separate circuit, common ground

[azazzera]

I know its not best practice but i am looking for an answer to this question. I wired two independent circuits with a common ground at first thinking that that would be ok. when i attached the positive wires to my block and checked to see if they had continuity between them i was surprised and shocked to see that they indeed showed continuity. I thought i had a short until i drew it out on paper and realized that the electricity flow was going through the common ground and through the other load and out the other positive wire. I now understand why I'm getting continuity through the two separate positive wires (because there not so separate) My question is can i keep it like this or is it a big no no and why. they will be both switched by circuit breakers on the panel. my only gap in understanding is what problems may happen when they are both on. I understand that if one light is on at a time the other positive wire will be back fed with electricity back to the other circuit breaker. Its a combination steaming deck light. deck light is halogen and steaming is incandescent. plan to change out steaming light with LED bulb.

Just me talking this out convinces me that i would rather have them both completely separate and will just run a new wire up the mast but am still curious if what i did is in code or not.

Thank you. AZ.

[ndutton]

A common ground for two DC circuits is allowable provided the ground wire is of sufficient size for the combined loads of the 2 circuits. No concerns over backfeeds with the common ground. An example is your car. Most of the circuits - and there are several - use the chassis or body as a common ground.

[azazzera]

That's good to know that it's permissible. It was a pain to wire that mast to start with. No problems with the incandescent bulb changed to an LED. If I have the halogen deck light on and the Led steaming light off no ill effects to the LED?

[ndutton]

The thing about LED's is they can be sensitive to low voltage conditions so the size of the wire is important. Did you use a voltage drop calculator when sizing the wires (figures load and circuit length)?

[azazzera]

I used the app that blue sea offers. It asks voltage,amps of load what voltage drop you want, I chose 3%,not in engine room, terminated on fuse, length of run, number of wires in bundle, type of scotch you prefer, you know questions like that and it spits out a gauge for you. Then I went up one gauge just to be safe. Are you familiar with the app and is it the type of tool you were asking about when you aske if I calculated voltage drop?

[BunnyPlanet169]

I used the app that blue sea offers. It asks voltage,amps of load what voltage drop you want, I chose 3%,not in engine room, terminated on fuse, length of run, number of wires in bundle, type of scotch you prefer, you know questions like that and it spits out a gauge for you. Then I went up one gauge just to be safe. Are you familiar with the app and is it the type of tool you were asking about when you aske if I calculated voltage drop?

Smoky single malt is the more robust solution - sounds like you are 100% on track.

[ndutton]

Yes, exactly what I meant, well done. The combination of the LED (minimal to insignificant load) and bumping the wire gauge one size beyond the calc, the ground wire should handle the sum of the loads easily.

I'm curious, did you already have your electrical design knowledge or did you pick it up on this forum? Either way, you're good.

[azazzera]

I wouldn't exactly refer to it as knowledge. I feel I know just enough to be dangerous. The difrence is I know that and am wise enough to as for advice and help when I need it. I new the gray hair ment something. Mostly curiosity and this forum to flesh it all out. If one never asks a question but simply reads the conversations it will be enough.

Endless gratitude. Adam z.

[ndutton]

Well said Adam.

[tac]

It's common to use the water/plumbing analogy to explain electricity. The analogy is not perfect, but for simple DC systems it's not bad. "Backfeeding" in both cases is unlikely. Compare your boat to the water system in your house. Your two light circuits are analogous to the kitchen and bathroom sinks Both have a common current source (battery electrons/water molecules), something to push them with (battery voltage/city or well pump water pressure), controls (breakers or switches/faucet valves), loads (lights/whatever you're washing), and a return to source (battery return at zero volts/waste drain at zero psi).

In the boat case the current source is the battery, whose "pressure" is the voltage that pushes electron current through wires when the switch is closed. Note when the switch is open, the voltage (pressure) at the source side of the switch is still there, but no electrons flow.

In the house the water source is water from city or well, whose pressure (psi) pushes water molecules as flow through pipes when a valve is opened. When the faucet is off, water pressure still exists in the pipe, but no water flows.

In both cases the fluid (electrons or water) flows (amps or GPM) from higher pressure (voltage/psi) to lower. The lower in these cases is circuit common (battery negative post, zero volts) and zero pressure at the house main sewage drain. These fluids can only flow from lower voltage/pressure to higher if pushed. In which case, they would again be flowing from higher to lower pressure. For a boat "pushed" means using a battery, battery charger, alternator, solar panel, etc.

In the case of your two light circuits with a common return, note they also have a common source (the battery). Continuity exits between their source wires because they are connected together. Continuity exits between their return wires because they are connected together. They are separate circuits because their loads are in parallel, each with its own switch (breaker). Note the similarity to your house sinks.

When you close the breaker for light A, leaving the one for light B open, only light A will turn on. After flowing through light A the voltage is zero, or near zero, since at that point it is essentially battery return (common). For electrons to flow from this point (common) back through light B would require two things that don't exist:
1. A voltage high enough to push electrons from battery return back through light B. However, the voltage at this point is already near zero, and electrons don't flow uphill without a pump.
2. In any case, the breaker/switch for light B is still open. Thus current cannot flow through light B because there is no path back to the battery, the source of the electrons.

In your case, I assume you disconnected the source (boat battery) from the panel, closed the breakers for lights A and B, and measured the resistance from the A to the B breaker. Keep in mind that the multimeter has a battery inside that provides the voltage and current to measure this resistance. Thus you are creating a series circuit from the red meter lead (higher voltage, meter battery +) through the A breaker and A load, to common return, and back through the B load and B breaker, to the black lead (lower voltage, meter battery -). Since both bulbs are incandescent (halogens are incandescents) I would expect to see a very low resistance - less than 10 ohms.

I've often heard the word "backfed" used in electrical and hydraulic circuits. It cannot exist unless there is a voltage/pressure to allow flow from a higher to a lower voltage/pressure.

[ndutton]

[Off topic]
Backfeeds are all too common and damaging in multi-phase AC circuits. Conditions that must exist are:

• Minimum of 2 circuits on different phases sharing a common neutral.
• Loads (circuits on) on the two circuits. These may be sentinel loads like a clock on a microwave appliance, small night light, illuminated switch, whatever.
• Loss of the neutral at the source.

Last one I worked on was a lost neutral at the power pole. It vaporized one dimmer, two GFCI receptacles and a stove circuit board. After a protracted claim and investigation process the serving utility will be paying for the homeowner's repair expenses.

Note: this applies to multiphase AC circuits. I repeat my comment in post #2, no concerns over backfeeds with the common ground in terms of on-board DC wiring and the reason this is off topic.