TRANSFORMER SECONDARY CONDUCTORS

[ohmhead]

Question the secondary conductors that feed a panel that has a main breaker installed what table is used or what article would be correct to size the grounding conductor ?
Lets give you a example of a 45 kva at 175 amp single run Alum feeder with 4/0 alum 3 phase 208 volts no voltage drop distance is 10 ft would it be a number 4#alu or 2#alu ?
What table would be used I understand that the branch breaker ground table in article 250 doesn't apply to this is that correct when you have unprotected feeders coming off a secondary transformer .

 

[hurk27]

I believe 450.6(C) direct us to 250.30 for separately derived systems.

250.30(A)(2) requires us to use 250.102(C) which directs us to use table 250.66 based upon the derived phase conductors, so with the fact you have 4/0 alu, table 250.66 tells us #4 copper or #2 alu.

I think I got this right?

 

[ohmhead]

I believe 450.6(C) direct us to 250.30 for separately derived systems.

250.30(A)(2) requires us to use 250.102(C) which directs us to use table 250.66 based upon the derived phase conductors, so with the fact you have 4/0 alu, table 250.66 tells us #4 copper or #2 alu.

I think I got this right?

Ok thanks so a number 2# alu would be the grounding conductor run with the secondary conductors to the load . Just interested in why they used that table and not the branch table 250.122 because table 250 .102(c) (1) it clearly states grounded conductor and not grounding conductor in addition iam also using multi paralleled on some transformers so I see taking the total conductors adding them up and using 12.5 % to calculate the grounding conductor in that raceway or is that the bonding jumper only . This is one code rule that to me is confusing to calculate by jumping around with many notes or articles .

 

[hurk27]

Ok thanks so a number 2# alu would be the grounding conductor run with the secondary conductors to the load . Just interested in why they used that table and not the branch table 250.122 because table 250 .102(c) (1) it clearly states grounded conductor and not grounding conductor in addition iam also using multi paralleled on some transformers so I see taking the total conductors adding them up and using 12.5 % to calculate the grounding conductor in that raceway or is that the bonding jumper only . This is one code rule that to me is confusing to calculate by jumping around with many notes or articles .

Disregard the reference to 450.6 as it was the wrong starting point

I was using the 2008 but the only thing that changed in the 2011 is the names they are calling the bonding jumpers, it seems they are now calling the bonding jumper that if you make the system bonding at the first disconnect, the the conductor that runs back to the transformer to bond the transformer case to the neutral or ground bar in the first disconnect is now called a supplied side bonding jumper, 250.30(A)(2) still points you to 250.102(C)(2) which still points you to table 250.66, but just the names have changed, you will be running your ungrounded phase conductors, and neutral, to the first disconnect in this you install your system bonding jumper between the neutral bar and the grounding bar which could be a screw and land your GEC's in this panel and you size the supply side bonding jumper that runs back to the transformer to bond its case, but the neutral is kept isolated from the case because the bond is made at the first disconnect, just like the main bonding jumper in a service, or you make the system bond in the transformer and run a supply side bonding jumper to the first disconnect and then you have to keep the neutrals and grounding separate , which if the first disconnect is a panel I would do the system bonding at it so the grounds and neutral don't have to be kept separate, and the GEC can land there, if you do it at the transformer then the GEC's have to land in the transformer.

But the sizing of both the system bonding jumper and the supplied side bonding jumper both have to be sized off table 250.66, so yes you need a #4 copper or a #2 aluminum run with your run to the first disconnect, some determine where to do the bonding from which is closest to the point of the grounding electrode, but I try to avoid doing it in the transformer if I have a multi-circuit panel for my first disconnect, the other reason is most panels come with a bonding screw that makes it easier to make the bond, but if you use this then you are limited to only landing the GEC on the neutral bar only because of 250.24.

Also the supply side bonding jumper can be a non-flexible metal raceway as stated in 250.30(A)(2) so if you are running a non-flexible metal raceway you don't need to run a supply side bonding jumper between the transformer and disconnect but you will be required to use bonding bushings or locknut's on each end of the raceway, could save some money.

Also the system bonding jumper requirements is in 250.30(A)(1) but also refers to 250.28 for sizing it but basically its the same as the other one.

Also 250.30(A)(1) exception 2 allows you to bond at both the transformer and the disconnect/panel if you use a non-conductive raceway such as PVC, this is just like a service.

Hope this clears it up a little, I don't know why I use the 450.6 reference as that it for a whole different install?

