208Y/120v Help me understand

[Strombea]

I'm just starting to get into these panels as i have always stuck with residential work. (so take it easy)

1. Why a high leg at all? What are you supposed to put on it (high leg) when all you need is 120v for computers and lights? And are you supposed to just put a blank filler on every 3rd breaker if no need for 208v because voltage is too high? (thus a third of the spaces are useless when no phase to phase load is needed?

2. Is there a difference between 208v from phase to phase and 208v high leg phase to neutral? example: a copy machine needs 208v, so do you run one single phase load to neutral or phase to phase load? the machine is getting 208v either way- Does it Matter?

I really don't understand the reason for a high leg.

Thanks for the wisdom shared!

 

[iwire]

If you have a high leg it is not 208Y/120.

That is delta.

 

[GoldDigger]

I'm just starting to get into these panels as i have always stuck with residential work. (so take it easy)

1. Why a high leg at all? What are you supposed to put on it (high leg) when all you need is 120v for computers and lights? And are you supposed to just put a blank filler on every 3rd breaker if no need for 208v because voltage is too high? (thus a third of the spaces are useless when no phase to phase load is needed?

2. Is there a difference between 208v from phase to phase and 208v high leg phase to neutral? example: a copy machine needs 208v, so do you run one single phase load to neutral or phase to phase load? the machine is getting 208v either way- Does it Matter?

I really don't understand the reason for a high leg.

Thanks for the wisdom shared!

The reason for the high leg is to provide 240V three phase to motors and other three phase loads. If you did not need that, then just putting in 120/240 single phase service would be easier except that POCO will be unhappy about the large single phase load imbalancing their system.

A 208Y/120 would give you all the 120V you need for lighting loads, but would not give you 240 for resistive heating and other loads that might need it. You would only get 208 instead.

For the most part, in the original plan, the high leg delta would not use the stinger to neutral for anything and the lighting loads would be 240 (phase to phase including using the stinger) instead of 208.

And yes, if you have only 120V line to neutral loads you will leave every third slot open except where you use three phase breakers for delta loads.

 

[Strombea]

The reason for the high leg is to provide 240V three phase to motors and other three phase loads. If you did not need that, then just putting in 120/240 single phase service would be easier except that POCO will be unhappy about the large single phase load imbalancing their system.

A 208Y/120 would give you all the 120V you need for lighting loads, but would not give you 240 for resistive heating and other loads that might need it. You would only get 208 instead.

For the most part, in the original plan, the high leg delta would not use the stinger to neutral for anything and the lighting loads would be 240 (phase to phase including using the stinger) instead of 208.

And yes, if you have only 120V line to neutral loads you will leave every third slot open except where you use three phase breakers for delta loads.

So maybe i have been looking at it wrong,

I thought in this config. the line to line is 208v and line to neutral is also 120v because i have seen this.

So the question now is if line to line is 208v does that mean there is no high leg and if line to line is 240v there is a high leg?

Is it that only B to A and B to C = 240v and that A to C is 208v?

So With out a high leg, there is no 240v unless using a booster, buck booster, etc.?

Sorry i have so many questions but this is an important config that i don't understand.

 

[hurk27]

With a 120/208 WYE you will have 208 volts A/B, A/C, B/C, and 120 volts A,B,C to neutral

With a 120/240 volt 4-wire delta also known as a high leg delta, You will have 240 volts A/B, A/C, B/C and 120 volts A&C to neutral, but 208 volts B to neutral, B would not be used for line to neutral loads, it would only be used for line to line loads, either 3-phase or single phase 240 volt loads.

Also with a 4-wire delta you have to look at the transformer configuration to know if it was designed for more 3-phase loads or more 120 volt loads by which transformer is bigger, if the 3 bushing transformer is the larger one then the service was design for more line to neutral loads and less three phase loads, if the two bushing transformer/s are the larger ones then it was design for a smaller line to neutral loads and more three phase loads, it was more common to have a larger three bushing transformer because most of the time the the 3-phase loads were added at a later date and the utility just added the other two bushing transformer/s to add the 3-phase capability, some times they are configured as an open delta where you would only have two tanks, but around here a closed delta is the common where we have three tanks.

Another little note is when the wire lands in the meter the high leg is always landed in the C lug, but when brought in to the panel it is moved to the B lug, some older services might have the high leg in the C lug at the panel before the NEC required it to land in the B lug, so always check your line to neutral voltages on each before ever selecting a breaker spot for a line to neutral load, it can be a costly mistake as I found out a long time ago when I wired a desiel pump up at our truck shop, the pump motor smoked in just a few seconds, also the NEC requires that the high leg to be orange or marked orange. (110.15) (230.56)

 

[norcal]

Just to add, if using the high leg & another phase for a 240V 1? load the circuit breaker would have to have a 240V rating, not the more common 120/240V rating.

 

[jimdavis]

Perhaps these diagrams will help.

 

[cadpoint]

Google is your friend, I use "Google", Image (insert) diagram 208/120;
The key part is to use diagram for results like this

 

[delectric123]

If you're dealing with a panel board, you probably have removable stabs, so if you run out of spaces for 120 volt loads, you can replace the B phase stabs with A or C phase.

 

[Strombea]

Thank You for the explanation, I get it now, i was confused between the two .

Just so i get i straight on another aspect:

Is it ever favorable on high leg delta to use B phase to neutral? Is high leg 208v to neutral sufficient for a 208v rated appliance on this system or are you better off running 240v phase to phase with a reducer? I understand that neutral doesn't provide input on its side of an appliance (coil for example) and phase to phase would provide input on both sides of a coil. Does this Matter? Depends on appliance?

 

[kwired]

Thank You for the explanation, I get it now, i was confused between the two .

Just so i get i straight on another aspect:

Is it ever favorable on high leg delta to use B phase to neutral? Is high leg 208v to neutral sufficient for a 208v rated appliance on this system or are you better off running 240v phase to phase with a reducer? I understand that neutral doesn't provide input on its side of an appliance (coil for example) and phase to phase would provide input on both sides of a coil. Does this Matter? Depends on appliance?

A 208 volt load should run just fine from high leg to neutral. Not really sure just what effects it will have on loading of the neutral conductor or the load distribution through the transformer if you would have a significant enough load to "neutral" on the high leg though.

One problem you will have is you are not likely going to find a 240 volt rated single pole breaker for a 120/240 volt series panelboard though.

This type of system is generally used either in places where there is a lot of three phase load - especially motors, and limited 120 volt loads, or sometimes where there is only limited three phase load - like maybe the air conditioning and the rest of the loads are all 120/240 single phase. Other times if in a remote area and three phase is needed it is used to save the POCO from needing to run all three phases plus neutral with their primary side - they can derive an "open delta" system from just two phases and the neutral. You can not do this with a wye connected secondary, and have a three phase secondary.

 

[GoldDigger]

One limitation on a high leg to neutral load is that all of the load current must pass through the two halves of the A-C winding, in addition to the A-B and C-B windings. This increases the resistive losses and may overload the A-C pot.

 

[kwired]

One limitation on a high leg to neutral load is that all of the load current must pass through the two halves of the A-C winding, in addition to the A-B and C-B windings. This increases the resistive losses and may overload the A-C pot.

If it is open delta there is only one path from B to N, but there is more winding (impedance) in that path - but also more volts then there is from A or C to N, so you can't really compare it to either of those in any way. But if there is significant B to N load - the A to C coil will be loaded heavier on the half that is carrying B current, plus any other 120 volt current connected to that side, as well as any A to C loads. So you could overload half that pot if you had significant B to N load as well as other load utilizing that half.