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Old 04-30-2019, 04:21 PM   #1
M2M
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Question Install disconnect between 1 and 2, or 3 and 4?

And please explain why you think your way is better.

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Old 04-30-2019, 04:29 PM   #2
Patrick L.
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Most install it in the ground lead. Many use more than one ground lead which makes that troublesome.
I'm one that uses 2 ground leads so I mount it in the starter lead. I don't see that much difference.
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Old 04-30-2019, 05:12 PM   #3
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Do whatever you want, just be aware that if you install it on the power side, you maintain a potential to chassis, as long as you keep that in mind it won't make a difference. The Model A uses the battery as a filter and voltage regulator so never run the car with battery dissconnected.
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Old 04-30-2019, 05:28 PM   #4
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

First off, put it in a convenient place so you can use it without opening the hood. I’ve put it on the ground side in RV’s, boats, hot rods and Model A’s.
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Old 05-01-2019, 07:21 AM   #5
marty in Ohio
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

I put mine in the ground between #1 and #2. I did that because Tom, at Snyder's, told me that was the best way to break the connection. Snyder's sold me the cables that fit perfectly when installing the switch in the seat riser in my '30 town sedan. Snyder's has never steered me wrong.
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Old 05-01-2019, 07:38 AM   #6
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Quote:
Originally Posted by eagle View Post
Do whatever you want, just be aware that if you install it on the power side, you maintain a potential to chassis, as long as you keep that in mind it won't make a difference.

Not to be argumentative, but how do you maintain a potential to the chassis? Once the flow of electrons is stopped by an open circuit, there is no longer a potential... Electrically, there is no difference which side of the battery you choose. I found that it is more important to place it conveniently so that it is easier to use than not to use...


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Old 05-01-2019, 07:46 AM   #7
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

The first consideration of a disconnect is security - you can leave the car open circuit and a thief of limited knowledge will need a tow truck to steal it.

The second consideration of a disconnect is protection from fire - if you suddenly have electrical problems you can reach down and hit the switch and remove the primary cause of fire.

I would consider the first rationale pretty incredible. Even if you leave the keys in the car most modern thieves would be hard put to even know how to start it. More than likely any potential thief today would bring a tow-dolly as his thievery tool of choice and carry the car off in the dark and covered by a tarp. No disconnect protects from that.

The second rational I would consider more credible - although 6V (or even 12V) isn't near the fire hazard that 120VAC is - but having a switch to disconnect might make fire-fighting efforts less restrained and more positive.

So I think I would put the switch within reach of the operator - or where an exiting operator might reach it quickly. Either 1 to 2 or 3 to 4 can accomplish this depending on the switch.

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Old 05-01-2019, 08:21 AM   #8
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Quote:
Originally Posted by Patrick L. View Post
Most install it in the ground lead. Many use more than one ground lead which makes that troublesome.
I'm one that uses 2 ground leads so I mount it in the starter lead. I don't see that much difference.
The switch works the same either way, it opens the conducting path. The argument against placing it on the hot side is that it exposes more surface area to a dropped wrench or other such object. Similar reason you disconnect the ground side of the battery first, reconnect last. Hot side won't throw sparks with the ground side disconnected.
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Old 05-01-2019, 09:28 AM   #9
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

two comments,

1. placing the switch on the hot side of battery means that you are pulling the starting current thru the switch. this may not be desirable, don't ask.

2. placement of the switch should be on outside of car somewhere. have a friend who's shop burned down, yep Model A caught fire from battery/wiring. in the racing world it is the rescue responders who will need to kill the power, if switch is inside and car on fire getting to switch is difficult.

in any event, it is a good idea to have the switch, John
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Old 05-01-2019, 10:02 AM   #10
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

If you put it on the ground side, between 1&2, be sure to tape off the cable from the switch to the battery. If that makes ground somehow, you are live again.
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Old 05-01-2019, 10:19 AM   #11
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Quote:
1. placing the switch on the hot side of battery means that you are pulling the starting current thru the switch. this may not be desirable, don't ask.
It doesn't matter which side of the battery the switch is in, the starting current is going to go through the switch either way.

