Neutral for 600amp open delta system

[jrannis]

We have a project with a 600 amp main breaker ATS. Each supply side has parallel 350kcmil including the neutral with 2/0 on the high leg. This is an older installation and I suspect that the original install was a fused main but, this may not be the case.
The load side has the same conductor sizes distributed in a load gutter to four small disconnects and lighting panels.
The commercial facility, about 5,000 sf has 3ph air conditioning equipment, a small kitchen and minimal lighting and normal receptacle loads as in nothing remarkable beyond what you may find in a house.
The historical load on the building is in the 50kw range due to the 3 phase loads.

Question here is this. What would be the minimum size neutral that can be used here?

It had been suggested that someone possibly swapped the high leg and neutral conductors during the original install as the smaller high leg is landed on the 600 amp.

Any thoughts on this theory?

 

[waedwards87]

How can you have an open delta AND a high leg? Open delta is a three phase construction from two single phase transformers. It is common to have a smaller conductor on the high leg (or lighting tap) because your maximum load is significantly lower than the transformer rating. You could be dealing with a SPLIT-PHASE configuration, which is tapping off the single phase split (240/120, specifically). It is a weird combination for sure. If you check out the Wikipedia article for split-phase electric power (I wont externally link on a forum), it discusses conductor sizing on such a system.

Can you confirm how many transformers are feeding this system and how many phases you have? I just really have a hard time believing this is a traditional high leg system.

 

[Smart $]

We have a project with a 600 amp main breaker ATS. Each supply side has parallel 350kcmil including the neutral with 2/0 on the high leg. This is an older installation and I suspect that the original install was a fused main but, this may not be the case.
The load side has the same conductor sizes distributed in a load gutter to four small disconnects and lighting panels.
The commercial facility, about 5,000 sf has 3ph air conditioning equipment, a small kitchen and minimal lighting and normal receptacle loads as in nothing remarkable beyond what you may find in a house.
The historical load on the building is in the 50kw range due to the 3 phase loads.

Question here is this. What would be the minimum size neutral that can be used here?

It had been suggested that someone possibly swapped the high leg and neutral conductors during the original install as the smaller high leg is landed on the 600 amp.

Any thoughts on this theory?

There's a good chance your are correct. 2/0 is the minimum size neutral, maximum calculated unbalanced neutral current permitting. Additionally, protecting a 2/0 conductor with a 600 OCPD is definitely a violation... hmy:

 

[Smart $]

How can you have an open delta AND a high leg? Open delta is a three phase construction from two single phase transformers. It is common to have a smaller conductor on the high leg (or lighting tap) because your maximum load is significantly lower than the transformer rating. You could be dealing with a SPLIT-PHASE configuration, which is tapping off the single phase split (240/120, specifically). It is a weird combination for sure. If you check out the Wikipedia article for split-phase electric power (I wont externally link on a forum), it discusses conductor sizing on such a system.

Can you confirm how many transformers are feeding this system and how many phases you have? I just really have a hard time believing this is a traditional high leg system.

Open delta can supply a 240/120V 3? 4W system. The high leg can even have lesser rated current (smaller stinger transformer rating), but using a smaller conductor requires a lesser-rated ocpd.

 

[jrannis]

How can you have an open delta AND a high leg? Open delta is a three phase construction from two single phase transformers. It is common to have a smaller conductor on the high leg (or lighting tap) because your maximum load is significantly lower than the transformer rating. You could be dealing with a SPLIT-PHASE configuration, which is tapping off the single phase split (240/120, specifically). It is a weird combination for sure. If you check out the Wikipedia article for split-phase electric power (I wont externally link on a forum), it discusses conductor sizing on such a system.

Can you confirm how many transformers are feeding this system and how many phases you have? I just really have a hard time believing this is a traditional high leg system.

I don't think I need to check wiki for anything concerning this. :roll:

Very common system here. Larger transformer is center tapped and the power transformer, usually half the size, develops the high-leg

Utility lands this high leg on C, we move it inside and land it on B for some reason.

I just checked your profile, you are an EE...

