240V split to 2-120V

[bbaker45152]

I hope someone out there knows the answer to this question. I have a need to take a 240V single phase line and make two seperate 120V lines. The two 120V lines will supply power to 2 electrical appliances that draw around 10 amps each. The existing wiring is 4 wire (2 hots, neutral and ground) and is run in 10ga wire. I need to have 2 seperate circuits. The two hots are not my issue or concern. It is the single neutral. Can one neutral work for both circuits? If not, how should it be wired.

 

[charlie b]

Can I ask you to clarify the nature of this project, and your role in it? Is this a machine that your company is manufacturing, or that your techs are maintaining? Or is this a project for your home?

 

[bbaker45152]

Clarification

Clarification

This is a system we have designed. It is quite simple and we have well over 100 of these in place at this time. But we have had a few units that have had issues with a circuit board in the microwave oven. The microwave manufacturer doesn't have a clue why the boards went bad. The other appliance is a heated display case. No issues with them at all. I am just trying to find out if it could be the single neutral causing the problem. Again, not wide spread and most of the units have never had any issues.

 

[bbaker45152]

Can I ask you to clarify the nature of this project, and your role in it? Is this a machine that your company is manufacturing, or that your techs are maintaining? Or is this a project for your home?

Charles,
This is a unit we have designed. We have over 100 of them placed for over a year. We have had issues with the microwave (part of the system) circuit board in a few of them. The microwave manufacturer can't figure out why we have lost these boards. My only thought is the single neutral may be the issue.

 

[rattus]

Charles,
This is a unit we have designed. We have over 100 of them placed for over a year. We have had issues with the microwave (part of the system) circuit board in a few of them. The microwave manufacturer can't figure out why we have lost these boards. My only thought is the single neutral may be the issue.

Is the AC supply hard wired or is it an appliance cord?

 

[bbaker45152]

Is the AC supply hard wired or is it an appliance cord?

The AC is hard wired to a 4 connector twist receptacle. The unit is wired with 10/4 cord that plugs into the receptacle.

 

[charlie b]

Sharing the neutral should not be the cause of your symptom, presuming the wiring is done correctly. If only one of the two appliances is operating at a given moment, current will flow from the ?hot? conductor on that side of the system, through the appliance, and back to the source along the neutral. With both appliances in operation, and with both drawing the same current (i.e., the 10 amps you mentioned), current will flow from the source along ?hot wire #1,? through appliance #1, then (?backwards?) through appliance #2, and back to the source via ?hot wire #2.? The neutral will not carry current in this situation. Finally, with both appliances in operation, but with an unequal load, what you will see is a mix of the two situations described above. The neutral will carry the difference in current, the ?imbalanced current,? from the two appliances. In no case will the current in the neutral exceed the current in either appliance.

So my first approach to troubleshooting would be to verify the units are wired correctly, with regard to the single phase 120/240V source. That would include verifying the system is set up to take power to the two appliances from different sides of the panelboard (i.e., one from Phase A and the other from Phase B). Next, I would verify that the components are indeed being fed from 120/240 single phase, and not from two phases of a 120/208 volt panel. Well, now that I have written that down, this might actually be the first thing I would check. If any portion of your component actually uses the 240 volt source, it might not take kindly to a supply of 208V.

Can you describe any troubleshooting efforts that have been made so far?

 

[rattus]

Maybe:

Maybe:

The AC is hard wired to a 4 connector twist receptacle. The unit is wired with 10/4 cord that plugs into the receptacle.

It is conceivable that the neutral is lost temporarily when connecting or disconnecting the twist lock. This could cause the neutral voltage to pull away from ground which might cause an overvoltage on the PC board.

If this is the case, separate neutrals would solve the problem. Hard wiring would also, but there is still the possiblity of losing the neutral.

 

[bbaker45152]

Sharing the neutral should not be the cause of your symptom, presuming the wiring is done correctly. If only one of the two appliances is operating at a given moment, current will flow from the ?hot? conductor on that side of the system, through the appliance, and back to the source along the neutral. With both appliances in operation, and with both drawing the same current (i.e., the 10 amps you mentioned), current will flow from the source along ?hot wire #1,? through appliance #1, then (?backwards?) through appliance #2, and back to the source via ?hot wire #2.? The neutral will not carry current in this situation. Finally, with both appliances in operation, but with an unequal load, what you will see is a mix of the two situations described above. The neutral will carry the difference in current, the ?imbalanced current,? from the two appliances. In no case will the current in the neutral exceed the current in either appliance.

So my first approach to troubleshooting would be to verify the units are wired correctly, with regard to the single phase 120/240V source. That would include verifying the system is set up to take power to the two appliances from different sides of the panelboard (i.e., one from Phase A and the other from Phase B). Next, I would verify that the components are indeed being fed from 120/240 single phase, and not from two phases of a 120/208 volt panel. Well, now that I have written that down, this might actually be the first thing I would check. If any portion of your component actually uses the 240 volt source, it might not take kindly to a supply of 208V.

Can you describe any troubleshooting efforts that have been made so far?

