480V 3 Phase 3 Wire Delta Service

Status
Not open for further replies.
K

KMeni

Member
There is a 1200A 480V 3 Phase 3 Wire Delta Main Service with a Distribution section.
Are there any NEC Code Section preventing you to feed a 400 Amp 480Y/277V 3 Phase 4 Wire Panelboard off of the 1200A 480V 3 Phase 3 Wire Delta Main Service?
Could you treat the 400A 480Y/277V 3 Phase 4 Wire as a new service new service and drive ground rod at that location??
 
Before you try to answer the code question, I'd suggest that you answer the _physics_ question: how will you get your neutral? If you have 480V 3 Phase 3 Wire Delta, then you don't have a neutral, and thus don't have 480/277V.

There are a number of different ways to get a neutral, and _how_ you do this will change the answers to the code questions.

-Jon
 
winnie said:
Before you try to answer the code question, I'd suggest that you answer the _physics_ question: how will you get your neutral? If you have 480V 3 Phase 3 Wire Delta, then you don't have a neutral, and thus don't have 480/277V.

There are a number of different ways to get a neutral, and _how_ you do this will change the answers to the code questions.

-Jon
Click to expand...

That's basically the question? There is no nuetral. The installation as described was discovered at a Pre Bid Job Walk. The installater, again has driven a ground rod at the 400A 480Y/277V 3 Phase 4 Wire Panelboard and it appears created at Bonding Jumper from the Ground Bus to the Neutral Bus??
Is that a Code Violation if so, reference code section!
 
In a wye configuration, three single-phase transformers are connected to a common point (neutral) via a lead from their secondaries. The other lead from each of the single-phase transformers is connected to the line conductors. This configuration is called a “wye,” because in an electrical drawing it looks like the letter Y. Unlike the delta transformer, it doesn't have a high leg.

Differences in wye and delta transformers....
Click to expand...
Understanding The Basics of Wye Transformer Calculations

The most basic of code references would be the differences between a groundED conductor and a groundING conductor. [Article 100]

We can continue on with 110.3(b)....
..and continue on to 250.4 ...

..and continue on.
 
Can you confirm what I believe you are saying:

The installer simply 'created' a neutral bus, with no connection at all back to the supply transformer, and then they bonded this 'neutral bus' to the ground bus? In other words, it is as if they were using the ground rod to 'create' the system neutral? This is an 'open neutral', and is generally considered a fault to be prevented, not a design to be installed.

I don't have a code book handy, but my _hunch_ is that you won't find a direct prohibition against such an installation, but rather lots of details that such a system inherently violates. For example, it would end up being an impedance grounded system (where the 277V loads acted as the grounding impedance), but would be serving line-neutral loads. In the event of a ground fault, the 'neutral' will be energized to 277V, and various 277V loads would suddenly see 480V. Any imbalance in the 277V loading would cause the neutral voltage to shift, putting more than 277V on some loads, less on others.

-Jon
 
winnie said:
Can you confirm what I believe you are saying:

The installer simply 'created' a neutral bus, with no connection at all back to the supply transformer, and then they bonded this 'neutral bus' to the ground bus? In other words, it is as if they were using the ground rod to 'create' the system neutral? This is an 'open neutral', and is generally considered a fault to be prevented, not a design to be installed.

I don't have a code book handy, but my _hunch_ is that you won't find a direct prohibition against such an installation, but rather lots of details that such a system inherently violates. For example, it would end up being an impedance grounded system (where the 277V loads acted as the grounding impedance), but would be serving line-neutral loads. In the event of a ground fault, the 'neutral' will be energized to 277V, and various 277V loads would suddenly see 480V. Any imbalance in the 277V loading would cause the neutral voltage to shift, putting more than 277V on some loads, less on others.

-Jon
Click to expand...
Yes!
There is no System Neutral back to the Utility Transformer!
Just the Ground Rod at the 400A 480Y/277 3P 4W Panelboard with a bond at between Ground Bus & Neutral Bus.
 
Looks like you need a 400 amp delta/wye XFMR to supply this panelboard. Or you need POCO to change the service to four wire.
 
KMeni said:
There is no System Neutral back to the Utility Transformer!
Just the Ground Rod at the 400A 480Y/277 3P 4W Panelboard with a bond at between Ground Bus & Neutral Bus.
Click to expand...
In that case, you cannot connect any 277v line-to-neutral loads. If you need to power any loads that require 277v, you need a neutral from the transformers.

If they're Y-connected secondaries, the system neutral can be run in with the service conductors. If they're Delta-connected, you'll need to derive a neutral.
 
KMeni said:
Are there any NEC Code Section preventing you to feed a 400 Amp 480Y/277V 3 Phase 4 Wire Panelboard off of the 1200A 480V 3 Phase 3 Wire Delta Main Service?
Click to expand...

250.24(C) says if the system is grounded at any point, then the groundedconductor must be run toall service equipment.
250.26 says which conductor must be the grounded conductor.

These two sections would not allow a single service to be both ungrounded and grounded.
 
Sorry, I'm still unclear if you actually need the 277 volts.
So far it sounds like the original install failry well follows NEC as far as grounding connections for an ungrounded system (per 250.24(E) )
The "ground" should be marked and treated as a grounding conductor and not grounded.
If you actually need the 277 then you will have to create a "neutral" as othes have stated. If you don't need the 277, the I believe the direct answer to your question is "no" on using a panelboard marked 480/277.
You would need to use a panelboard listed and labeled for the 480
ungounded or corner grounded system and you could not use slash rated breakers (480/277).
 
Status
Not open for further replies.
Top