bure961
Senior Member
- Location
- Farmingham, MA
Some people use color codes. So if its red and black wiring it a very good chance its dc.
How to tell AC/DC in the field
- Thread starter Eddy Current
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K8MHZ
Senior Member
- Occupation
- Electrician
If you mean the off the shelf 'tick' tracers, no, they do not detect DC.
There are similar detectors that need contact made to a single metal probe that will detect DC, but not without making true contact.
The closet thing I have seen (and owned) was a no contact DC ammeter. You place the insulated wire in two notches on the back of the meter and read out the current. It also shows the direction of the current. It's function is to determine if an automotive electrical system is functioning properly.
K8MHZ
Senior Member
- Occupation
- Electrician
I see more red and black conductors in 3 conductor NM than I do in DC wiring.
Using insulation color is a bad way to determine voltage and current type.
I have seen every color there is used on both AC and DC systems. Ford even used red for the ground on some of their Ranger trucks in the 90's.
Always test, never guess.
kwired
Electron manager
- Location
- NE Nebraska
I never said it could be. It is very common to transform AC to the needed voltage then rectify it to DC though.
My point was that sometimes if you recognize the source you will know whether you have AC or DC before any testing or reading nameplates, etc. If you have equipment that has a battery as the source it is DC. If it connects to premises wiring it is probably AC supplied, but may have rectifier at the front end and the majority of the thing operates on DC voltage. Your typical desktop computer would be this way. The AC input immediately hits the power supply unit which transforms the incoming voltage then rectifies it to DC. There is no AC utilized beyond the power supply component.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
Or L1 and L2 of 240V split phase.
Maybe not transformed but it can be inverted to produce AC.
The output stage of a variable frequency inverter is one pretty common example of this.
K8MHZ
Senior Member
- Occupation
- Electrician
Even tougher to ID is pulsating DC. Pulsating DC is connected to a transformer, but it is called a coil. An automobile ignition coil for example.
It takes pulsating 12 volt DC (yes, it has polarity, on a vehicle it will be that the polarity of the frame of the vehicle is used on the switching side) and outputs high voltage (10 - 30 kV) DC. This could also be called pulsating, but since the distributor makes for a longer break between pulses, it's more like intermittent DC.
One reason cars switched from positive to negative ground was to make the spark plugs last longer by switching the polarity of the electrode. Actually, that probably was the main reason.
It takes pulsating 12 volt DC (yes, it has polarity, on a vehicle it will be that the polarity of the frame of the vehicle is used on the switching side) and outputs high voltage (10 - 30 kV) DC. This could also be called pulsating, but since the distributor makes for a longer break between pulses, it's more like intermittent DC.
One reason cars switched from positive to negative ground was to make the spark plugs last longer by switching the polarity of the electrode. Actually, that probably was the main reason.
- Location
- Massachusetts
If it is DC, grounded and under the NEC one conductor would have to be white.
White could be positive or negative, you would have to test to tell.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
This reminds me of the high school physics assignment to describe a way to measure the height of a tall building using a barometer.
Among the answers given were:
1. Drop the barometer off the roof and measure the time it takes to hit the ground.
2. Attach a string to the barometer, lower it from the roof almost the ground and measure the period of the resulting pendulum.
3. Climb the stairs with the barometer and a pencil, measuring the height of the building in units of "barometers".
But my hands down favorite was: "Find the building superintendent and offer to give him the barometer if he tells you the height of the building."
Eddy Current
Senior Member
There would be no white ie. neutral wire in a DC system right? Just positive and negative.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
White (or gray, or a few other combinations) does not identify a neutral under the NEC. It indicates a grounded conductor.
If the DC circuit is negative ground, the negative will be white and the positive may be almost any other color, including black.
If the system is positive ground, the positive will be white and negative may be almost any other color, including red.
For an ungrounded DC system, there is no requirement that the negative be black and the positive red. But neither of them can be white or green.
In some non-NEC DC circuits, negative will be green and positive will be red.
And finally, in a balanced DC system of the type Edison introduced there can indeed be a neutral. Something like three wires: {+120, 0, -120}.
But if instead it is {0, +120, +240}, under the basic definition of neutral, the +120 wire is a neutral, since it is offset by the same amount (120V) from both of the other circuit conductors.
So a neutral does not even need be be a grounded conductor, except under some specifically identified situations.
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fmtjfw
Senior Member
- Location
- Fairmont, WV, USA
Neon tester
Neon tester
Using a neon test light, one side will light for a suitable DC voltage, two sides for a suitable AC voltage.
Neon tester
Using a neon test light, one side will light for a suitable DC voltage, two sides for a suitable AC voltage.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I would still use my DVM.
As would I. See post #18.
broadgage
Senior Member
- Location
- London, England
DC can not be transformed from one voltage to another with a traditional transformer.
DC can be transformed from one voltage to another by rotating machinery, basicly a DC motor to suit the supply voltage, that drives a dynamo to produce the desired voltage. Adds considerable cost, complication and losses and therefore avoided if possible.
These days, DC can be transformed from one voltage to another by means of DC to DC converters that use electronics, these are a relatively new invention, certainly not available in the Edison era.
And, no, non contact voltage tracers do not work on DC.
broadgage
Senior Member
- Location
- London, England
Many DC systems had a grounded neutral, often white in colour.
In DC days it was common practice to distribute 3 wire DC, with a grounded neutral, a positive outer, and a negative outer.
This reduced the amount of copper required and gave two voltages. In the USA normally 120 volts from either outer to the neutral, with 240 volts between the outers.
UK systems were similar in principle, but commonly at twice the voltage.
K8MHZ
Senior Member
- Occupation
- Electrician
Some clarification.
DC can be transformed into higher voltages using a transformer if the DC is pulsed. That's how automotive ignition systems work.
Pulsating DC will be detected with a no contact tracer rated for the voltage. They make them to check for current on spark plug wires.
DC can also be dropped from one voltage to another by the use of solid state voltage regulators.
Technically, DC to DC converters don't convert DC to DC. They change DC to AC using electronics, then they change the AC voltage and rectify it back to DC.
It can also be increased.
Step up or step down choppers. We've done them up tp 3000V and 6000kW.
K8MHZ
Senior Member
- Occupation
- Electrician
That's some big choppers!!
Quite so.
Our first forray into it was on paper machine drives.
Conventionally, they were DC using phase-controlled rectifiers for speed control.
There were two major drawbacks with this system.
The power factor, particularly at reduced speeds, was poor to very poor.
And the harmonics drawn from the supply were high.
Our approach was to provide a common DC bus fed typically from a a 24-pulse diode rectifier. That fixed the poor power factor and high harmonic content at a stroke.
Original DC motors were generally retained with the obvious cost savings.
Speed control was by a semiconductor switching device between the DC bus and the DC motor. A step down chopper.
Some of the drives have to be regenerative - the process drives them rather than being driven by them. Because the motor voltage is lower then the DC bus, they had step up choppers to stuff the leccy back into the DC bus.
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