Calculating amperage

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AJones

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Location
Houston Texas
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Maintenace Electrician
When using an amp meter on parallel runs load side on a 600 amp disconnect is it accurate to read each leg, L1 a + b etc to calculate total amps?
Or do I need to read all parallel runs per each leg together. I added each cable on each phase together to give my total. example: L1 a=105 b=65 Total 170 amps,
also repeated L2 and L3.
 
AJones said:
When using an amp meter on parallel runs load side on a 600 amp disconnect is it accurate to read each leg, L1 a + b etc to calculate total amps?
Or do I need to read all parallel runs per each leg together. I added each cable on each phase together to give my total. example: L1 a=105 b=65 Total 170 amps,
also repeated L2 and L3.
Click to expand...
If you are getting significantly different current readings on the two conductors of the same leg in a parallel set, then it seems to me that they are not a parallel set. Current applied to a pair of identical conductors tied together at both ends should split evenly between them. Some small variation is normal, I guess, but 105A on one conductor and 65A on the other sounds excessive to me.
 
ggunn said:
If you are getting significantly different current readings on the two conductors of the same leg in a parallel set, then it seems to me that they are not a parallel set. Current applied to a pair of identical conductors tied together at both ends should split evenly between them. Some small variation is normal, I guess, but 105A on one conductor and 65A on the other sounds excessive to me.
Click to expand...
Thanks. Actual numbers are L1 a70 b40; L2 a70 b30 and L3 a55 b42. This disconnect is feeding 480 panel(s) and 2-480/208 step down transformers
all fed to a gutter with polaris taps...all available equipment was running.
 
LarryFine said:
That's a huge difference! If they're really in parallel, they should be very close, the goal being identical.
Click to expand...
You are correct I shouldn't have said parallel as these are actually splitting off from the disconnect and energizing separate systems.
 
AJones said:
Thanks. Actual numbers are L1 a70 b40; L2 a70 b30 and L3 a55 b42. This disconnect is feeding 480 panel(s) and 2-480/208 step down transformers
all fed to a gutter with polaris taps...all available equipment was running.
Click to expand...
Sounds like they are not actually in parallel, but each set is supplying separate loads.
 
AJones said:
You are correct I shouldn't have said parallel as these are actually splitting off from the disconnect and energizing separate systems.
Click to expand...
Okay you're forgiven. Yes, simply add the currents. The totals should match the line sides for each phase.
 
LarryFine said:
Sounds like they are not actually in parallel, but each set is supplying separate loads.
Click to expand...
You are correct. I know the difference I but really just described it incorrectly. I actually have no true parallel runs here other than the main shop.
They read within 5 amps of each other. This does tell me that I do have lots of room to add equipment safely and that is the initial goal.
 
A little bit of a diversion. I'm a Brit as many of you know. We don't use Amperage. We use current for our circuits.
 
If you are metering the RMS current feeding two _different_ loads and then adding the numbers, then you can potentially have a significant error.

If the two loads have the same power factor then the numbers just add. Otherwise the total current will be less than the sum of the RMS values.

Jon
 
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