FAO ARW or other electrically-minded souls

If it its DC and all the cables start from the battery then yes, clamping across all the cables will give the total current as all the wire magnetic fields will be in the same direction and can't cancel to give an erroneous reading.

You will get the total current through all three in your instance. If the positive and negative leads ran together at some point and you clamped around both you would get zero (ok, probably with a small error term).

In the same way if you clamp around a normal AC cable with two conductors, you will see zero (even on an AC meter).

Clamp meters work best on big currents, the accuracy you get will depend on the meter.

Thanks Andy and Newshound, that's brilliant.

if current is small, you can put multiple turns through the clamp if you have sufficient slack and can fit it in ... then divide down by number of turns.

As long as the three currents are running in the same direction it should measure the total.

However be aware that *most* clamp meters are AC only, there are some that do DC as well but they are more expensive and much less common.

I think most clamp ammeters these days, regardless of price, can measure both AC and DC currents. They tend to use cheap hall effect sensors and can measure much lower currents accuratly provided the meter is zeroed before measurement to cancel out any local dc magnetic fields.

The last time I looked on eBay suggests that AC only clamp ammeters were far cheaper than one that can do DC as well.

I suspect the rate of drift of the sensor is the crucial issue.

This guy would know - he does respond to questions:

Aaah! the infamous DC transformer.

Works best on fusion generators or those free electricity you tube projects :-)

AB From the well pummelled keyboard of Archibald Tarquin Blenkinsopp Esq

F*ck yeah, I'm so glad that increasing the turns on a DC solenoid doesn't increase the magnetic field.

The clamp detects the average magnetic field, so if the currents are all in one direction the sum of them all.

Obviously. Think of a multistrand wire...

Well that of course depends on other factors as well.

As I got constructively dismissed by Clive Sinclair for explaining to him. There is only so much magnetic field that can be stored in a lump of ferrite, and if the frequency you operate it at is fixed, that limits its power output no matter how many turns you put on it.

IMHO, on a car, they're a waste of time. Unless measuring starter current.

Crikey. You certainly mixed with the top drawer. ;-)

My clamp meter claims to do AC and DC, and indeed it can - but it can only measure DC using the probes and not the clamp.

Strange - mine measures DC via the clamp, and I wouldn't say it's expensive - it's a UNI-T UT210E

I got mine a while back now. Measures to 400A, have measured car starter at 150A. DC has two ranges, 40A FS and 400A FS. There are four AC ranges. I've only measured around 15 amps on an AC range.

Paul

For constant 'I', then field is proportional to N.

But for any given voltage driving the solenoid, increasing the number of turns using the same diameter wire the magnetic field stays the same. Because NI stays the same; where I is proportional to 1/N. Of course the power consumed does go down proportional to N.

So it depends, but making a blanket sarcastic statement doesn't do much for you.

A DC Clamp meter measures the flux in the core and can't be considered a transformer.

Only someone ignorant of magnetic fields would call the magnetic circuit of a clamp meter a 'transformer'. They best stick with what they know, presumably re-arranging 1's and 0's in what they hope is the right order for some of the time.

A DC Clamp meter measures the flux in the core and should never be considered a transformer. More turns increases the flux correspondingly.

Some of us live in the real world and don't have time to waste at silly projects.

That explains it.