"Phase or line and system" 210.5

[jetlag]

Hypothetical: I have a 480/277 service. The service feeds two transformers that output 120/208.

How many systems are present?

according to the definition of seperately derived systems , they can not be called that because they have connection to supply conductors of the main system . But can the two outputs be called a system? What about small low voltage tranformers for a/c and door bell alarms , is each to be counted as a system ? I am leaning toward the 480/120 is the only system. But even a 120/24 volt transformer does create a 24 v system I quess.

 

[kwired]

Agreed. That is why I interpret the requirement as letting the electrician know if he or she is dealing with 120 or 480. Yes, both are dangerous. But we do treat them differently (e.g., working clearances and the "prohibited approach zone" are different). So there is some value in knowing which you are looking at, when you open a box.

After you open the box it is too late to find out what is there if you have encroached a prohibited approach zone. You would need to know what is there before opening it to comply with this.

 

[acrwc10]

according to the definition of seperately derived systems , they can not be called that because they have connection to supply conductors of the main system . But can the two outputs be called a system? What about small low voltage tranformers for a/c and door bell alarms , is each to be counted as a system ? I am leaning toward the 480/120 is the only system. But even a 120/24 volt transformer does create a 24 v system I quess.

They are separately derived because they are magnetically connected but not "bolted" for lack of a better discription.

Art.100 Def. Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

From the Handbook;
Some examples of a separately derived system may include a generator, a battery, converter windings, a transformer, and a solar photovoltaic system, provided they “have no direct electrical connection” to another source

A separatly derived systems only connection to the supply system is through the bonding of the grounding system.

 

[hurk27]

They are separately derived because they are magnetically connected but not "bolted" for lack of a better discription.

Art.100 Def. Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

From the Handbook;
Some examples of a separately derived system may include a generator, a battery, converter windings, a transformer, and a solar photovoltaic system, provided they ?have no direct electrical connection? to another source

A separatly derived systems only connection to the supply system is through the bonding of the grounding system.

Wow someone who thinks like me

 

[jetlag]

They are separately derived because they are magnetically connected but not "bolted" for lack of a better discription.

Art.100 Def. Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

From the Handbook;
Some examples of a separately derived system may include a generator, a battery, converter windings, a transformer, and a solar photovoltaic system, provided they ?have no direct electrical connection? to another source

A separatly derived systems only connection to the supply system is through the bonding of the grounding system.

they are "bolted to supply" 480/277 in 208/120 out

 

[hurk27]

they are "bolted to supply" 480/277 in 208/120 out

You never connect the neutral on the supply side of a transformer, only bond the neutral to ground on the secondary side.

 

[acrwc10]

they are "bolted to supply" 480/277 in 208/120 out

If they were bolted to the supply, you would have 480/277 in and 480/277 out. The output is connected to the XFMR but it is insulated from the input.
You only have 480 in, no 277 on the input.

 

[jetlag]

If they were bolted to the supply, you would have 480/277 in and 480/277 out. The output is connected to the XFMR but it is insulated from the input.
You only have 480 in, no 277 on the input.

I know the transformer secondary is not directly connected to the supply, but I thought the code meant even the primary could not connect to the 480/277 supply to be a sperately derived system. If that is not the case why didnt the code list transformers with magneticly connected secondary along with a battery , generator and other things . So you are saying every small low voltage like 24 v transformer is a seperately derived system and must be identified as such ?

 

[acrwc10]

Art.100 Def. Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

A direct electrical connection would be, bussing to bussing, of cable to cable, etc. an XFMR uses an "indirect connection" magnetic fields. That is why I posted the Handbook commentary along with the definition from the NEC.

 

[jetlag]

If they were bolted to the supply, you would have 480/277 in and 480/277 out. The output is connected to the XFMR but it is insulated from the input.
You only have 480 in, no 277 on the input.

ok I see now , the code was not talking about the tranformer primary could not be connected to suppy to be seperately derived. While we are on the subject how is it even possible for a transformer secondary to connect to the supply ? I believe some florescent fixtures have the supply reconnected to the secondary wires someway.

 

[jetlag]

Hypothetical: I have a 480/277 service. The service feeds two transformers that output 120/208.

How many systems are present?

I believe it is 3 systems, it doesnt matter that two have the same voltage, they are not in phase with each other , if you dont believe it try to connect their secondarys together . , that could be wrong, if the transformers are exactly the same it might be possible for the outputs to stay in phase with each other.?

 

[Smart $]

ok I see now , the code was not talking about the tranformer primary could not be connected to suppy to be seperately derived. While we are on the subject how is it even possible for a transformer secondary to connect to the supply ? I believe some florescent fixtures have the supply reconnected to the secondary wires someway.

Autotransformers and buck/boost-connected transformers are direct connected to the supply system.

 

[Smart $]

I believe it is 3 systems, it doesnt matter that two have the same voltage, they are not in phase with each other , if you dont believe it try to connect their secondarys together . , that could be wrong, if the transformers are exactly the same it might be possible for the outputs to stay in phase with each other.?

Typically-connected same-voltage delta-wye transformer secondaries are in-phase.