12v Conductor Orientation

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batch

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I have a job that is a large car dealer with many sets of LED Reverse Channel Letters. They shipped the letters with the 12v secondary conductors being black and white. I assumed the black was the + and the white the -.

I was wrong...

The other location from the same sign manufacturer had black and red conductors. The crew installing them thought black + and red -.

I told them the opposite.

No big deal but how would you have guessed the colors?
 
Documentation, or a call to the manufacturer. Never connect DC when the correct polarity is less than clear. Not all electronic components come with protective diodes.

When it comes to low-voltage DC, red is universally positive and black negative. However, forced to choose with white and black, I would again have taken black as negative.
 
I agree with Larry, when guessing about the polarity in DC, Black will always be negative and the "other" color will be positive. I used to see a lot of this confusion with fire alarm cables that contained black and white conductors. Electricians can't seem to get away from AC where white is the grounded conductor so they use white as the ground in DC.
 
infinity said:
I agree with Larry, when guessing about the polarity in DC, Black will always be negative and the "other" color will be positive.
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I would not count on that.


In RI the fire alarm code has chosen the colors we must use. Blue and white are used for signals and we must make blue positive and white negative.

In machine wiring DC circuits are often blue and white with a blue stripe.....I wish I could remember which is negative.

My point is that we can not assume anything in this global economy. Assuming is likely to let the smoke out.
 
iwire said:
I
In machine wiring DC circuits are often blue and white with a blue stripe.....I wish I could remember which is negative.
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Blue with white is a grounded DC conductor. It is supposed to be white with a blue stripe but most people do blue with a white stripe.

If ungrounded, both are blue.
 
On my RV, and all RVs, for the DC, white is ground and black is hot. It drives me nuts.
Its this way because on a RV, and elsewhere, black is hot and white is neutral or ground for the 120V systems, which on a RV exist together.

I remarked my battery leads red and green.
 
Good thing no one's mentioned positive-ground systems, like most communications systems. Oops, sorry.

FWIW, I've always done LV positive ground and ungrounded systems with black as negative (with respect to the other lead). Past that, marking and documentation is essential. (Haven't done FA circuits for years; and the old systems were all black wire, anyway.)
 
The only documentation was on the power supply. It called for black and red. I guess in retrospect black for ground could still make sense.

So is using a white conductor that has not been re-designated ok in this case?
 
Color-coding of low voltage wiring in RV's is extremely inconsistent. Most of the wiring I work with is colored as follows (the wires also have numbered labels):

House and chassis battery wiring: Red positive, black negative (grounded to chassis)

Wiring harnesses built in-house: Red +12V power, green ground, blue switch leg, orange signal (or additional switch leg).

Outsourced wiring harnesses: All wires are white, with tiny numbers printed on them.

Wiring supplied with devices: whatever colors the manufacturer felt like using.

The RVIA 12V standard does not specify which colors to use. It requires only that "All ground returns to chassis or power source shall be uniquely identified as separate from the supply conductors (i.e. color, marking, etc)" and that "Each circuit conductor shall maintain its established identification throughout its run."
 
I received a power supply today for the same job. This one shows a black(-) and white(+) on the secondary side. The same brand as the first one, just isn't showing red and black on the schematic. Also there is a brown conductor coming out of the primary side for a dimmer.
 
Not always true, but I've had luck using the darker color for negative and lighter for positive. I tested a crew of my guys years ago. We had been working together awhile and normally used certain colors for certain things.

Example:
Red and Blue
Red + Blue -

Brown and Blue
Brown - Blue +

So blue could be either one depending on which color it was mated with. It was just our unwritten system.

My color scheme seems to work with you wiring color code also.
Black = darker color -
White = lighter color +

Black = darker color -
Red = lighter color +
 
The answer is: there is no answer. Take something as simple as 12VDC in a vehicle. + = Red, - = Black (gnd) until you run it through a seven pin trailer connector. SAE specs for the trailer are Ground (-) is white, Red is for a turn signal and Black is hot. Of course, that's only true is somebody followed the SAE spec. YMMV
 
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