480v wye service neutral size

[S9arky]

Hi, I am building a 800 amp 480/277 volt wye service. All breakers in this service will be feeding step down transformers to 208/120 volt. My question is what size conductor do I need to run from the utility ct can to my panelboard? Since the transformers have no neutral connection on the line side, I don't have a number to base the neutral load on.

Thanks!

 

[infinity]

Take a look at 250.24(C).

 

[dionysius]

Hi, I am building a 800 amp 480/277 volt wye service. All breakers in this service will be feeding step down transformers to 208/120 volt. My question is what size conductor do I need to run from the utility ct can to my panelboard? Since the transformers have no neutral connection on the line side, I don't have a number to base the neutral load on.

Thanks!

You might supply more info. Where is the 800A 480/277 being sourced from. Since I assume it is 3 phase w neutral you need 800A per phase so run 2 500 kcmil in parallel for each phase in 4" PVC conduit. A little overkill but ok. Same for neutral.

For the step down trafos here is some info. This is what to use to get a 240/120 from the 480/277
This explains all voltage derivations with hi leg etc.

https://www.youtube.com/watch?v=dlscWJRm38M

I assume you may need 240 but if not the info will still help. The derived neutral will be very low current if you do a good job with balancing. If you want super big margin make derived neut same as phase. You will connect it to ufer(concrete encased rebar) ground if you have that.

This is Mike Holt explaining so well the derived neutral in Delta to Y which describes a 400A service dry trafo

https://www.youtube.com/watch?v=a8LjxEtd8N8


Let me know more about how it is going since I have a similar job and I want to compare notes with you......I have to run many many 230V indoor units (HVAC) from 277/480 like you. The outdoor units all are 480 3 phase.

 

[dionysius]

You might supply more info. Where is the 800A 480/277 being sourced from. Since I assume it is 3 phase w neutral you need 800A per phase so run 2 500 kcmil in parallel for each phase in 4" PVC conduit. A little overkill but ok. Same for neutral.

For the step down trafos here is some info. This is what to use to get a 240/120 from the 480/277
This explains all voltage derivations with hi leg etc.

https://www.youtube.com/watch?v=dlscWJRm38M

I assume you may need 240 but if not the info will still help. The derived neutral will be very low current if you do a good job with balancing. If you want super big margin make derived neut same as phase. You will connect it to ufer(concrete encased rebar) ground if you have that.

This is Mike Holt explaining so well the derived neutral in Delta to Y which describes a 400A service dry trafo

https://www.youtube.com/watch?v=a8LjxEtd8N8


Let me know more about how it is going since I have a similar job and I want to compare notes with you......I have to run many many 230V indoor units (HVAC) from 277/480 like you. The outdoor units all are 480 3 phase.

If you know all of the following please bear with me so others can read also. Add your own notes/corrections to what I have. For step down trafo use 3 phase 2:1 and use "hi leg" to get multiple voltages: 240;208;120. Supply breakers for trafo need to be ganged mechanically. Panel needs to be marked. A ground is derived by connecting the B node to uffer (concrete encased rebar) ground if you have one. Case is also connected to this ground. The center tap of C phase winding is conn to grd.

Do a good job of balancing the phases. I would stay away from single phase trafos unless someone can justify with reason.

If I get time I will go into sizing so let me know the loads you anticipate.

As I read your post it is a delta-delta hi leg that you may select. If however you want all 120 volt and very little or no 240/208 then state that.

I have to solve this problem myself so any info will be welcome.

 

[iwire]

Since I assume it is 3 phase w neutral you need 800A per phase so run 2 500 kcmil in parallel for each phase in 4" PVC conduit. A little overkill but ok. Same for neutral.

Why do you say that a pair of 500 kcmil is overkill. Even if we assume you mean copper in separate raceways the result is not an 800 amp circuit it would only have an allowable ampacity of 760 amps.

240.4(B) would allow the next size up breaker but the loading can only be 760 amps.

For the step down trafos here is some info. This is what to use to get a 240/120 from the 480/277
This explains all voltage derivations with hi leg etc.

https://www.youtube.com/watch?v=dlscWJRm38M

I assume you may need 240 but if not the info will still help. The derived neutral will be very low current if you do a good job with balancing. If you want super big margin make derived neut same as phase. You will connect it to ufer(concrete encased rebar) ground if you have that.

Why are you assuming the OP wants 240 with a high leg when they stated they are going 208Y/120?

As I read your post it is a delta-delta hi leg that you may select. If however you want all 120 volt and very little or no 240/208 then state that.

Maybe you should reread the OPs post.

 

[dionysius]

Points well taken, Iwire. I responded about 20 mins after I read the post.....mean maxima culpa

go with 600 Kcmil not 500 and use 1/0 EGC

do not use hi leg, I crossed my lines on this one. Do go 3 phase 480 to 208 with centre taps if you can locate a good one. Let me know what you pick.

