Was wondering how to calculate how much amperage will be on a grounded conducter. I understand in theory it's purpose on a mutli-wire branch circuit, but how is it different in 3-phase? I was working on a 225 amp main lug only panel today and the phases were somewhat balanced and it made me think.

Black leg had 42 amps.
Red leg had 53 amps.
and Blue leg had 41 amps.

But the nuetral had 25 amps. Does this seem right to you?

quick answer:

In=SQRT[(Ia^2+Ib^2+Ic^2)-(Ia*Ib+Ib*Ic+Ic*Ia)]

In=SQRT[(42*42+53*53+41*41)-(42*53+53*41+41*42)]

In=SQRT(6254-6121)=SQRT(133)= 11.5A

So, 25A seems too high.

What kind of loads do you have, and what type of equipment are you using to measure the nuet?

Harmonic currents????

The loads are mostly emergency 2x4 lay-in lights that are always on.

I used my Fluke Model 332 amprobe/ voltage/ resistance tester.

Well, from a "Basic" standpoint, it would be easy to say that the 25 Amps found on the Common Grounded Conductor (AKA "Neutral", or "Noodle" for a more Technical description ...), would be the highest L-N Load value of all the 3 Lines,...

BUT...


Thought #1:

The Emergency Ballasts may introduce a more Linear / True Power looking Load characteristic (they look a lot more like a Pure Resistance Load, than the Standard Ballasts do) - and therefore attribute a significant part of the L-N Load Amperes across the 3 Phase Circuitry, with a resulting 40% to 55% remaining as an unbalanced Noodle Load,

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Thought #2:

In addition to the Fluorescent Lighting Fixtures, there may be a large percentage of Incandescent &/or Quartz-Halogen type Lighting driven from the same Panelboard,

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Thought #3:

In addition to the Fluorescent Lighting Fixtures, there may be some Resistance Heaters driven from the same Panelboard,

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Thought #4:

The highest L-N (unbalanced) Load being 25 Amperes, found on any single Line (A, B, or C), two Lines (A-B, B-C, or A-C), or from all three Lines (A, B & C), with the remaining Load Amperes being L-L &/or L-L-L.

This kind of looks feasible, since the Loads across ØA and ØC are nearly equal (41 and 42 Amps), and the Load on ØB is quite different (53 Amps).
There could be L-L Loads across A-B, and B-C, along with each Line having L-N Loads, which would result in figures such as the ones you found,

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Thought #5:

The Overall Power Factor(s) of the Loads on all three Lines is very close to Unity, therefore the L-N vs. L-L-L balance of Current may be "Partially" figured as a Vector Sum (as described by JBD's formulas).

If the Loads were _ALL_ Unity Power Factor (1.0 PF,100% True Power, or Pure Resistance), then the Formulas posted by JBD would apply, however, the results show an Imbalanced Load of 11.50 Amperes, so there must be something else driven from this Panelboard, which has yet to be discovered.

Your readings sound OK to me, only that they appear to be "hinting towards a mixture of Power Factors" found on the connected Loads, or the possibility of L-L / L-L-L connected Loads, or both scenarios.

Could you post a reply with some additional information - such as:

* Are there any 2 or 3 Pole Circuits from this Panelboard,

* Are there any True Power Loads on any Circuits from this Panelboard (Incandescent / Quartz Lamps, or Pure Resistance Heating Elements),

* What do the Loads on each 1 Pole Circuit readout as, when the previously mentioned Load levels are found on the Panelboard's Feeders,

* Do the Load levels on the Feeders stay fairly steady over time, or do they fluctuate,

* What are the results on the Feeder's Common Grounded Conductor ("Noodle" / "Neutral"), when a Single Pole Circuit is turned off (if this is possible to do, without getting somebody pissed off!!!),

* Any other data you feel like posting.

Please note that, while I am asking for a bunch of stuff - and mentioning a lot of possible reasons for the Load readings, there are _LIKELY_ no problems with the System you are dealing with.*
I am just setting the scene for some "Academic Fun-Time", plus to promote an interactive testing situation.

(*I include this disclaimer to point out that from what you posted, these would be readings found under one of many conditions - however, due to Murphy's Law, there may be something installed by "Sparks-A-Flyin' Electric, Inc." which could be viewed as "Having A Very Intense Sucks Value" - so I need to indemnify myself...)

Scott35

edited for one of many grammatical miss-steaks found "post-posting"...
If additional blunders exist, please "mentally edit" as needed...

Me...

[This message has been edited by Scott35 (edited 06-17-2006).]

I would have expected a bit less with those phase loadings but I'd only be concerned if the neutral was carrying more than half the value of the heaviest loaded phase.

Scott35, thought #4 seems to be the most likely scenario. I am pretty sure there was at least one 3-phase load (L-L-L-) operating from the panel. I'm not exactly sure what that load is though because of course nothing is identified except for the fancy red tape beside each of the SP circuit breakers. Also my Fluke tester is a 322, not a 332

Any chance you can turn off the 3-phase load(s) and do another set of current readings?

Not likely. Unless I am called back there to do some other work. I suppose I could when the business is closed.

Thinking about this particular problem leads me back to the discussions of neutrals and 310.15(B)(4).

The neutral here is generally not considered a current carrying conductor.

But consider: if you had a 40A, 120V purely resistive load connected from phase A to neutral, and a 40A, 208V purely resistive load connected between phases A and B, you would end up with 40A on each conductor, including the neutral, with absolutely nothing wrong and purely resistive loads.

-Jon