 

[Smart $]

I don't think I need to check wiki for anything concerning this. :roll:

Very common system here. Larger transformer is center tapped and the power transformer, usually half the size, develops the high-leg

Utility lands this high leg on C, we move it inside and land it on B for some reason.

Code requires you to (can't remember the section offhand).

 

[jrannis]

Code requires you to (can't remember the section offhand).

Of course but, since it is physically outside of A and B, how did our industry decide this was a good idea? I can imagine the fist fights at that code panel discussion.
Maybe they proposed it only to trade it for something crazy like ground rods.

 

[jrannis]

There's a good chance your are correct. 2/0 is the minimum size neutral, maximum calculated unbalanced neutral current permitting. Additionally, protecting a 2/0 conductor with a 600 OCPD is definitely a violation... hmy:

Thanks Smart,
I am proposing to swap the neutral and high leg conductors and am getting some resistance from someone about a 70% neutral rule. I don't think it applies here but more for something like a range.

 

[Smart $]

Of course but, since it is physically outside of A and B, how did our industry decide this was a good idea? I can imagine the fist fights at that code panel discussion.
Maybe they proposed it only to trade it for something crazy like ground rods.

Can't say for certain, but AFAIK, the high leg has always been B. POCO puts it on C because that's the way 3? meters are designed to work with a high leg system. Additionally, the secondary phases may not even be "abc" with respect to the primary (or grid). They may be ABC/bca or ABC/cab. POCO may rotate the phases to balance the load on their distribution system.

 

[Smart $]

Thanks Smart,
I am proposing to swap the neutral and high leg conductors and am getting some resistance from someone about a 70% neutral rule. I don't think it applies here but more for something like a range.

70% is an allowed reduction beyond maximum calculated unbalanced neutral current, for single phase 3W equipment such as ranges. It is not a minimum reduction rule.

You need to do some verification before swapping. You could swap and still have a non-compliant condition.

 

[waedwards87]

LOL Youre right jrannis. EE. Designed transformers for a while though.. which is why it still sounds odd.

The center tap youre tapping off we would consider a split phase. It acts like that lighting tap when you go to open delta. Not odd in the connection sense, but odd in that youre going to have to de-rate the crap out of that connection. In my nerd glasses world im cringing at the voltage regulation on a system like that.

 

[augie47]

250.24(C) is your reference. If your 350s are in separate conduits your grounded conductor could be as small as a #2. Based on the older installs I see I would think your guess as to the orginal system being fusible is accurate. It was not uncommon back when to see a 400-600 amp 3 pole fusible disconenct with a 100-200 amp high leg fuse

 

[jrannis]

250.24(C) is your reference. If your 350s are in separate conduits your grounded conductor could be as small as a #2. Based on the older installs I see I would think your guess as to the orginal system being fusible is accurate. It was not uncommon back when to see a 400-600 amp 3 pole fusible disconenct with a 100-200 amp high leg fuse

Interesting you should mention the #2. I believe that is what the parallel conductors that come from the POCO are on the high leg.

 

[jrannis]

LOL Youre right jrannis. EE. Designed transformers for a while though.. which is why it still sounds odd.

The center tap youre tapping off we would consider a split phase. It acts like that lighting tap when you go to open delta. Not odd in the connection sense, but odd in that youre going to have to de-rate the crap out of that connection. In my nerd glasses world im cringing at the voltage regulation on a system like that.

The phase to phase 240 is rock solid in this setup and the split phase( I learned a new word today) is only used for single phase loads 120/240.
I was surprised how many areas don't use this type of setup.

 

[waedwards87]

The phase to phase 240 is rock solid in this setup and the split phase( I learned a new word today) is only used for single phase loads 120/240.
I was surprised how many areas don't use this type of setup.

I learned a little bit today too! Ill be talking to our sales engineers tomorrow.. we couldve been crushing that market! Never thought of that configuration but I like it. :thumbsup:

 

[kwired]

How can you have an open delta AND a high leg? Open delta is a three phase construction from two single phase transformers. It is common to have a smaller conductor on the high leg (or lighting tap) because your maximum load is significantly lower than the transformer rating. You could be dealing with a SPLIT-PHASE configuration, which is tapping off the single phase split (240/120, specifically). It is a weird combination for sure. If you check out the Wikipedia article for split-phase electric power (I wont externally link on a forum), it discusses conductor sizing on such a system.

