How do you calculate total 3 phase current draw?

[Steve Kelly]

Hi All,

I've looked all over the internet for a simple answer to this question and have found the answers overly complicated. I'm dealing with a 5 wire 208 3 phase system. There's a mix of inductive and resistive loads so the load is unbalanced. Using a clamp on ammeter I get the following:

L1: 5.1A
L2: 2.6A
L3: 1.0A
N: 3.4A

How do I figure the total amperage draw of the system? Is it an average of the 3 phases, or do I add them all up? The websites I've visited have calculations involving pf, KVA, Kw etc. Is there a way to figure total amperage draw based on the readings from the 3 phases alone?

Thanks
Steve

 

[Jraef]

For what purpose? The amperage draw is what you see. Amperage is always per phase, whether it's single phase or three phase. So what are you trying to determine here?

 

[iceworm]

For three phase amperage - FORGET the term "Total Current". That concept is useless.

If the three phases are reasonably balanced - list that current. For example, if A = 5.1A, B = 5.0A, C = 5.2A, you would say, "The current is 5.1A" That phrase means the currents are reasonably balanced and 5.1A is an average number. Do not say "Amps per phase", or "Amps per leg". Neither of those terms have any meaning.

If the currents are unbalanced as in your example, list the currents just as you did.
L1: 5.1A
L2: 2.6A
L3: 1.0A
N: 3.4A

That describes perfectly what is happening.

ice

 

[kingpb]

As stated by others forget amps.

Always work in KVA

 

[Steve Kelly]

Thanks!

Thanks!

Hi Guys, Thanks for your help!

Steve

 

[glene77is]

How do you calculate total 3 phase current draw?

Steve

Good question, and one that I have calculated before, including the unbalanced neutral loading.

My question , to extend your query, is this :

Given : a 100 year old commercial building, with 3 phase four-wire service.
Given : 3 phase 135 A cables pulled to this Sub-Panel Load-Center used as a big sub-panel.
Given : each of the bus bars appears to be of the 135 A size.
Given : total existing load pulled would max at 90 A.

Question : How much current load can be placed on this Sub-Panel, on the three Legs combined ?
An example of the question is 'Could we place loads like'
A=90A,
B=90A,
C=90A


Of course, we would be derate everything, and measure/calculate the unbalance, and can provide monitoring equipment.
We are only looking for an additional 50 A sub-panel load, and would never try to use the total available current.

Our other option is a new, long, additional service run to a new ( additional ) sub-panel, which is do-able.

Any thoughts ?