Some basic electrical questions

[GallenH]

I have some questions about electrical wiring in general. My plan is to put a cutoff switch for the cabin battery in my 97 PW and adding to that electrical the Victron battery monitor. I’m running a cable from the battery negative to the switch—a cable from the switch to the Victron shunt—a cable from the shunt to a bus bar that will have the cabin load/grounds. So here goes:

1. Switch: Blue Sea. The m-series is 300A; The e-series is 350A. With my single battery setup I figure the greatest draw is probably the 12v heater element on my old Dometic fridge. I have no inverter and the rest of the loads are the usual suspects (water pump, heater fan, ceiling fan and interior lights). Are the amp ratings of either switch acceptable?

2. Busbar: Currently there are 3 wires connected to the battery negative. Do I need to get a 4-post bus bar or will a single-post or double-post be ok? I don’t need the cabin load/grounds separate. They never were. Again I’ll be going with Blue Seas product. They have ones rated at 250A and 600A. Would the 250A work ok?

3. Cables: Amp ratings on the Blue Sea products assume 4/0 wire. Is that a good choice or is it over kill? I’m estimating the total wire loop will be 8 feet. But is 4/0 a safe bet for minimizing loss? Also does anyone have any recommendations for brand?

4. Currently I have wires going from the battery neg and pos to a cable that exits the battery box and terminates in MC-4 connectors at the hitch on the rear of the van. I plug my solar suitcase into these connectors when I hook it up. Do you recommend including this in the “cut-off switch loop” or just leaving them? There’s never any load unless I connect up the solar panel.

I think that’s it and, as always, appreciate greatly any advice.

thanks.glenn

[markopolo]

It seems unlikely that the house loads in your van would exceed 40A. Even that would likely require the fridge to be on DC, the water pump running, the furnace fan running and some lights. That's also assuming that the charger you choose is rated 40A or less.

6 gauge wire would be adequate.
4 gauge wire would be more than adequate.

I think you've already mentioned that you have no plans to add a second battery or large inverter.

The 250A rated stuff is fine.

I like the idea of using a 4 post or buss bar.

These terminal fuse blocks are easy to use:
https://www.bluesea.com/products/519...k_-_30_to_300A

or dual fuse block https://www.bluesea.com/products/215...k_-_30_to_300A

The dual fuse block would mount on the battery positive terminal. One fuse could be coach main wire and the second fuse could be for the wire from the isolator / separator.

[mck150stone]

Don't have advice; but curious as to why switch the negative as opposed to switching the positve?

Is there a reason that switching negative is required or advantageous; Done a lot of electrical work over the years but the only RV I've worked is mine.

Just wondering
Paul J

[Sehc]

What is the actual amps you expect? What is the distance for the battery cable? 4/0 sounds very heavy. My max amps are 200 and I use 2/0 for 4 feet over all one way. I went with the larger Blue Sea 600 amp bus bar and the Blue Sea M series switches. It is about voltage drop. Note; 4 awg (21 sq mm) is not 4/0 awg (107 sq mm).

[GallenH]

@Paul J. Now that you bring it up: I'm not sure. When putting the RV in storage I always disconnected the battery negative. That probably comes from the car procedure that I follow where you don't want a disconnected positive cable accidentally touching the grounded chassis/frame. With a switch, however, that wouldn't be the same issue. Blue Sea's wiring diagram does show it connected to the positive, between the battery and the boat engine. I'm not sure that it makes a difference. Anyone else have thoughts?

[GallenH]

@markopolo. Many thanks! I know from your other posts that you've done a lot of wiring and your advice is greatly appreciated.

[GallenH]

@Sehc. Yes! I did mean simply 4ga wire.

[avanti]

I switch positive. If you are literally disconnecting everything, I don't see where it matters one way or the other. However, I have two items upstream of the master switch: (1) the solar controller, and (2) the Trik-L-start. I want these always connected, since I park outdoors and want them doing their job all the time. If anything remains connected, I think it is important to maintain the integrity of the chassis ground, which means positive switching.

[mck150stone]

Most systems if not all systems that I've work on disconnected the load from the source on the hot side (hot in relation to neutrals and grounds); and most maintain solid connection on grounds and neutrals.

If a system starts getting a little complex it seems like things might get a little confusing.

Again not giving advice so much as just talking about normal practices.

Thanks for your response,
Paul J

[Scottie409]

Originally Posted by mck150stone

Don't have advice; but curious as to why switch the negative as opposed to switching the positve?