FWIW, In house wiring and other electrical, by code the switch is always in the "hot" side.
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Old 05-01-2019, 11:55 AM   #12
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

If you put the switch in the hot cable, the cable between the battery and the switch will still be hot even with the switch off. If the cable rubs through and touches a ground there may be a fire. If you put it in the ground cable even if that cable touches the a ground nothing will happen because it is just going to ground.
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Old 05-01-2019, 12:48 PM   #13
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

I put it on the ground Side , 1-2. My disconnect is on the seat riser so it was easier to wire that way.
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Old 05-01-2019, 12:59 PM   #14
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

On the ground as what I have worked on in the past that was OEM, it was always in the ground cable. That's not saying a disconnect switch won't fail, I've replaced a few $50 switches in my lifetime, but not on a Model A.
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Old 05-02-2019, 02:24 AM   #15
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Quote:
Originally Posted by wmws View Post
If you put the switch in the hot cable, the cable between the battery and the switch will still be hot even with the switch off. If the cable rubs through and touches a ground there may be a fire. If you put it in the ground cable even if that cable touches the a ground nothing will happen because it is just going to ground.

I asked this question also on Facebook and someone had a similar reply:

"The best side is on the chassis connection (1-2 in this case)and the reason is that the live pole of the system is only exposed a few places at wiring terminals. The chassis ground, however, is almost everywhere on the car. Hence if you drop a tool on the disconnected ground pole, the probability that the other end of the tool will by chance contact the live side wiring is almost non existent except for the case of shorting across the battery poles themselves, which a disconnect switch will anyway not guard against. On the other hand, if you with the switch on the live wiring side of the battery drop something on the disconnected pole of the battery, chances are pretty high that the other end of the thing will contact the chassis creating a monumental short."

So I think I'll go between 1 and 2 and I already have a good disconnect switch. Which leads to the question; what do you use from the battery to the switch, and then from the switch to the chassis? Coppers straps or wires? If you use wire for all or part of the job what AWG wire size do you use?
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Old 05-02-2019, 03:31 AM   #16
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Hallo,
I use isolated copper wire. 25 mm˛, that has about a thicknes 10 mm. Up to 180 Ampere.
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Old 05-02-2019, 09:00 AM   #17
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Werner, try insulated, not isolated.

I would use insulated cable, minimun 1/0 AWG

https://www.theautomationstore.com/w...version-chart/
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Last edited by katy; 05-02-2019 at 09:01 AM. Reason: Addition
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Old 05-02-2019, 10:00 AM   #18
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Quote:
Originally Posted by M2M View Post
... Which leads to the question; what do you use from the battery to the switch, and then from the switch to the chassis? Coppers straps or wires? If you use wire for all or part of the job what AWG wire size do you use?
Same size wire in and out - same current. Straps work too on the ground side, so long as they are equivalent size. Bratton's sells #1 battery cables, Snyder's 2/0, which is probably overkill, but won't hurt.

Quote:
Originally Posted by Werner View Post
Hallo,
I use isolated copper wire. 25 mm˛, that has about a thicknes 10 mm. Up to 180 Ampere.
I assume 10mm diameter includes insulation. 25 mm˛ is close to #3 AWG, which is rather small. 50 mm˛ is a better choice. Overheating your cable should not be a problem because of the short duration of starter operation, but the insulation may be compromised if it needs to be run under load for an extended time.

Last edited by Badpuppy; 05-02-2019 at 10:22 AM.
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Old 05-02-2019, 10:06 AM   #19
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

Just spit balling. For the switch in 1 -2 battery/frame ground, for the dual ground lead issue. Could you not run one heavy duty lead from the battery to the switch, and then on the other side of the switch go to two places with 2 cables?
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Old 05-02-2019, 10:37 AM   #20
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Default Re: Install disconnect between 1 and 2, or 3 and 4?

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Originally Posted by 30 Closed Cab PU View Post
Just spit balling. For the switch in 1 -2 battery/frame ground, for the dual ground lead issue. Could you not run one heavy duty lead from the battery to the switch, and then on the other side of the switch go to two places with 2 cables?
Yes you can.

You need the heavy cables only between battery and starter (both hot and ground). For the rest of the system only a 12 AWG wire to the frame is required, but it's usually already common with the starter ground. Where is this other ground wire going?
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