How do I derive my Neutral when I use Step Up/ Step Down transformers?

[Installer]

Here's a panel with a 2 pole 240 VAC Breaker feeding a 240/600 step up transformer followed by a 600/240 step down transformer. and a second panel.
How do I derive my Neutral at the second panel?
Is my grounding correct?
Is there a text book out there or any reference information where I can find this type of information?
Thank you in advance
Respectfully
Installer

 

[jaggedben]

Consult the transformer instructions to be sure, but it looks like your neutral at the step down secondary can be brought out from either X2 or X3. That is not shown in the drawing. If you don't do this you'll fry the loads.

The NEC gives you two options for bonding and grounding: either do both at the transformer, or do both at the first disconnect. I believe you also need the panel on right to have a main breaker or a disconnect in between it and the transformer.

You're currently showing grounding, but not bonding, at the step down transformer.

There is also the issue of grounding the 600V system. The NEC requires you to ground a conductor or have ground detectors. If you ground a conductor you would do that at the secondary of the step up.

 

[don_resqcapt19]

Connect X2/X3 to a grounding electrode, run the neutral from that point to the panel neutral bar. Install a system bonding jumper at either the transformer or the panel and install a supply side bonding jumper between the panel and the transformer.
Note that since the 600 volt system is an ungrounded system, 250.21(B) will require a ground detector.

 

[LarryFine]

The same way you do for a service.

 

[Installer]

How do I ground a conductor at the secondary of the step up here? Do I tie it to the EGC?

 

[electrofelon]

How do I ground a conductor at the secondary of the step up here? Do I tie it to the EGC?
View attachment 2573848

IF this is outside, you can use 250.30(A)(1) exception #2. So you ground one of the 600V legs and treat it like a service, so its a grounded conductor and your EGC. This saves you having to run a third conductor.

 

[winnie]

You can ground H1 or H4 of the step up.

Note that the GEC cannot be isolated from the EGC.

 

[petersonra]

Do you need to ground one of them?

I think g and n need to be bonded at the remote pb.

 

[electrofelon]

Just have to say, seems doubtful transformers are worth it here. I get about 3k for 1/0 Al wire that would do the run at 240. Even using the exception so only needing 2 wires, I get #4 AL at 600V, which is about $1000. Sure conduit will be a little larger and more expensive, I get $1400 more for conduit. The best quick and dirty price I found on these transformers is about $2300. Remember, losses forever, the gift eternal

 

[winnie]

Definitely worth doing the calculation to compare using fat wire vs two transformers.

I don't see how 1/0 AWG compares to 4 AWG 600V vs 240V; I would expect 250 kcmil for similar voltage drop performance. Additionally the system being supplied is a very small 120/240V system, I'd expect a reasonable chance of 120V loading.

Jonathan

 

[electrofelon]

I don't see how 1/0 AWG compares to 4 AWG 600V vs 240V; I would expect 250 kcmil for similar voltage drop performance. Additionally the system being supplied is a very small 120/240V system, I'd expect a reasonable chance of 120V loading.

Jonathan

To do a fair analysis, you allow extra voltage drop for the 240 version because the 600 volt version will have voltage drop in the transformers. IIRC I used 6% for the 240 and 3% for the 600. Thats why everyone jumps to the using transformers because they ignore their voltage drop and it makes it look like the best deal since sliced bread . I also used a full 5 KVA for the load which is likely high.

 

[petersonra]

Just have to say, seems doubtful transformers are worth it here. I get about 3k for 1/0 Al wire that would do the run at 240. Even using the exception so only needing 2 wires, I get #4 AL at 600V, which is about $1000. Sure conduit will be a little larger and more expensive, I get $1400 more for conduit. The best quick and dirty price I found on these transformers is about $2300. Remember, losses forever, the gift eternal

I didn't look at 600 V xfmrs but the 480/240-120 xfmrs for 5 KVA are maybe $600 each.

#4 AL is about 60 cents a foot these days. 1800 foot run X2 is about $2000. I don't know where you are getting #4 AL wire for 30 cents a foot.

 

[electrofelon]

I didn't look at 600 V xfmrs but the 480/240-120 xfmrs for 5 KVA are maybe $600 each.

