3 phase feeding single phase panels

[electricalist]

120/208 3 phase service that feeds single phase panels.
If you use all 3 phases how do you feed the panels? a/b panel 1 b/c panel2 c/a panel3
Is this a good practice?

 

[ActionDave]

Yup.

 

[Smart $]

120/208 3 phase service that feeds single phase panels.
If you use all 3 phases how do you feed the panels? a/b panel 1 b/c panel2 c/a panel3
Is this a good practice?

Exactly... and yes.

Commonplace nowadays for apartment complexes.

Nomenclature....

Service: 208Y/120 3? 4W

Apartments: 120/208 1? 3W

 

[electricalist]

Thanks,,
Most of the service I ve seen are straight forward hi volt in xfmr to low volt 3 phase panels
Is my op to save money on panels or some other reason?

 

[Smart $]

Thanks,,
Most of the service I ve seen are straight forward hi volt in xfmr to low volt 3 phase panels
Is my op to save money on panels or some other reason?

I'm not really sure. I'm not an estimator so have never compared costs.

It would reduce the service rating compared to 1?, and perhaps result in a better $/kWH rate from POCO.

 

[Carultch]

120/208 3 phase service that feeds single phase panels.
If you use all 3 phases how do you feed the panels? a/b panel 1 b/c panel2 c/a panel3
Is this a good practice?

That's exactly correct. Your three panelboard (or other equipment) that distributes the loads to the single phase panelboards would have two pole breakers for each single phase feed. And you would strategically stagger among the three phases in order to balance the phase currents, and reduce the maximum expected current.

There are two main difference between 120/208 3-wire single phase feeders, and the equivalent 120/240 3-wire single phase feeders.
1. The two phase conductors of the 120/208 system are only 120 degrees out of phase. This means that even with the line currents balanced, you'll have a significant current on the neutral, in order to form a complete circuit path. So if you are sharing a conduit among multiple circuits of this type, the neutral counts as a current-carrying conductor for ampacity adjustment purposes.

2. Line-to-line connected loads (ovens, driers, HVAC systems) will operate with less voltage and slightly more current. Therefore you cannot simply swap in a 240V appliance and expect it to work the same. Household appliances usually are rated this way, but you need to verify that it can work at the lower 208V voltage, after voltage drop is considered. This is usually not a problem for resistive heat loads like ovens, however it is much more important with motor loads.


It is common practice for this setup in apartment buildings, where each dwelling unit is given a single phase feed as its "service". You'd then distribute the dwelling units among each pair of phases, and if you do not have a multiple of 3, then you can adjust the House Loads panel to compensate.

 

[electricalist]

Amazing.
The ec that did that job knew all that i guess and all i thought was check that out that looks like a good idea.

 

[electricalist]

There are two main difference between 120/208 3-wire single phase feeders, and the equivalent 120/240 3-wire single phase feeders.
1. The two phase conductors of the 120/208 system are only 120 degrees out of phase. This means that even with the line currents balanced, you'll have a significant current on the neutral, in order to form a complete circuit path. So if you are sharing a conduit among multiple circuits of this type, the neutral counts as a current-carrying conductor for ampacity adjustment purposes.

As i read this again it sunk in more.
For mwbc i prefer 3 hots 1 neutral. I just didnt approach thinking of the service that way..
I must have heard 1k times someone say in commercial we upsize single phase SEC but nobody said why,,,,,i guess derating would be to upsize..


Is 3 phase line to line loads operating at lower voltage and slightly more current why some say 3 phase is cheaper to use?

 

[iwire]

There are two main difference between 120/208 3-wire single phase feeders, and the equivalent 120/240 3-wire single phase feeders.
1. The two phase conductors of the 120/208 system are only 120 degrees out of phase. This means that even with the line currents balanced, you'll have a significant current on the neutral, in order to form a complete circuit path. So if you are sharing a conduit among multiple circuits of this type, the neutral counts as a current-carrying conductor for ampacity adjustment purposes.

As i read this again it sunk in more.
For mwbc i prefer 3 hots 1 neutral. I just didnt approach thinking of the service that way..
I must have heard 1k times someone say in commercial we upsize single phase SEC but nobody said why,,,,,i guess derating would be to upsize..


Is 3 phase line to line loads operating at lower voltage and slightly more current why some say 3 phase is cheaper to use?

You are charged for power not current.

 

[electricalist]

Yea I guess that makes sense too. Thanks for all the good info.
One you'll read something I post and say yep he way listening.

 

[iwire]

Posting from mobile while watching wife play slots. Can't be to detailed from here.

 

[electricalist]

Ok then bet 1 line at lowest denomination spin. If you hit any amount respin. If not bet 2 lines ..continue this process through all bet lines avaliable..example..5 liner...after you cycle through those lines but the denomination to the next amount and repeat cycle till you've cycled all denim the go back to lowest denominations and start over.
With this you will have covered every line bet at every denomination there is and there for these are the most equal odds mood not losing..
Good luck

 

[Carultch]

Is 3 phase line to line loads operating at lower voltage and slightly more current why some say 3 phase is cheaper to use?

When you connect a 208 V load from phase A to phase B, that is a single phase load.
When you connect a 208V load to all three phases, that is a three phase load.


For single phase line-to-neutral loads, current travels from source to load on the line wire, and back to the source on the neutral wire. You are requiring two wires to carry the current.