 

[Cow]

Take a look at 250.24(C).

:thumbsup:

We do quite a few 480v farm services with no neutral load, so it's very common for us to size the neutral to the minimum.

 

[dkidd]

Points well taken, Iwire. I responded about 20 mins after I read the post.....mean maxima culpa

go with 600 Kcmil not 500 and use 1/0 EGC

do not use hi leg, I crossed my lines on this one. Do go 3 phase 480 to 208 with centre taps if you can locate a good one. Let me know what you pick.

The transformer secondary voltages have nothing to do with the OP question of service neutral size.

 

[dionysius]

:thumbsup:

We do quite a few 480v farm services with no neutral load, so it's very common for us to size the neutral to the minimum.

Can you say what trafos you use to derive 208 and 120 most efficiently?.....

 

[dionysius]

The transformer secondary voltages have nothing to do with the OP question of service neutral size.

I do not believe I said they did. It is the balancing of the phases that keep neutral to low current. I suggested he use 1/0 for neutral and tie to uffer having done a good balancing of loads between phases. As you know a perfect balance means 0 neutral current. I am just repeating myself....sorry

 

[dkidd]

I do not believe I said they did. It is the balancing of the phases that keep neutral to low current. I suggested he use 1/0 for neutral and tie to uffer having done a good balancing of loads between phases. As you know a perfect balance means 0 neutral current. I am just repeating myself....sorry

No amount of imbalance will cause current in the service neutral in the installation as described.

 

[kwired]

I do not believe I said they did. It is the balancing of the phases that keep neutral to low current. I suggested he use 1/0 for neutral and tie to uffer having done a good balancing of loads between phases. As you know a perfect balance means 0 neutral current. I am just repeating myself....sorry

There are no connections to neutral so how does it get any unbalanced current?

If phase conductors are not balanced (with no neutral loads) then there will be unbalance at the neutral point inside the transformer, but we are not selecting a conductor for that portion of the circuit.

 

[infinity]

:thumbsup:

We do quite a few 480v farm services with no neutral load, so it's very common for us to size the neutral to the minimum.

I think that was what the OP was asking.

 

[roger]

use "hi leg" to get multiple voltages: 240;208;120.

The 208 voltage measured in a center grounded Delta configuration is not intended to be used for anything, it's simply a bi product of the system. It is not the same as the 208 volts of a Wye.

A ground is derived by connecting the B node to uffer (concrete encased rebar) ground if you have one. Case is also connected to this ground.

Only if you're wanting a corner grounded Delta and in that case you would not measure 120 or 208 volts anywhere in the system. As an aside, it can connect to any or all grounding electrodes

The center tap of C phase winding is conn to grd.

Not if you already have a corner grounded delta.

I would stay away from single phase trafos unless someone can justify with reason.

They can eliminate problems associated with additive harmonics although in reality that's not much of a concern


Roger

 

[don_resqcapt19]

Can you say what trafos you use to derive 208 and 120 most efficiently?.....

It would most often be a 480-208Y/120 transformer, would typically be a delta/wye.

 

[don_resqcapt19]

You might supply more info. Where is the 800A 480/277 being sourced from. Since I assume it is 3 phase w neutral you need 800A per phase so run 2 500 kcmil in parallel for each phase in 4" PVC conduit. A little overkill but ok. Same for neutral.
....

The OP said it was for the service, and he said that he had no line to neutral loads, so why suggest a neutral that is the same size as the phase conductors?

 

[S9arky]

My ungrounded conductor size is parallel 600mcm cu for each phase. I submitted my line drawing to the state chief electrical inspector with minimum sized neutral conductor at one 3/0 cu for each phase, so three parallel 3/0 cu. This is for service feeders only.

 

[infinity]

My ungrounded conductor size is parallel 600mcm cu for each phase. I submitted my line drawing to the state chief electrical inspector with minimum sized neutral conductor at one 3/0 cu for each phase, so three parallel 3/0 cu. This is for service feeders only.

As long as your neutral is at least 12.5% of the phase conductor size you're good.

 

[winnie]

My ungrounded conductor size is parallel 600mcm cu for each phase. I submitted my line drawing to the state chief electrical inspector with minimum sized neutral conductor at one 3/0 cu for each phase, so three parallel 3/0 cu. This is for service feeders only.

This description is a bit confusing.

Do you have 3 parallel 600mcm for each phase, or do you think that because you have 3 phases you need three separate neutral conductors?

-Jon

 

[iwire]

This description is a bit confusing.

Do you have 3 parallel 600mcm for each phase, or do you think that because you have 3 phases you need three separate neutral conductors?

-Jon

The OP states it is an 800 amp service, the chance of that being done with one set of conductors is pretty low.