 

[infinity]

You're asking about the supply side bonding jumper. {250.30(A)(2)} It's hard to find an answer in the NEC when using generic terminology because if you search for grounding conductor you won't find it. A 45 kva transformer will typically have an equipment grounding conductor (EGC), grounding electrode conductor (GEC), supply side bonding jumper (SSBJ) and a system bonding jumper (SBJ).

As Hurk mentioned you would use table 250.66 based on the secondary or derived conductor size for the SSBJ. For this application that table would also be used to size the GEC and SBJ.

 

[augie47]

A simple question to ask yourself is "what overcurrent device will operate when the "ground" I am installing comes into play". If there is not one supplying the phase conductors directly, then Table 250.66 comes into play.
If one of the conductors between your transformer and your panel happens to fault to ground, then panel main will not see that fault so that OCP device is not relative.
With such a fault, there is no overcurrent device immediately ahead of the conductors so it will depend on the transformer primary OCP to open often requiring a greater current.

 

[ohmhead]

Thanks for the information I understand what you are pointing out . There is no protection on that feeder from transformer secondary to panel in my case MCP in panel . I have at least 38 transformers from 500 KVA down to 30 KVA on this project some single runs some multi paralleled runs on secondaries . Some secondaries have two separate runs which feed two separate loads . So I guess each feeder grounding & bonding must be calculated separate to each supplied load on each separate feeder leaving transformer .

On each transformer in house we use a bonding jumper inside transformer from ground case to neutral continuous this jumper is tied to the ground room bus plate mounted in each electrical room there all tied together at Grd bus plate on project and all go back to the main electrical room ground bar which goes to cold water build steel ground rods and uffer grounds so each panel or switchboard has no bonding jumper its done in the transformer one time .

We are using table 250.102(c) (1) for bonding jumper and grounding conductor that they call a bonding jumper with conductors on each secondary we are using the 12.5 % of total on the paralleled secondarys .

I feel I have a better understanding to that part of the NEC after reading your posts thanks again to each response made .

 

[texie]

Thanks for the information I understand what you are pointing out . There is no protection on that feeder from transformer secondary to panel in my case MCP in panel . I have at least 38 transformers from 500 KVA down to 30 KVA on this project some single runs some multi paralleled runs on secondaries . Some secondaries have two separate runs which feed two separate loads . So I guess each feeder grounding & bonding must be calculated separate to each supplied load on each separate feeder leaving transformer .

On each transformer in house we use a bonding jumper inside transformer from ground case to neutral continuous this jumper is tied to the ground room bus plate mounted in each electrical room there all tied together at Grd bus plate on project and all go back to the main electrical room ground bar which goes to cold water build steel ground rods and uffer grounds so each panel or switchboard has no bonding jumper its done in the transformer one time .

We are using table 250.102(c) (1) for bonding jumper and grounding conductor that they call a bonding jumper with conductors on each secondary we are using the 12.5 % of total on the paralleled secondarys .

I feel I have a better understanding to that part of the NEC after reading your posts thanks again to each response made .

I think I'm following your installation but want to clarify.
Table 250.102(C) indicates you are using the 2104 NEC. I thought FL was on the 2008 although for this calculation it would be the same, just found in a different area of Art 250. Keep in mind that 250.102(C) only applies to the neutral/ground bond not the GEC. For the GEC (Table 250.66) it would be the same except it maxes out at 3/0.
Also, these secondaries are not feeders until after the first disconnect, rather a transformer tap and have to meet the 10 foot or 25 transformer tap rules if inside and the ones that have multiple taps have to meet the rules.

 

[ohmhead]

I think I'm following your installation but want to clarify.
Table 250.102(C) indicates you are using the 2104 NEC. I thought FL was on the 2008 although for this calculation it would be the same, just found in a different area of Art 250. Keep in mind that 250.102(C) only applies to the neutral/ground bond not the GEC. For the GEC (Table 250.66) it would be the same except it maxes out at 3/0.
Also, these secondaries are not feeders until after the first disconnect, rather a transformer tap and have to meet the 10 foot or 25 transformer tap rules if inside and the ones that have multiple taps have to meet the rules.[/QUO

Yes its the 2014 code book its ok we understand were not using that for another 5 years . Each neutral to ground yes it can be 3/0 max . If you had 6 runs of 600 mcm with double 200 % neutrals at 500KVA at 2000 amps would it be a larger bonding jumper or would that just pertain to the grounding conductor . Just to clarify yes all our transformers are within 25 foot tap rule .

 

[infinity]

Each neutral to ground yes it can be 3/0 max . If you had 6 runs of 600 mcm with double 200 % neutrals at 500KVA at 2000 amps would it be a larger bonding jumper or would that just pertain to the grounding conductor . Just to clarify yes all our transformers are within 25 foot tap rule .