For single phase line-to-line loads where the lines are 180 degrees out of phase (120/240), current travels from source to load on the black wire, and back to the source on the red wire. You require two wires to carry the current. You might also require an additional neutral, for carrying any imbalance of the load, or for instrumentation purposes. This is two, maybe three wires required to carry the current.

For single phase line-to-line loads, where the lines are 120 degrees out of phase (120/208), current travels from source to load on perhaps the black wire, but it cannot all travel back on the red wire due to the phase timing. Some of it travels back on the neutral. All three wires are actively involved in the circuit, thus all needed. Only one phase of the three phase system is used, so you don't get any more value out of it than you would with an ordinary single phase circuit.

For three phase loads, current travels from source to load on all three line wires. It can travel back on the neutral, but doesn't need to, since it adds up to zero for a balanced system. The power transmitted is equivalent to the phase current multiplied by phase-to-neutral voltage, and added up among all three phases. This gives you 1.73 times as much power as an equal current/equal voltage single phase system, but you only need 1.33 times as much wire. You have more power utilization for each kcmil of copper.

 

[ActionDave]

Thanks,,
Most of the service I ve seen are straight forward hi volt in xfmr to low volt 3 phase panels
Is my op to save money on panels or some other reason?

Since an individual apartment load is tiny compared to a commercial site and the the apartment dwellers don't have any three phase equipment there is not much to gain by bringing all three phases into each apartment.

 

[GoldDigger]

For single phase line-to-line loads, where the lines are 120 degrees out of phase (120/208), current travels from source to load on perhaps the black wire, but it cannot all travel back on the red wire due to the phase timing. Some of it travels back on the neutral. All three wires are actively involved in the circuit, thus all needed. Only one phase of the three phase system is used, so you don't get any more value out of it than you would with an ordinary single phase circuit.

For three phase loads, current travels from source to load on all three line wires. It can travel back on the neutral, but doesn't need to, since it adds up to zero for a balanced system. The power transmitted is equivalent to the phase current multiplied by phase-to-neutral voltage, and added up among all three phases. This gives you 1.73 times as much power as an equal current/equal voltage single phase system, but you only need 1.33 times as much wire. You have more power utilization for each kcmil of copper.

That requires prompt refutation!
If you connect a single load between two lines (say black and red), all of the current that flows through that load will be carried entirely by the black and red wires.
For a resistive load the current will be in phase with the voltage between those two wires. *But* if you look at the line to neutral voltage on the red and black wires, the load current will not be in phase with either of those voltages. That does not mean the current is forced to flow somewhere else. It is simply out of phase with a voltage which does not appear directly across the load.

 

[Carultch]

That requires prompt refutation!
If you connect a single load between two lines (say black and red), all of the current that flows through that load will be carried entirely by the black and red wires.

Thanks for mentioning that. That is true for individual loads.

Where it is not true, is for a 120/208 single phase feeder, that feeds numerous single phase 120V loads. Even if you balance the loads across this panel, the neutral still will carry current equal to the phase currents. And it does this, because the currents on the two phases do not add up to zero.

So if you are using a single raceway or cable tray to carry the 120/208 single phase feeders to several dwelling units, the neutral wires must also be counted as a CCC for derating purposes.

 

[masterinbama]

Price some large (3000 amp) single phase gear. You will see why 3 phase services and single phase feeders are the norm on large complexes.

 

[electricalist]

Could that be a product of supply and demand due to electrical reasons that dictate 3 phase be standard for.commercial applications

 

[electrofelon]

Since an individual apartment load is tiny compared to a commercial site and the the apartment dwellers don't have any three phase equipment there is not much to gain by bringing all three phases into each apartment.

I wonder about sending three phase vs single phase to units where there is a three phase service and how it works out in terms of wire cost, equipment cost, conduit size/cost/fill etc. considering three phase 60 amp is about equal to single phase 100 amp. I have a project coming up where I will need to send a subfeed to about 40 units. I haven't gotten far enough along to run numbers yet.

 

[kwired]

Price some large (3000 amp) single phase gear. You will see why 3 phase services and single phase feeders are the norm on large complexes.

Possible you may not be able to find that size of gear in anything but three phase, but also remember if you need 3000 amp single phase you only need roughly 58% of that for three phase (of same voltage - 208 vs 240 volts makes a some difference also) I have seen 2000 amp three phase gear used for single phase service.

I wonder about sending three phase vs single phase to units where there is a three phase service and how it works out in terms of wire cost, equipment cost, conduit size/cost/fill etc. considering three phase 60 amp is about equal to single phase 100 amp. I have a project coming up where I will need to send a subfeed to about 40 units. I haven't gotten far enough along to run numbers yet.

You use smaller conductors, but more of them. 100 amp 2 pole breakers typically will still cost less then 60 amp three pole breakers, panels with three bus will usually be more then panel with two bus - and both will normally be 125 amp bus construction. If separate meters for each feeder three phase meters/sockets will be more costly then single phase as well - again both would be 125 amp constructed.

Not saying it wouldn't ever be cost effective to supply a dwelling unit in such case with all three phases, but seems it likely is not going to be plus there would seldom be a need for a load that uses all three phases.

Now if you don't have separate metering but still have a large facility, you may benefit by sending some larger three phase feeders out to certain areas and then maybe splitting into single phase feeders from that point, but metering sometimes gets trickier with the POCO and they would not want those unmetered feeder conductors that may be easy to tap some load from.