3 Phase Current Question

[bristoluktraining]

Hi all my first post so go gentle, I am only training!!1

System spec

3 phase + Neytral 400v star system

I1 = 10A
I2 = 3A
I3 = 2A

What is the total 3 phase current?

I make it 7.5 A but a collegue makes it 10A can anyone offer some assistance? and explain how and why they get to their conclusion

Thanks in advance

 

[jghrist]

The concept of "total 3 phase current" has no meaning. If you are concerned with capacity of equipment, use the highest phase current.

 

[Dennis Alwon]

Are you looking for the current on the neutral?

 

[mivey]

You can see from the other posts that you calculated neutral current and your colleague calculated equipment capacity requirements.

Is this your question or question you were given?

 

[charlie b]

Much depends on the reason the question was raised in the first place. If this were to be a test question, then I would want to slap the person who wrote the question. As has already been mentioned, the notion of "total current" has no meaning whatsoever. If the question were intended to ask about the total load (not total current, total load), I would average the three currents, multiply by the voltage (400 volts) and multiply again by 1.732 (the square root of 3), and say the total load was 3,464 VA. If the question had to do with sizing the wires for this situation, I would start by sizing it for the largest load (as has already been suggested). But then I would look into ways to balance the loads, so that a smaller wire might be used.

Welcome to the forum.

 

[shamsdebout]

The concept of "total 3 phase current" has no meaning. If you are concerned with capacity of equipment, use the highest phase current.

What would you say if a PE (15 years experience) with the result of a line test from an electrician on each of the three phase on a 208/120V system tell me to add the currents up to determine the system capacity?

 

[Besoeker]

Much depends on the reason the question was raised in the first place. If this were to be a test question, then I would want to slap the person who wrote the question. As has already been mentioned, the notion of "total current" has no meaning whatsoever.

I totally agree on both points.

If the question had to do with sizing the wires for this situation, I would start by sizing it for the largest load (as has already been suggested). But then I would look into ways to balance the loads, so that a smaller wire might be used.

A little off topic from the question actually asked...but for what it's worth...
It isn't common in UK to use anything smaller than 1.5 mm^2 (about #14AWG) for installations which is rated at more than 10A for the least favorable installation method. For commercial and industrial installations it is usually steel wire armoured clipped direct or on a perforated cable tray and that give a rating of around 20A. In short, for the loads given, I personally wouldn't fiddle around trying to balance them.

 

[Besoeker]

Are you looking for the current on the neutral?

That works fine for the specific case of all loads having the same power factor.

 

[Besoeker]

Hi all my first post so go gentle, I am only training!!1

System spec

3 phase + Neytral 400v star system

I1 = 10A
I2 = 3A
I3 = 2A

What is the total 3 phase current?

I make it 7.5 A but a collegue makes it 10A can anyone offer some assistance? and explain how and why they get to their conclusion

Thanks in advance

Hi fellow Brit and welcome the forum!
It's a good place with lots of helpful information.

I quite like Bristol - it has life and diversity. In fact, I like the West Country generally. Off topic but maybe the mods will let it go if I ask nicely?

 

[shamsdebout]

That works fine for the specific case of all loads having the same power factor.

And when they are not?

 

[LarryFine]

Welcome to the forum!

I1 = 10A
I2 = 3A
I3 = 2A

What is the total 3 phase current?

In my opinion, the answer to the question is 10a. You must be able to supply the greatest load.

 

[bob]

And when they are not?

you don't get the correct result. It may be close but not correct. Depends on the PF.

 

[charlie b]

What would you say if a PE (15 years experience) with the result of a line test from an electrician on each of the three phase on a 208/120V system tell me to add the currents up to determine the system capacity?

Well, I would tell him or her to go back to school, and relearn what three phase systems are all about. But then, I am a PE with 35 years experience, so I can get away with that. :grin:

What I would suggest to you, should you get such a request, is that you ask for clarification of the question. Ask if he or she is looking to size the conductors, or to determine if the source has capacity for adding load, or trying to figure out whether it is acceptable to leave that degree of imbalance in the loading, or what.

 

[charlie b]

Hi fellow Brit and welcome the forum!

It's a good place with lots of helpful information.

I quite like Bristol - it has life and diversity. In fact, I like the West Country generally. Off topic but maybe the mods will let it go if I ask nicely?

As long as it doesn't hijack the thread. But I am left to wonder why there is a UK-related member name and a zip code from Los Angeles.

 

[skeshesh]

As long as it doesn't hijack the thread. But I am left to wonder why there is a UK-related member name and a zip code from Los Angeles.

I see you know about the law prohibiting the British from living here in LA :grin:

And when they are not?

Basically it makes it more complicated. Assuming unity PF you're assuming balanced angles (i.e 0,-120,-240) between the phases. PF of each phase will effect this (i.e. phase C@-120deg with PF1.0 ----> -180deg with PF0.5 (-120+inv. cos(.5)). Using the corrected angle you can use use x & y components of each phase current. Add up x & y components and with the result you can find the magnitude and angle of the neutral current.

I hope I remember this correctly and won't have to be schooled too hard (as usual) by the usual suspects...

 

[Besoeker]

And when they are not?

Often.
A motor, resistive heating, power factor correction.

 

[Besoeker]

As long as it doesn't hijack the thread. But I am left to wonder why there is a UK-related member name and a zip code from Los Angeles.

Charlie
Where did you see the LA zip code?
I have looked at all the posts and can't see it.

 

[bob]

Originally Posted by shamsdebout
What would you say if a PE (15 years experience) with the result of a line test from an electrician on each of the three phase on a 208/120V system tell me to add the currents up to determine the system capacity?

It's likely what he told you is correct. When you have unbalanced currents with a 120/208 volt system you can calculate the capacity by taking each phase amps x 120 volts and add them together. You can add the amps together x 120 volts and get the capacity. This may be what he was telling you.
If you have 10 amps per
phase 10 x 208 x 1.73 = 3600 w
10 x 120 x 3 = 3600 w
30 a x 120 = 3600 w

 

[Besoeker]

And when they are not?

My apologies. I misread your question as
"And when are they not?" and answered it accordingly.
To answer the question you did ask, you have to add the currents vectorially taking account of the relative phase angles.
It isn't too difficult if you have a reasonable grasp of trigonometry.