Wye to low zig-zag on DELCO E-5399M2 3

[mkteng]

I purchased a rebuilt rail car genset with a 20kW/25kVA 3phase DELCO E-5399M2 (powered by a Detroit Diesel 2-71) as standby power for my home. I intended to use two lines (plus neutral) to provide 120V/208V to my split-phase 120V/240V panel through a transfer switch. I am concerned how the unbalanced load will affect the generator; therefore, I would prefer to rewire the output to single phase (zig-zag). Unfortunately, I received no documentation with the unit and when I removed the wiring panel I only viewed the four power and neutral wires coming from inside the generator outer shell. Does anyone have information on this unit that would enable me to rewire it from 3phase wye to single phase zig-zag? Thanks in advance for your assistance.

 

[davidbeach]

In order to make that change you need to be able to access both ends of all three windings. If you are stuck with only three phases and a neutral point there is no connection that will give you single phase. This topic has been discussed several times on this forum, a search should turn up many reference to wiring diagrams.

 

[mkteng]

I have generic 12-wire diagrams and am familiar with the rewiring process but not on this old-style Delco generator. I was looking for some help to determine if I should move forward with the unbalanced loading scheme outlined above or if someone with experience with this particular unit could advise me on the zig-zag rewiring process. I believe that there were tens of thousands of these units deployed over the years, so I expect that someone would be familiar with them.

 

[waross]

I would just use two phases and not worry about unbalance.
If you have a single phase voltage regulator, put your load on the same lines as the voltage regulator.
It sounds as if your set has been rewound at some time and only four leads brought out.
Many of the old Delco sets used 10 lead connections and could not be reconnected for single phase.
Whether you use two phases of three phase, a zig-zag connection or a double delta connection, your KVA capacity on single phase will be 2/3 of the 3 phase capacity. The KW output will be limited by the maximum KVA.
When I say that it cannot be reconnected, I mean with the existing leads. A winding shop may be able to open the neutral star point and bring out three leads.
respectfully

 

[mkteng]

Thanks for the advice Waross. I will move forward with the unbalanced load approach, since that is the simplest one now that I know it is safe. I sized the genset at 20kW knowing that I would only need to support a maximum load of 12kW at single phase 120/240 (should it be deemed necessary to convert from 3phase to single phase and lose 1/3 supply capacity). So, I am not worried about exceeding 12kW on the two supply phases from the genset. Do you have any other hints or cautions that I should be made aware?

 

[waross]

Check any 230 volt appliances for compatibility with 208 volts. The apartment that I live in is supplied with 120/208.
The electric stove is slow heating but acceptable. The voltage is set for about 124/215. I have no problems, and I am probably the only one who is aware of the 208 volt connection.
I would suggest a similar voltage setting.
This is worth repeating.
Remember to take the power off the leads that supply the voltage regulator. If you don't you may experience severe voltage drops under load.
respectfully

 

[mc5w]

And make sure that the neutral from the generator to the transfer switch and then to the panelboard is 100% of the hot wire capacity. Unlike 120/240 volts, a 3-wire network circuit has full load current flowing in the neutral for a balanced set of 120 volt loads.

Mike Cole

 

[waross]

Good catch, Mike. (lps)
I always start designs of systems of this size by assuming a 100% neutral. I then only reduce the neutral if it can be justified. In this case the neutral must be 100% (or more if the load has a lot of harmonics.)
respectfully

 

[mkteng]

Thanks Mike and waross! I was planning on connecting the three existing utility supply lines to my house (120/240V, split phase plus neutral) directly to the output of the transfer switch. The inputs to the switch would come from the genset (L1, L2 and neutral) and new 120/240V split phase lines from the utility transformer (through the power meter). Will the standard utility neutral line support full load??? I would expect that it would have to, but I am uncertain now that you raise the topic. Thanks again for your advice.

 

[waross]

You will have to judge that for yourself.
However, most of your large loads will not use the neutral.
Water heater, electric stove, clothes drier, 230 volt air conditioners, will draw either no current or very little current on the neutral.
Estimate your single phase loads and compare to the size of your neutral conductor. The neutral should be large enough to support the single phase load on either line 1 or line 2.
I will be surprised if your neutral needs to be upgraded in North America.
I haven't done residential work in North America for a couple of decades now and I am not familiar with present neutral derating practices.
respectfully

 

[mkteng]

Thanks waross! I agree that the large loads are 230V and would not draw significantly from the neutral (as with normal split phase). My single phase 120V loads consist mainly of lighting and some small electronic equipment. Nothing that would exceed a total load of 50A. I expect that the existing neutral will suffice, but will bring it to the attention of the electrical contractor and the utility linemen who will be doing the actual installation. Thank you again for sharing your valuable experience!

 

[mc5w]

You would have 2 calculations and actual instances of neutral current. When running on utility 120/240 volts, neutral current from 120 volts will tend to go towards zero depending on your phase balance. When running on 120/208 volts network from your generator neutral current from 120 volt loads will tend to maximize. Therefore, the neutral in your utility service and going from the service to the transfer switch can be reduced in size. The neutral from the transfer switch to the panelboard would be the greater of what you need when running on utility power and what you can get from the generator.

However, in most electrical service applications the neutral size is not based on heating of the neutral but rather good voltage regulation for 120 volt loads as load balance is never perfect. You also need low enough neutral impedance so that when starting motor loads the shift in neutral voltage is not bad - too small of a neutral would cause lights on the same side of the line to dim when a motor starts and lights on the other side to brighten when a 120 volt motor starts.

Mike Cole

 

[mkteng]

Hi Mike. Thank you for the feedback. I understand that the neutral from the transfer switch to the panel would have to support the greater load as you mention above. However, I am still uncertain what you mean exactly by the statement, "When running on 120/208 volts network from your generator neutral current from 120 volt loads will tend to maximize". It has been 25 years since my EE course on power and motors, but I was under the impression that the neutral sizing (and cable) supplied by the utility company was for the worst case scenario when 100% load (100A) was supplied to a single supply (of the two split-phase inputs) and thus would support any neutral load the generator would require. I would expect that the balancing of the 120V loads at the panel would be better than that but there will be times when a fraction of the 120V load would come from a single supply line (i.e. refrigerators and lighting). Am I missing something important here?

 

[waross]

The code in Canada requires that the neutral have sufficient ampacity to support the maximum unbalanced load. It is often sized at 100%.
In any event, if it is sized for the unbalanced load it will still be adequate.
I think that mkteng was refering to neutral currents in a single phase installation compared to the neutral currents with two phases of a three phase system in use.
For example, if you have a resistive load of 40 amps on each leg of your main panel;
On single phase power from the utility the currents will cancel on the neutral and you will have zero current on the neutral.
When you switch the same load over to your generator, the neutral currents will not cancel but will add vectorilly and the neutral current will equal the phase or line currents.
Did I get that right, mteng?
respectfully

 

[mkteng]

Now I remember... I agree that when a 120V load is balanced the neutrals would cancel out in single phase, but would ADD vectorilly in 3phase as Mike stated earlier. I will ensure that the neutral is sized accordingly. Many thanks guys.