Electrical Junction Boxes
In many projects with voltage problems there may be a junction box that has not been discovered which is between the panel and the modern live line . I saw this because of the difference in colors of the wires. It would be great to find this junction and identify the wiring transition, that is - how they are joined together, what color goes to what, and where do they go from there. I would also inspect the integrity of all the wiring methods and come up with a plan to upgrade the wiring and bring it all up to code. Most of the time it is more cost effective to pull out all the original wiring and start from scratch, possibly with a new panel as well, but I'll let you be the judge of that.
Indications of a Lost Connection
This has all the indications that the black wire at the modern live line has lost its connection to power for what ever reason, and that you are experiencing a voltage feed back condition which commonly occurs through a 220 volt electrical device such as a water heater, clothes dryer or electric range.
What happens in a case like this is the power flows through the device, typically equipped with a heating element, and power is fed back to the panel supplying reduced voltage depending on the size of the 220 volt electrical device?
This can be due to a few faults, the easiest being a partial tripped main breaker which supplies power to the panel. I would start at the main panel and test for voltage across the two main lines or the secondary side of the main breaker. Your looking for 120 to ground or neutral from both hot legs, and 240 volts between the two hot legs. If a leg has been lost you will not get the 240 (or 220 volt - the power varies) reading, instead you will receive a weak 120 volt reading. This would indicate a lost leg of power.
Are there other circuits in this panel?
Is this old building in a residential area and not fed with a 3-phase or industrial service? Be aware that partial power can damage electrical and electronic devices and they should be unplugged. Tom, lets start with this - and let me know what you find out - ok? Let me know what you find. If you do not feel comfortable working on this, consider calling a professional.
At the Old Building Jobsite:
OK, here's what I know thus far. It's very difficult to tell exactly where the wire is coming from because, while the conduit is surface-mounted on the ceiling, the ceiling is about 15 feet high, so I can't easily access it to open all the potential junction boxes. This building is about 100 years old, and, as far as I can tell, all the old cloth-wrapped wiring has been abandoned and redone with metal conduit (rigid and flex), some flexible light blue plastic conduit, and some in Romex. Most of the wire in the conduit is just insulated wire, not Romex. There are some runs of Romex in the building, especially in the stud walls. Back to the circuit we are dealing with: The two black and two white conductors leaving the box in the same conduit do not appear to be related. In fact, upon closer inspection, one of the two black wires is actually gray. However, both of the black wires are fastened to two separate circuit breakers, which are on the same side of the box. I believe this means that they can not be 220v. Is that right? If the breakers are full size - not twins and they are side by side they will be on separate busses thus 220v between them
Identifying the Electrical Wires
The gray wire may be a ground wire, you should verify this. Noting that the circuit is not 240v, how is it possible that one conductor is reading 42 volts? Is it still feedback ? One way to see what is affecting the possible feedback is to turn off any 220 volt circuits and see if the 42 volt reading goes away. If so the one 220 volt circuit that is re energized and brings back the 42 volt reading will be the one to investigate for a possible open or short. This 220 volt device should be identified and tested at the device. You will most likely not have a 220 volt reading at the device and it the device will not be functioning properly. Is this a hazardous condition? If it is not a hazard, I will just cap off the black wire, and utilize the Red (hot) and White (common) then ground to the metal box. Would this be OK? It should be identified and corrected because there could be a hazardous condition. Now, I did notice that the breaker for the circuit in question powers the back room light bulb, two light bulbs in the basement, one electrical outlet in the middle section of the store, and whatever it is that turns on the A/C unit. Now, I realize that the HVAC is a 240v component, but this is the 120v feed that powers, I think, the HVAC switch solenoids and also the transformer which powers the thermostat (neither the HVAC nor thermostat were operational while I had the power cut off to the circuit in question). This HVAC Unit should be checked to see if it is functioning properly.
Back to the circuit panel-since the aforementioned conduit had only black and white wires, but no ground wires leaving the box, am I to assume that I should be sure to use metal boxes and ground each receptacle, switch, etc. to the boxes via pigtail? Is this a safe alternative to a non-grounded system? That is, using 3-prong outlets, but pigtail the ground screw to the boxes? If the conduit is a solid continuous run and you do in fact get a good voltage reading when checking to the metal box then yes - you should install a grounding bond screw with a pigtail to the receptacles. Regarding the panel's condition: I believe that both panels are A-OK. They are modern, circuit breaker types, Cutler Hammer, with all new wiring entering and exiting both.
As mentioned earlier, most of the wires are in conduit or are Type NM Cable Romex Brand.
Keep in mind that Romex should not be present in areas where it is vulnerable to damage, rather it should be protected by a wall covering such as sheet rock, otherwise the wiring should be replaced with a conduit protected wiring method. Regarding the 30 amp breaker: Seems to me that, if this is a 120v circuit, this should be changed to a 20 amp. Maybe someone was trying to save money by not spending $12 on an additional breaker. Is this what you were saying, to just change the breaker to get into spec? Yes - you should replace this breaker with a 20 amp.
Are there other circuits in this panel?
Yes. There are two panels fed off of one main from the alley. Both wires entering our building are black, appear to be 4 gauge, I'd guess. I will check the voltage on them tomorrow. Is this old building in a residential area and not fed with a 3-phase or industrial service? This is an old building on the town square. I doubt it is 3-phase, because most businesses here on the square would not have required it (restaurants, offices, Laundromat, hairdressers, etc. However, I do not know for sure. Is there a way to tell by looking? Usually you will see a 3-pole Main Breaker or 3-pole breakers for equipment such as HVAC, Refers, Large Mixers or Motors etc.