situation is unique.

Most times you guys give a great answer but sometimes you have to bicker and b*tch to get there. It's nice to see people correcting each other for the greater good but sometimes I get the feeling some of you may be out to prove yourself. I could be wrong and some just don't have etiquette when typing.

None the less, to those who are out to help me, especially Roger in this case, MUCH thanks!

Originally Posted by Shawn Willy

I should mention that my example is on a non-inspected vessel so we are using 4/0 TEW in free air (no derating).

I am a qualified electrician (not too much experience with TXs coming off of generators on a vessel) and #2 is sufficient. I called the transport authorities (TC) and they confirmed it.

My situation is unique.

Most times you guys give a great answer but sometimes you have to bicker and b*tch to get there. It's nice to see people correcting each other for the greater good but sometimes I get the feeling some of you may be out to prove yourself. I could be wrong and some just don't have etiquette when typing.

None the less, to those who are out to help me, especially Roger in this case, MUCH thanks!

Hi Shawn

I'm glad your an electrician and never meant to imply otherwise just wanting to be sure you understood this ain't something for the unqualified to be fooling with....

Vessel .... that would be an area I know zero about....

I don't think it matters though whether vessel or dry ground as far as the calculations. Other than I do not think you would use table 310.16 for the conductor ampacities as I did ... liklely table 310.17.

Anyway glad this worked out for you.

My intention with the long winded reply earlier was to raise any flags in case the information you received on the other forum might be in error. If you came back to this forum after getting the answer then you would see what I had posted and possibly seeked further advice.....certainly not mine ... because I was not real confident in my calculations. Jumped into the fire anyway in hopes it would lead to the correct answer for you.

Smooth sailing or whatever it is you do on a vessel.....

Roger

Ken

This is the link to the ECM article I have on file ... written in 2002.

Transformer Installation Made EasySort of

Roger-
Thanks for the link. I will also look up one that I have and post it here for others to use. It is basically a transformer "report" written by an inspector and the things he finds wrong in transformers and the proper way to wire them.

You are correct in referring to the secondary conductors as tap conductors. Article 240.21(B)(3) Taps supplying a transformer [primary plus secondary not over 25 feet long.]

Your understanding of the tap rules in reference to the length of the conductors are correct also - 10' tap rule = conductors not over 10' long.

Here is that link I spoke of. It is a two part "report" thats why the 2 links

Electrical Knowledge Repository - Transformer article, part 1
Electrical Knowledge Repository - Transformer article, part 2

Originally Posted by ken horak

Roger-
Thanks for the link. I will also look up one that I have and post it here for others to use. It is basically a transformer "report" written by an inspector and the things he finds wrong in transformers and the proper way to wire them.

You are correct in referring to the secondary conductors as tap conductors. Article 240.21(B)(3) Taps supplying a transformer [primary plus secondary not over 25 feet long.]

Your understanding of the tap rules in reference to the length of the conductors are correct also - 10' tap rule = conductors not over 10' long.

Here is that link I spoke of. It is a two part "report" that's why the 2 links

Electrical Knowledge Repository - Transformer article, part 1
Electrical Knowledge Repository - Transformer article, part 2

Hi Ken

Thank you ...they are now bookmarked and will enjoy going over those articles.

You got me thinking that maybe I was in error on the secondary conductors being tap conductors but that being correct was probably more luck than knowledge. That is ,however, how I understood my sources.

Anyway I know this went way off the normal path for the forum but that happens with me a lot....LOL

Thanks for your interest

Originally Posted by Roger Frazee

Not all transformers are a separately derived systems (SDS). The key is if the grounded neutral conductor is carried/connected by both the primary and secondary. If it is it is not separately derived. If it isn't then it is a separately derived system.

A delta primary/wye secondary is an SDS. Grounded conductor is not solidly connected between primary and secondary.

Yep, that was one of my typing-in-my-sleep-and-not-thinking-it-through moments - sorry about that.

Originally Posted by ken horak

When using the tap rules the length of the tap ( 10' 25' ) is the length of the entire conductor from termination point to termination point.

Originally Posted by Roger Frazee

My understanding of the 10 foot tap rule is ...Taps not over 10 feet in length.

Correct, the tap rule is regarding the actual length of the conductors from terminal to terminal, not the horizontal distance between the enclosures.

Roger and Ken, thank you for those links.

Yep, that was one of my typing-in-my-sleep-and-not-thinking-it-through moments - sorry about that.

Believe me I know the feeling.

All to often I'm up late, very tired, and trying to reply on this forum when I should be sleeping. It has caused more than one over sight ....

Lesson ... when responding to this forum best be alert and focused ....

When I made my long reply I was actually worried I was going to get plucked like a duck but so far still have most my feathers.....

Roger and Ken, thank you for those links

.

And thank you for the many that you have supplied