Is there a reason that switching negative is required or advantageous; Done a lot of electrical work over the years but the only RV I've worked is mine.
lll
Just wondering
Paul J

swtching negative is standard practice in automotive systems as it prevents arcing

[avanti]

Originally Posted by Scottie409

swtching negative is standard practice in automotive systems as it prevents arcing

I believe what you are thinking of is the rule that you should always remove the battery terminals negative-first. This practice is only pertinent to service operations such as replacing the battery. It has to do with the danger of shorting a hot wire to a still-connected chassis.

It has nothing at all to do with the design of a switching system (which was OP's question). The current being switched is exactly the same at either terminal. There is no difference in arcing.

[browngw]

My 1997 Pleasure-Way STW had a DC cutoff switch under the passenger side bed near the rear. It was in the area of the external breaker (with reset) for the frig.

[mck150stone]

Originally Posted by Scottie409

swtching negative is standard practice in automotive systems as it prevents arcing

Interesting; automotive systems are not in my area of expertise.

Paul J

[booster]

I agree with Avanti, there should be no difference in arcing from switching positive or negative unless it is an electronic module that won't turn on internally until it sees a ground path, and I don't recall any of them in vehicles.

[Bruceper]

There are three schools when it comes to battery switching.

1) Doesn't matter

2) Electrons flow from negative to positive, so disconnect the negative and no worry about drains. Unless of course something shorts out, makes it own ground and decides it doesn't need a battery ground and drains your battery into oblivion.

3) Disconnect the positive, usually due to sanctioning rules for racing which also includes the alternator going through the disconnect switch to de-energize the electrical system. Done for safety reasons and not practicality. There is usually an internal switch, an external switch and an inertia switch to kill the system in case of collision.

Here's my take, disconnect the negative if you just don't want the battery to drain while stored due to parasitic losses. However if you are concerned about fire, disconnect the positive.

[avanti]

Originally Posted by Bruceper

There are three schools when it comes to battery switching.

1) Doesn't matter

2) Electrons flow from negative to positive, so disconnect the negative and no worry about drains. Unless of course something shorts out, makes it own ground and decides it doesn't need a battery ground and drains your battery into oblivion.

3) Disconnect the positive, usually due to sanctioning rules for racing which also includes the alternator going through the disconnect switch to de-energize the electrical system. Done for safety reasons and not practicality. There is usually an internal switch, an external switch and an inertia switch to kill the system in case of collision.

Here's my take, disconnect the negative if you just don't want the battery to drain while stored due to parasitic losses. However if you are concerned about fire, disconnect the positive.

I don't understand this at all. It doesn't make the slightest difference which direction the electrons move. Without a complete circuit between the battery's negative and positive terminals, there can be no current. It can't "make its own ground". A switch on either side will break the circuit.

The racer's rules make sense. If you have two current sources, then switching positive is safer in a negative ground system.

[Bruceper]

Originally Posted by avanti

I don't understand this at all. It doesn't make the slightest difference which direction the electrons move. Without a complete circuit between the battery's negative and positive terminals, there can be no current. It can't "make its own ground". A switch on either side will break the circuit.

The racer's rules make sense. If you have two current sources, then switching positive is safer in a negative ground system.

Try starting your car and disconnecting the negative and tell me what happens. Some modern vehicles have protection against this, lots don't.

Originally Posted by avanti

It doesn't make the slightest difference which direction the electrons move.

Of course it does. What you probably meant to say was "it doesn't matter to the reader which direction electrons flow". I added fluff that shouldn't have been in there, too much information leads to confusion.

Originally Posted by avanti

Without a complete circuit between the battery's negative and positive terminals, there can be no current.

This is entirely incorrect. The chassis is, for all intents and purposes, ground. It has nothing to do with positive and negative. For current to happen there needs to be a voltage potential. A battery's positive connection meeting a ground source provides potential. This is why cars that are parked sometimes catch on fire due to the presence of a ground loop causing the potential to exist.

I'm going to take a stab and again say I think you meant "A car can't start without the complete circuit between the battery's negative and positive terminals."

Originally Posted by avanti

It can't "make its own ground". A switch on either side will break the circuit.

Correct. But since this seems like a discussion on semantics "making it's own ground" is a poor choice of words, but it's simple language that people with little understanding of electricity will understand. Shall we call it a short? A ground fault? A ground loop? How many people have been killed or hurt touching an RV because of a ground loop?

tldr;
For the most part everyone is really part of group 1. It doesn't matter which side you disconnect. All other decisions are technical mumbo jumbo.

[avanti]

I am not trying to be argumentative at all, and I'm pretty sure that we are simply not communicating. However, if so, we are REALLY not communicating...

Originally Posted by Bruceper

Try starting your car and disconnecting the negative and tell me what happens. Some modern vehicles have protection against this, lots don't.