Not sure what the bold means?

For the 6 set of 600 kcmil the system bonding jumper has to be a minimum of 12.5% of the derived ungrounded conductors.

 

[ohmhead]

Not sure what the bold means?

For the 6 set of 600 kcmil the system bonding jumper has to be a minimum of 12.5% of the derived ungrounded conductors.

OK so the system bonding jumper is 12.5 % this is the conductor or strap which bonds the neutral to the ground .
Now what is the size or the grounding conductor for that 6 runs of 600 mcm alu that's my question table 250.102 (c) (1) if go to that table alu I see 500 to 900 aluminum 3/0 so what iam asking is that the size of my grounding conductor running with my phase & neutral wires . This is not a service must have a grounding conductor with phases its feeding a indoor switchboard .

Or is the grounding conductor to be 250.122 breaker size if we had a 1600 amp main on the switchboard it feeds

 

[Joe Villani]

If you look at the last part of 250.102(C) you will see that your "ground wire" or system bonding jumper in each conduit will be sized based on the derived phase conductors in each conduit.

Does your job spec copper ground conductors?

If so another step is involved.

You have to find the equivalent copper conductor to, in this case 6 runs of 600 kcmil aluminum.

Based on the number of runs I am assuming these conductors terminate at a 2000 amp overcurrent device?

Equivalent in copper conductors on a 2000 amp ocpd would be 6 runs of 400 kcmil copper in each raceway.

Based on 250.66 I came up with a 1/0 Copper conductor or 3/0 aluminum conductor run in each raceway.

 

[infinity]

OK so the system bonding jumper is 12.5 % this is the conductor or strap which bonds the neutral to the ground .
Now what is the size or the grounding conductor for that 6 runs of 600 mcm alu that's my question table 250.102 (c) (1) if go to that table alu I see 500 to 900 aluminum 3/0 so what iam asking is that the size of my grounding conductor running with my phase & neutral wires . This is not a service must have a grounding conductor with phases its feeding a indoor switchboard .

Or is the grounding conductor to be 250.122 breaker size if we had a 1600 amp main on the switchboard it feeds

Yes, the system bonding jumper (SBJ) is based on 12.5% of the ungrounded conductors.
The supply side bonding jumpers (SSBJ's) (the "grounds" run with each set of the secondary conductors) are based on the size of the ungrounded conductors in each parallel raceway using T250.66. [This is for the 2011 NEC.]

6*600 kcmil = 3600 kcmil * 12.5% = 450 kcmil Al SBJ
600 kcmil ungrounded conductor in each raceway w/ Table 250.66=#3/0 Al SSBJ in each parallel raceway as Joe mentioned in the previous post.

 

[ohmhead]

Yes, the system bonding jumper (SBJ) is based on 12.5% of the ungrounded conductors.
The supply side bonding jumpers (SSBJ's) (the "grounds" run with each set of the secondary conductors) are based on the size of the ungrounded conductors in each parallel raceway using T250.66. [This is for the 2011 NEC.]

6*600 kcmil = 3600 kcmil * 12.5% = 450 kcmil Al SBJ
600 kcmil ungrounded conductor in each raceway w/ Table 250.66=#3/0 Al SSBJ in each parallel raceway as Joe mentioned in the previous post.

Iam trying to get this in my head clearly iam not understanding 6 runs of 600 mcm from a secondary of a transformer which feeds a switchboard this is not a service so I will run my phase & neutral and a grounding conductor to a main breaker in switchboard in 6 raceways .

I understand the 12.5 % of total paralleled conductors and I see the 450 mcm now this is a jumper from case of transformer to neutral bus yes / no .


Next question to clearly understand this your saying that when I run my phase & neutrals in my 6 runs to switchboard I use a 3/0 as the grounding conductor table 250.66 ?



And that I will not use table 250 .122 which is the equipment ground conductor raceway ground .

Just to make this clear I have a transformer and a switchboard with a main breaker 1600 amps and iam going to use a 3/0 grounding conductor in my raceway as the grounding conductor .

 

[infinity]

Iam trying to get this in my head clearly iam not understanding 6 runs of 600 mcm from a secondary of a transformer which feeds a switchboard this is not a service so I will run my phase & neutral and a {SUPPLY SIDE BONDING JUMPERS}grounding conductor to a main breaker in switchboard in 6 raceways . {Yes}

I understand the 12.5 % of total paralleled conductors and I see the 450 mcm now this is a {SYSTEM BONDING JUMPER}jumper from case of transformer to neutral bus yes / no . {Yes}


Next question to clearly understand this your saying that when I run my phase & neutrals in my 6 runs to switchboard I use a 3/0 as the {SUPPLY SIDE BONDING JUMPERS}grounding conductor table 250.66 ? {Yes}



And that I will not use table 250 .122 which is the equipment ground conductor raceway ground .

