How much maximum current by using 2 lines out of 3phase?

[RHeTTRoNiCS]

Hello all...have a quick question...we've got a 3phase AC line source 208Vac 35Amps. As we know, 3phase have L1, L2, L3, and Neutral. What if I want to use L1, L2, and Neutral only...so how much maximum current I can get. Is it still 35Amps or less? Thanks.

 

[FvM]

35 A if neutral wire has same cross section as phases.

 

[D.A.(Tony)Stewart]

Even using resistive loads , PF degrades with a missing phase and phase imbalance can result in six(6) times increase in line loss if using only one (1) phase but only 50% more with 2 phase working with same load.

Also 2 of 3 Phase motor windings can burn out from stall current.

Consider a constant linear load on 3 phases, 2 then 1, so I will increase as # of phases reduces.

With 3 phases, winding loss = 3x I²R with no loss on Neutral
With 2 phases, winding loss = 2x (I*1.5)²R on both Neutral and Line x2 =4.5x/3x or 1.5x normal
With 1 phases, winding loss = 1x (I*3)²R on both Neutral and Line x2 =18x/3x or 6x normal
Similar calculations can show increase in conduction losses from reduced power factor from excitation of 3rd phase from other 2 imbalance, effectively pulling down the neutral away from the other 2 phases.

With temperature rise from conduction losses, then derating of transformer must factor this gain by constant linear power demand from 3 phase loads moved to 2 phases.


My calculations conclude use < 66% of rating of each phase current , depending on PF effects
However this is based on lost phase from transformer not a breaker.

Judging by question limit of 35A being small might not result in DT temp rise, but startup up 3rd phase can cause significant overvoltage from sag starting up phase 3.
I have seen ATE PSU"s blow for this reason in production as TE Mgr.

 

[RHeTTRoNiCS]

Thanks for the response...I attached the drawing to show what I wanted...Really need to know if it is safe or not? Thanks a lot.

 

[FvM]

Means your AC line is 120/208V. 208 volt is -13% for a 240V device, but should stll work.

Do you really have common neutral+protective earth wire? That's very unusual these days because it's rather unsafe.

The available power is 208V/35A.

 

[schmitt trigger]

In addition to what Sunnyguy mentioned, you must be aware that a neutral connection is NOT equal a ground connection.

The ground connection is for safety purposes, and unless there is a fault, it only carries minimal (leakage) current.

On the other hand, the neutral wire must carry the full current unbalance between the two phases.

There are some electrical codes that allow it for low power loads, but in general it is not good practice.

 

[FvM]

Post #4 shows a 4 pin CEE-connector. There's however no CEE connector defined with common neutral and ground, only three phase and ground without neutral or 5-pin connector with separate neutral and ground.

But a 240 V device connected phase-to-phase won't use neutral at all.

 

[RHeTTRoNiCS]

Hello Guys....yes it is confirmed Neutral and Ground is shorted by wire inside circuit breaker...So do you think it will work and it is safe?

 

[schmitt trigger]

It will work.
And if the internal load is solely a single 240v load, and not a pair of split 120v loads, it is also relatively safe.

Not the best practice, though.

For instance in the area where I live electric clothes dryers operate from a 240v line (for the heaters), but nevertheless have a 120v motor which uses a separate neutral. For many years they used a three-prong connector, and installers would tie the neutral to the appliance's metal chassis.

Unfortunately, since clothes dryers operate in wet environments, sometimes mild shocks would occur. It did happen to my wife.

Now the electrical code has been updated to ensure that clothes dryers must have four-prong connectors, with separate ground and neutral conections.

 

[D.A.(Tony)Stewart]

Hello Guys....yes it is confirmed Neutral and Ground is shorted by wire inside circuit breaker...So do you think it will work and it is safe?

It will work as long as there are no huge inrush when the 3rd phase breaker is activated. Even when 3 phases are restored after a power outage, if the phase loads are not balanced for inrush on one (1) phase you can get excessive voltages on the others.

recall I said "I have seen ATE PSU"s blow for this reason in production as TE Mgr." This was due to imbalance of similar load types on each phase, which was corrected by Maintenance after the PSU blew and caused a line stop much longer than the brief power outage. In some cases 10% imbalance is OK and others , too much, depending on surge currents.


3 Phase rectifiers with 6 diodes are inherently low ripple, so consult with your ATE supplier about operation of the PSU on 2 phases, as capacitor ripple current rises significantly. Which may be OK for short term.

 

[FvM]

Now the electrical code has been updated to ensure that clothes dryers must have four-prong connectors, with separate ground and neutral conections.

I'm not so familiar with US electrical codes, but I assume "now" has been sometime in the last century? According to German standards, a common PE/N wire can be used in TN-C-S earthing inside a building up to a switchboard if it has at least 10 sq mm cross section. Usually PE and N are only connected at the central distribution point in a building. See also https://en.wikipedia.org/wiki/Earthing_system

 

[RHeTTRoNiCS]

It will work as long as there are no huge inrush when the 3rd phase breaker is activated.

So what happen when there's huge current on turn-on? Any major risks? Thanks

 

[FvM]

I don't see a specific inrush current problem with 2-phase load.

 

[D.A.(Tony)Stewart]

I don't see a specific inrush current problem with 2-phase load.

A missing phase does not cause the inrush as these are independent. But when a system is imbalanced, inrush of the 3 phase can cause sag on that phase creating and overvoltage on the other phases by loading factor on the distribution transformer. It may occur when the other 2 active phases are already at nominal voltage.

If surge currets are 5 to 10x nominal load and the load is already high , if started at peak voltage, this conditon will occur. Howmich depends on many system variables in load regulation or transfer impedance of the transformer. Since mechanical breakers are asynchronous with phase, the conditon may not be detected frequently.

So beware of imbalance and extreme loading factors when restoring one of 3 phases alone. Transfer impedance and load factor are some of the distribution variables.


this applies moreso to industrial applications with highly reactive loads, such as capacitive front end PSU chargeup and motor reactance for many cycles.

 

[FvM]

Unbalanced load and related inrush current problems matter if the load current is a significant fraction of rated transformer current. I won't expect this in the present case. Single phase loads and respective inbalance are common in power distribution systems and transformers are designed to handle it.

Is U.S. residential power distribution still using multiple single phase transformers as I have seen it in the 90er years?

 

[D.A.(Tony)Stewart]

Yes North American residential power is still single phase, but if there is a line fault in any phase all 3 phases that are converted to single phase DT's for distribution with 3 outputs. I mentioned my ATE experience, because I thought it was relevant to this application. Possibly for ATE and likely a similar small percentage of the transformer capacity,

It wasn't clear if this was the master single phase down or just one of many breakers/