To date we have just confirmed that the receptacle installed in the unit has power and that the power is correct, ie hot side is where it should be and that we have a good mechanical ground. We have always found that we have a range of 119 to 123 volts on each duplex installed in the unit. I hope my terminology is correct. By duplex I mean there is a 4" square box with two industrial grade outlets in it. One hot feeds one duplex (outlet) and the other feeds the other. The neutral goes to the neutral side on the one outlet and then a jump wire is run to the other. The ground wire is screwed to the box and the ground lug on one of the outlets. I can send you a bad third grade drawing of how these boxes are wired if that would help you.

 

[bbaker45152]

It is conceivable that the neutral is lost temporarily when connecting or disconnecting the twist lock. This could cause the neutral voltage to pull away from ground which might cause an overvoltage on the PC board.

If this is the case, separate neutrals would solve the problem. Hard wiring would also, but there is still the possiblity of losing the neutral.

This what I have thought all along. Unfortunately, these units are rolled out and used for about 6 hours during the day and then un-plugged and put out of the way. There is no way to hard wire. Could I add a (master) disconnect switch that must be off before the unit is plugged and un-plugged. It seems that the units never quit while they are working. The reports are "it worked fine yesterday and today it is dead". Afterthought..................... Is there some type of commercially available dealy job that I could plug in to the microwave circuit that would protect it? Almost like a surge protector??

 

[rattus]

Yes, but:

Yes, but:

This what I have thought all along. Unfortunately, these units are rolled out and used for about 6 hours during the day and then un-plugged and put out of the way. There is no way to hard wire. Could I add a (master) disconnect switch that must be off before the unit is plugged and un-plugged. It seems that the units never quit while they are working. The reports are "it worked fine yesterday and today it is dead".

That should work, but it might also be possible to rewire each unit to draw power from only one 120V leg, then rewire the outlets to balance the load between L1 and L2. Not sure if this is code compliant though. You should hire a master electrician to be sure it is done right.

 

[LarryFine]

The AC is hard wired to a 4 connector twist receptacle. The unit is wired with 10/4 cord that plugs into the receptacle.

. . . there is a 4" square box with two industrial grade outlets in it. One hot feeds one duplex (outlet) and the other feeds the other. The neutral goes to the neutral side on the one outlet and then a jump wire is run to the other.

If this was premises wiring, we would be required to use separate jumpers for each receptacle, joined to the incoming neutral conductor. While it may not be required here, it would certainly be a good idea.

. . . these units are rolled out and used for about 6 hours during the day and then un-plugged and put out of the way. There is no way to hard wire. Could I add a (master) disconnect switch that must be off before the unit is plugged and un-plugged.

You probably don't need a redundant disconnect switch, but you should make sure you unplug each 120v cord before unplugging the twist-lock each day, and vice versa in the morning, plugging in the 120v cords first and the twist-lock last.

 

[220/221]

The neutral connections are EXTREMELY important here. There is NO room for error...ie: loose connections.

 

[smithacetech]

It is conceivable that the neutral is lost temporarily when connecting or disconnecting the twist lock. This could cause the neutral voltage to pull away from ground which might cause an overvoltage on the PC board. QUOTE]

I may be just guessing but, isn't the neutral blade on a twistlock plug longer or bigger than the hots for this reason.

This problem could be anywhere from the power source,(over/under voltage, spikes, etc.) or the wiring on the unit or even then appliances being plugged in. Unless you can see some thing like an overheated neutral wire, it's just gonna take good ole fashion elimination via troubleshooting.

 

[smithacetech]

After re-reading this post Im inclined to think its the microwave boards that are faulty. Perhaps a bad component or a bad solder point. Are the microwaves all the same? Do they all have the same boards, same dates of manufacturing? Maybe you can find a common problem & narrow it down?

 

[bbaker45152]

Larry,

Thanks for you input. When you say seperate jumpers for each receptacle, do you mean simply running 2 short wires from the neutral line to each receptacle?

 

[bbaker45152]

I was wondering when someone would ask or suggest that. The microwaves are all the same. They are commercial Sharp ovens. I have been trying to tell Sharp that they have an issue. They claim they do not see this issue anywhere else. I am just trying to get to the bottom of this all. I've got fingers pointing in all directions.
Oddly enough, there was a model change after the first 25 units. The original 25 had never gone down. The second wave of 75 has been where the issue is. I have not been able for anyone to tell me what was changed in the new model. The wiring was done exactly the same way in ALL of the units. Mystery time.......................

 

[smithacetech]

hmmmmmmmm.......

I would definately check into these microwaves. Or you could even replace the bad microwave with a comparable model or brand in the same unit & see if the problem is still there.

 

[Dennis Alwon]

I would run a separate neutral. I have seen problems with microwaves years ago on MWBC. That neutral has to be right on a MWBC-- this was stated earlier.

 

[rattus]

Yes, but:

Yes, but:

It is conceivable that the neutral is lost temporarily when connecting or disconnecting the twist lock. This could cause the neutral voltage to pull away from ground which might cause an overvoltage on the PC board. QUOTE]

I may be just guessing but, isn't the neutral blade on a twistlock plug longer or bigger than the hots for this reason.

That may be true, but with worn or pitted contacts in the receptacle, you might have an intermittent condition.