Try disconnecting the POSITIVE and tell me what happens. The issue has to do with regulators that depend on the battery load for proper operation. IT DOESN"T MATTER WHICH SIDE YOU DISCONNECT.

Of course it does. What you probably meant to say was "it doesn't matter to the reader which direction electrons flow". I added fluff that shouldn't have been in there, too much information leads to confusion.

Not only doesn't it matter to the reader, it doesn't matter to ANYONE who is interested in practical electrical engineering of automotive systems. It matters to people doing esoteric physics experiments, but going there would be pedantic.

This is entirely incorrect. The chassis is, for all intents and purposes, ground. It has nothing to do with positive and negative. For current to happen there needs to be a voltage potential. A battery's positive connection meeting a ground source provides potential. This is why cars that are parked sometimes catch on fire due to the presence of a ground loop causing the potential to exist.

As far as I can see, this is nonsense. You appear to be deeply confused.

Are you saying that in this configuration, the bulb will light?:

notacircuit.jpg

If so, you are wrong. If not, I just don't understand what you are saying. You are correct that for a current to happen you need a voltage potential. But there IS NO SUCH POTENTIAL between the battery and ground (ignoring trivial edge cases such as tiny static charges).

If the battery were a capacitor, then it WOULD light (very briefly). But a battery is a galvanic cell. It has no charge, so there is no current. A galvanic cell requires a circuit in order for it to create a charge/current.

I'm going to take a stab and again say I think you meant "A car can't start without the complete circuit between the battery's negative and positive terminals."

Correct. But since this seems like a discussion on semantics "making it's own ground" is a poor choice of words, but it's simple language that people with little understanding of electricity will understand. Shall we call it a short? A ground fault? A ground loop? How many people have been killed or hurt touching an RV because of a ground loop?

Sorry, I don't understand any of the above. This is not semantics, it is physics. In the case of batteries: no circuit, no current.

[booster]

Semantics may be getting in everybody's way.


Some points:


The minus side of a battery is not ground as such, it is just the negative.


The chassis of a motorhome is not earth ground unless it is on shore power or something is connecting it to earth. It is called chassis ground for some reason or safety ground, and is also the negative of the batteries, so take your choice.


The only ground that really matters is earth ground, and AFAIK you don't have any alternate names for it, and it is the universal reference for power systems.


In an RV, you can have a voltages on any or all wires in respect to earth ground and or chassis ground as long as the plus and minus combine for the correct voltage for whatever they are running. I think it is called "push/pull" voltage generation.


"Ground loops" as I was taught would be when a voltage/signal get connected to a ground that also does not have a true ground connection to bleed the signalpower off to. It then feeds back into other stuff. Very common in audio and video stuff. For things like in a van, I would call it a short, either to ground or to another wire of different voltage.

[avanti]

Originally Posted by booster

The only ground that really matters is earth ground, and AFAIK you don't have any alternate names for it, and it is the universal reference for power systems.

I don't completely agree with this. There is nothing special about earth ground. It is simply a handy reference point that is available to everybody. Voltages are relative, so you always have to ask "voltage relative to what?". In order to have some kind of standard, we by convention often say "relative to earth ground". But, "Mars ground", or "RV chassis ground" are just as valid, and work the same way.

In an RV, you can have a voltages on any or all wires in respect to earth ground and or chassis ground as long as the plus and minus combine for the correct voltage for whatever they are running. I think it is called "push/pull" voltage generation.

Well, sometimes. If the chassis of the RV is somehow bonded to earth, then this statement is correct. But if not, then the whole system is floating relative to earth, and is is not meaningful to say that the wires in the RV have a voltage with respect to earth. This is my basic point: No circuit, no current.
[Again, this ignores static charges of the kind you perceive when you touch a doorknob. That can and does happen between the vehicle and earth, but any current is fleeting and not relevant to this discussion, AFAICS.]

"Ground loops" as I was taught would be when a voltage/signal get connected to a ground that also does not have a true ground connection to bleed the signalpower off to. It then feeds back into other stuff. Very common in audio and video stuff. For things like in a van, I would call it a short, either to ground or to another wire of different voltage.

Not exactly. Ground loops have nothing to do with "true ground". They occur when there is more than one connection to a "ground" (ANY ground, including a floating chassis). Since any ground (even Earth!) has finite resistance, the potential will be different at different connection points. If there is ALSO a connection within the device being powered (forming a phantom circuit), current will flow through the ground, and create issues such as hum in an audio device. There are various techniques for avoiding ground loops. In an RV, one good one is to always create a strict branching "tree" of grounds, with the chassis as the "root".