neutral current calc

[electgut21]

I don,t have a code book at this time and I need to find out how to size the neutral conductor in a 3 phase 4 wire system. I plan on attending classes soon to increase my understanding in this area. Right now I am trying to figure out a bad setup .

 

[augie47]

I don,t have a code book at this time and I need to find out how to size the neutral conductor in a 3 phase 4 wire system. I plan on attending classes soon to increase my understanding in this area. Right now I am trying to figure out a bad setup .

a day late... see :

http://forums.mikeholt.com/showthread.php?t=120102&highlight=neutral

 

[bob]

I don,t have a code book at this time and I need to find out how to size the neutral conductor in a 3 phase 4 wire system. I plan on attending classes soon to increase my understanding in this area. Right now I am trying to figure out a bad setup .

Is this for a 3 phase service? If so 250.24(C1) requires the neutral be sized not smaller than shown in 250.66 .

 

[Dennis Alwon]

Is this for a 3 phase service? If so 250.24(C1) requires the neutral be sized not smaller than shown in 250.66 .

This is true if it is single phase also. The grounded conductor of a 3 phase 3 wire delta service shall have an ampacity not less than that of the ungrounded conductors.

 

[ptonsparky]

For multiwire branch circuits. At least equal to the phase wires. (If you are allowed to use this method at all in your area.)
For SE with all motor load. Minimum allowed 250.66. Other types of loads up to 100%. If an engineer is involved maybe an oversized neutral.

 

[electgut21]

Yes it is for a 3 phase system . I don,t quite understand the formulas shown. But thats on me. IF shown a sample using readings I think I will understand.
Got to go to work. Take care.

 

[bob]

Yes it is for a 3 phase system . I don,t quite understand the formulas shown. But thats on me. IF shown a sample using readings I think I will understand.
Got to go to work. Take care.

What formula? You just read table 250.66.

 

[Smart $]

Yes it is for a 3 phase system . I don,t quite understand the formulas shown. But thats on me. IF shown a sample using readings I think I will understand.
Got to go to work. Take care.

The formula(s) posted on that other thread have nothing to do with properly sizing a neutral.

Are we talking the neutral of service or feeder conductors?

If so, Article 220 only permits a couple methods to determine size of a service or feeder neutral. For the calculation mehtod, you would total the calculated line-to-neutral loads on each line and use the highest value, aka maximum unbalanced current, to size the neutral, but it canot be smaller than a comparable GEC sized per Table 250.66... nor is it required to be larger than the largest phase conductor of the service or feeder

 

[electgut21]

I like to know how to calc. the unbalanced current in the neutral wire . I don't know what the correct current should be.

 

[ptonsparky]

I like to know how to calc. the unbalanced current in the neutral wire . I don't know what the correct current should be.

There is no correct current for a neutral other than not exceeding its ampacity. It will vary as lighting is turned on, coffee pots cycle, and seasons change.

 

[Varczar]

Elecgut21,

I often dont have a code book within reach either. The link below is to the 2008 NEC. I use this all the time since someone on this forum posted it a while back - so I am just passing it on. Good luck!

http://nfpaweb3.gvpi.net/rrserver/browser?title=/NFPASTD/7008SB

 

[Smart $]

I like to know how to calc. the unbalanced current in the neutral wire . I don't know what the correct current should be.

I'm still not sure what your objective is. Please elaborate more. Is this about properly sizing a service or feeder neutral conductor or determining what the current value is through a specific neutral conductor is when subject to line-to-neutral loads connected thereto...???

 

[electgut21]

Tom, let me say it this way. L-1 = 96A.
L-2= 118A
L-3=112A
What should N=? I should know this but I don,t. 208/120 4 wire.

 

[Smart $]

Tom, let me say it this way. L-1 = 96A.
L-2= 118A
L-3=112A
What should N=? I should know this but I don,t. 208/120 4 wire.

Provided each line exhibits the same power factor, 19.7A. This can be determined using the formula posted in the other thread linked to earlier in this one.

However, if you read the entire other thread, you'd know that the neutral current can vary widely with differing power factors. With a variance of .1 pf (lagging) on L2, the neutral current shoots up to 65.3A... and to get this answer, you'd need to possess a means to determine the power factor on each line and then use vector math (or it's diagramming equivalent).

 

[mivey]

Tom, let me say it this way. L-1 = 96A.
L-2= 118A
L-3=112A
What should N=? I should know this but I don,t. 208/120 4 wire.

Listen to Smart$ and the others. To use the formulas given you have to make some assumptions. You will only know what the neutral current is by taking into account the complete vector, not just the magnitudes of the current vectors.

We can calculate a worst case, best case, etc. but you really need to know what kind of loads you have (i.e. the load impedance, which gives you the pf).

Why not just measure the neutral current (and ground if needed)?

Alternately, you could also measure the phase angle between the voltage and the current associated with that voltage. Then, you could also measure the phase angles between the voltages, but it would be close enough to assume an even displacement for most cases. With this you could calculate the real neutral current.

 

[electgut21]

Thanks fellas for all the help.