High Leg Delta

alixenos

alixenos

Member
Location
Florida
Occupation
Electrical Design Engineer
Hello,

Im using a high leg delta panel in my design and I wanted to connect some 208V on the high leg phase and neutral.

The senior engineer whom Im working with discouraged me from using that 208V and the reasoning behind it was to get 208v from two hots and not from a hot and a neutral!!

Any explanation as to why is that?
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
The high leg is a 4-wire Delta is pretty useless for single phase 208 volt loads. For one you cannot use a standard single pole circuit breaker.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
alixenos said:
Hello,

Im using a high leg delta panel in my design and I wanted to connect some 208V on the high leg phase and neutral.

The senior engineer whom Im working with discouraged me from using that 208V and the reasoning behind it was to get 208v from two hots and not from a hot and a neutral!!

Any explanation as to why is that?
Click to expand...
Using the 208V L-N connection will reduce the amount of 120V L-N loading you can have, especially if it is an open delta.
 
alixenos

alixenos

Member
Location
Florida
Occupation
Electrical Design Engineer
infinity said:
The high leg is a 4-wire Delta is pretty useless for single phase 208 volt loads. For one you cannot use a standard single pole circuit breaker.
Click to expand...

Why is that? My understanding is the neutral grounded and a single circuit breaker will open and protect the ungrounded conductor for a single phase circuit.


Sent from my iPhone using Tapatalk
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
alixenos said:
Why is that? My understanding is the neutral grounded and a single circuit breaker will open and protect the ungrounded conductor for a single phase circuit.


Sent from my iPhone using Tapatalk
Click to expand...
Standard 120/240V circuit breakers are not rated for 208 L-N circuit interruption, you would need a 277V single pole breaker.
 
R

ruxton.stanislaw

Senior Member
Location
Arkansas
Occupation
Laboratory Engineer
There is also the reason of how a connection like that does not utilise the transformers effectively. For a very small load, it would be okay, though anything more than a couple % of the transformer VA rating is not recommended.
 
tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
If its all in packaged equipment there are L-N rated '240V' din rail breakers, but besides that I would never use that 208 L-N, seems like it would not be a stable voltage.


I'd say we need more info:
Is it a open or full delta?
If say its open on your load calculation the B-N (Single phase 208V) load VA adds to both the A-B winding and the A-C winding.

Is it a customer owned or utility transformer?
Whats the calculated load?
What is the 208 single phase equipment?
 
tom baker

tom baker

First Chief Moderator
Staff member
Location
Bremerton, Washington
The straight rated breakers are rated to clear fault at 240v, more expensive and harder to find, please check on this so you know
Also there is a code section about breaker type for high leg systems
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I believe that this is the section Tom mentioned.

240.83(E) Voltage Marking.
Circuit breakers shall be marked with a voltage rating not less than the nominal system voltage that is indicative of their capability to interrupt fault currents between phases or phase to ground.
240.85 Applications.
A circuit breaker with a straight voltage rating, such as 240V or 480V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker’s voltage rating. A two-pole circuit breaker shall not be used for protecting a 3-⁠phase, corner-grounded delta circuit unless the circuit breaker is marked 1φ–3φ to indicate such suitability. A circuit breaker with a slash rating, such as 120/240V or 480Y/277V, shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker’s voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker’s voltage rating.
Click to expand...
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
jim dungar said:
Standard 120/240V circuit breakers are not rated for 208 L-N circuit interruption, you would need a 277V single pole breaker.
Click to expand...
Just to elaborate a bit, I am nearly certain no one makes a singe pole 240 rated plug on breaker. 2 poles are available (i.e for siemens its a R suffix, Q230R, for QO its an H suffix QO230H). All 3 pole breakers are straight rated. So to do this one would need one of the special 2 poles or a 3 pole, and the 2 poles are usually more expensive than the 3's. IF its a siemens true/bolt on panelboard, the BQD breakers are 277/480 slash rated so you could indeed get a single pole and they are the same form factor as the 240V bolt ons so will fit unlike some brands where the 240 vs 480 bolt ons are physically different.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
alixenos said:
Hello,

Im using a high leg delta panel in my design and I wanted to connect some 208V on the high leg phase and neutral.

The senior engineer whom Im working with discouraged me from using that 208V and the reasoning behind it was to get 208v from two hots and not from a hot and a neutral!!

Any explanation as to why is that?
Click to expand...
You can't get 208 volts from two hots of a high leg delta. The line to line voltage for all combinations is 240 volts.
 
You must log in or register to reply here.
Top