#4 AL is about 60 cents a foot these days. 1800 foot run X2 is about $2000. I don't know where you are getting #4 AL wire for 30 cents a foot.

IT was all quick and dirty, I am open to corrections . Yeah I googled the part number for the transformer, but you can go with another brand and they are much cheaper, I See one for $685.

Nassua national shows #4 XHHW for .31/foot. Will be cheaper if you ask them for a quote.

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[Strathead]

Here's a panel with a 2 pole 240 VAC Breaker feeding a 240/600 step up transformer followed by a 600/240 step down transformer. and a second panel.
How do I derive my Neutral at the second panel?
Is my grounding correct?
Is there a text book out there or any reference information where I can find this type of information?
Thank you in advance
Respectfully
Installer

View attachment 2573845

Let's start with the proper terminology. Your second transformer is a 600V to 120/240V transformer, otherwise you don't need to generate a neutral. Second, Your grounding isn't correct. The system bonding jumper (250.102) is required to be installed in the transformer. Then you will run a system bonding jumper (250.102) in the conduit with the transformer secondaries and land it on the ground bus in the panel. The neutral will run to the neutral in the panel. The neutral and ground will not be bonded in the panel.

 

[don_resqcapt19]

Let's start with the proper terminology. Your second transformer is a 600V to 120/240V transformer, otherwise you don't need to generate a neutral. Second, Your grounding isn't correct. The system bonding jumper (250.102) is required to be installed in the transformer. Then you will run a system bonding jumper (250.102) in the conduit with the transformer secondaries and land it on the ground bus in the panel. The neutral will run to the neutral in the panel. The neutral and ground will not be bonded in the panel.

There is no requirement that the system bonding jumper be in the transformer. It is permitted to be in the transformer or at the first OCPD enclosure. In either case a supply side bonding jumper, not a system bonding jumper, must be run between the transformer and the panel.

 

[Elect117]

There is no requirement that the system bonding jumper be in the transformer. It is permitted to be in the transformer or at the first OCPD enclosure. In either case a supply side bonding jumper, not a system bonding jumper, must be run between the transformer and the panel.

I thought the terminology was system bonding jumper if you bond in the panel and supply side bonding jumper if you bond at the transformer?

 

[don_resqcapt19]

I thought the terminology was system bonding jumper if you bond in the panel and supply side bonding jumper if you bond at the transformer?

The system bonding jumper is conductor that connects the neutral and system bonding jumper and or EGCs.
The supply side bonding jumper is like an EGC but it bonds things on the line side of the first OCPD, and is sized per Table 250.102(C)(1).

Bonding Jumper, System. (System Bonding Jumper)
The connection between the grounded circuit conductor and the supply-side bonding jumper, or the equipment grounding conductor, or both, at a separately derived system. (CMP-5)

Bonding Jumper, Supply-Side. (Supply-Side Bonding Jumper)
A conductor installed on the supply side of a service or within a service equipment enclosure(s), or for a separately derived system, that ensures the required electrical conductivity between metal parts required to be electrically connected. (CMP-5)

 

[electrofelon]

One issue I see is I believe we need primary protection for the second transformer. Although electrical theory says it can be protected by the first transformer primary OCPD, I dont see an allowance in the code for that. May want to put in a fused disconnect.

 

[petersonra]

One issue I see is I believe we need primary protection for the second transformer. Although electrical theory says it can be protected by the first transformer primary OCPD, I dont see an allowance in the code for that. May want to put in a fused disconnect.

I wondered about that. I was going to look it up but forgot. Good catch.

 

[Installer]

I forgot to mention that we are using an existing 20 year old buried conduit that was a straight 120 Run with H1, N and G.
We are upgrading the exterior system and increasing the loads which is why we are using the transformers.
So we have three conductors which we hope to repurpose as H1, H2 and G with 240/600 to 600/240 transformers.
There is a one grounded conductor present coming from the Feed Panel. (Grounded Conductor 1)
There is a second grounded conductor created by tying X3 and X2 together , connecting it to the Neutral of the second panel and an exterior ground rod. (Grounded Conductor 2)
What do I do with Grounded Conductor # 1?
Since this is a Delta Delta transformer, is it a correct statement to say that there are no ground leads to tie Grounded Conductor # 1 to?