Can you confirm how many transformers are feeding this system and how many phases you have? I just really have a hard time believing this is a traditional high leg system.

LOL Youre right jrannis. EE. Designed transformers for a while though.. which is why it still sounds odd.

The center tap youre tapping off we would consider a split phase. It acts like that lighting tap when you go to open delta. Not odd in the connection sense, but odd in that youre going to have to de-rate the crap out of that connection. In my nerd glasses world im cringing at the voltage regulation on a system like that.

Interesting you should mention the #2. I believe that is what the parallel conductors that come from the POCO are on the high leg.

You said it was 2/0 earlier, you are not supposed to parallel conductors smaller then 1/0 - so that still leaves you with some issues if you swapped the high leg and neutral conductors.

I learned a little bit today too! Ill be talking to our sales engineers tomorrow.. we couldve been crushing that market! Never thought of that configuration but I like it. :thumbsup:

Now that you crawled out from under your rock....

Open delta is all over the place around these parts, both 240 and 480 volt systems. They cost less to build for limited load applications, or for instances like the OP has where there maybe is a heavy 120/240 single phase load but a limited three phase load - then they will use a large transformer for the 120/240 and a small transformer for deriving the high leg that has limited load on it, and it is all built from standard 120/240 single phase transformers that are commonly in stock in the POCO warehouses as well. You do need to pay attention to how load is distributed, just because you have a certain kVA net does not mean all that kVA can be balanced on the system.

 

[jrannis]

You said it was 2/0 earlier, you are not supposed to parallel conductors smaller then 1/0 - so that still leaves you with some issues if you swapped the high leg and neutral conductors.

Now that you crawled out from under your rock....

Open delta is all over the place around these parts, both 240 and 480 volt systems. They cost less to build for limited load applications, or for instances like the OP has where there maybe is a heavy 120/240 single phase load but a limited three phase load - then they will use a large transformer for the 120/240 and a small transformer for deriving the high leg that has limited load on it, and it is all built from standard 120/240 single phase transformers that are commonly in stock in the POCO warehouses as well. You do need to pay attention to how load is distributed, just because you have a certain kVA net does not mean all that kVA can be balanced on the system.

Yes, we are also proposing to change the small entry conductors from the CT can to the line side of the utility breaker. It was just way too much to add on the OP an the decision was made on that undisputed issue.

 

[augie47]

kwired: My error... old age brain fart.... You are correct a parallel ground would have to be 1/0. I zeroed in on a #2 being adequate for a 350 and let the parallel slip my mind. Good thing I wasn't inspecting this one

 

[waedwards87]

You said it was 2/0 earlier, you are not supposed to parallel conductors smaller then 1/0 - so that still leaves you with some issues if you swapped the high leg and neutral conductors.

Now that you crawled out from under your rock....

Open delta is all over the place around these parts, both 240 and 480 volt systems. They cost less to build for limited load applications, or for instances like the OP has where there maybe is a heavy 120/240 single phase load but a limited three phase load - then they will use a large transformer for the 120/240 and a small transformer for deriving the high leg that has limited load on it, and it is all built from standard 120/240 single phase transformers that are commonly in stock in the POCO warehouses as well. You do need to pay attention to how load is distributed, just because you have a certain kVA net does not mean all that kVA can be balanced on the system.

Im VERY familiar with open delta applications and configurations.. just not with the high leg, precisely for the reasons you've stated. Typically we have the inverse problem: heavy three phase load and small single phase load. Of course, for every application there probably is its inverse.

 

[jrannis]

Im VERY familiar with open delta applications and configurations.. just not with the high leg, precisely for the reasons you've stated. Typically we have the inverse problem: heavy three phase load and small single phase load. Of course, for every application there probably is its inverse.

Ok now, let's talk about using a delta breaker as a main.