Just to make this clear I have a transformer and a switchboard with a main breaker 1600 amps and iam going to use a 3/0 grounding conductor in my raceway as the {SUPPLY SIDE BONDING JUMPERS}grounding conductor . {The 1600 amps is irrelevant because the derived conductors are ahead of the OCPD}

I added some comments directly to your post. As you can see the terminology is somewhat important to figuring out what is what.

 

[ohmhead]

I added some comments directly to your post. As you can see the terminology is somewhat important to figuring out what is what.

Thanks this is what our engineer has typically the one line he has shows a 500 kva transformer 480- 208/120 3ph the secondary is 2000NNG which is a feeder code with 6 sets of 600 mcm ABCNN 200% neutrals that raceway grounding conductor is a 500 mcm this is all aluminum wire . He shows a 3/0 which bonds from the room ground bar which is located in every electrical room and we run a 3/0 copper from that copper room ground bar bare to transformer case non spliced and it attaches to the neutral bus in transformer non spliced .
We also install a aluminum jumper from metal case of transformer to neutral which we size at 12.5 % of the phase conductors on the secondary side.
In the smaller transformers we just run the neutral non spliced from neutral directly back to metal case so its a full size jumper.

So when I read your post I see the total opposite your stating 3/0 in raceway ?
and in my case a 500 bonding jumper from case to neutral is that correct ?

 

[infinity]

Thanks this is what our engineer has typically the one line he has shows a 500 kva transformer 480- 208/120 3ph the secondary is 2000NNG which is a feeder code with 6 sets of 600 mcm ABCNN 200% neutrals that raceway {SUPPLY SIDE BONDING JUMPER}grounding conductor is a 500 mcm this is all aluminum wire {That's OK but only a #3/0 Al is required based on the ungrounded conductors in the raceway.}

He shows a 3/0 which bonds from the room ground bar which is located in every electrical room and we run a 3/0 copper from that copper room ground bar bare to transformer case non spliced and it attaches to the neutral bus in transformer non spliced .{This is something different, it's the GEC which is from T250.66 based on the total size of the parallel derived conductors and not required be larger than #3/0}.


We also install a aluminum {SYSTEM BONDING JUMPER}jumper from metal case of transformer to neutral which we size at 12.5 % of the phase conductors on the secondary side. {Yes}

In the smaller transformers we just run the neutral non spliced from neutral directly back to metal case so its a full size jumper. {Yes that's fine because your GEC and SBJ would be required to be the same size based on the secondary conductors and T250.66.}

Hope this helps.

 

[don_resqcapt19]

...
In the smaller transformers we just run the neutral non spliced from neutral directly back to metal case so its a full size jumper. ...

You need both a neutral and a supply side bonding jumper between the transformer and the first OCPD.

 

[texie]

Not sure what the bold means?

For the 6 set of 600 kcmil the system bonding jumper has to be a minimum of 12.5% of the derived ungrounded conductors.

Not sure how that came up as my quote. That is the opposite of what I said in my post #8. The OP responded in #9 and stated the opposite of what I said.
In any event, the point I was trying to make was, as you said, the bonding jumpers must be 12.5% of the ungrounded but the GEC max would be 3/0.
It seems that the OP is confused about bonding jumper sizes VS GEC sizes. Apparently he is not alone a this seems to be one of the reasons that the requirements were rearranged in the 2014 NEC to include new Table 102(C). The end result is the same, just presented differently.

 

[infinity]

Not sure how that came up as my quote. That is the opposite of what I said in my post #8. The OP responded in #9 and stated the opposite of what I said.
In any event, the point I was trying to make was, as you said, the bonding jumpers must be 12.5% of the ungrounded but the GEC max would be 3/0.
It seems that the OP is confused about bonding jumper sizes VS GEC sizes. Apparently he is not alone a this seems to be one of the reasons that the requirements were rearranged in the 2014 NEC to include new Table 102(C). The end result is the same, just presented differently.

This can be very confusing and using incorrect terminology only makes it more so. Not to pick on Ohmhead but I used the capital red lettering in my responses to improve the clarity of his questions. Using terms like bond or ground or grounding conductor is confusing and makes it harder to find the answer in the NEC.