What is the definition of the "Maximum Phase Current"? 1

[plangetee]

Hello all

I am producing a One-Line and am asked to find the "Maximum Phase Current" based on all the loads on the circuit. In my case loads consist of some Transformer Rectifiers, several motors (all less than 10 HP), & some heaters.

I know HOW to find the Maximum Phase Current. My question is WHAT does it actually mean? What useful information would it give to, say, a customer who knows what it is?

 

[rbulsara]

It helps them size the source circuit, service and/or transformer/ generator etc.

If the load is unbalanced, one of the phases will have higher current than the other two, assuming a 3 phase source. The 3 phase source still needs to be sized based on the phase with highest current.

Say, the calculated load is 15, 10 and 10 amps on phase A, B and C respectively. The service has to be 15A, 3 Pole minimum or even greater based on local codes.

This also illustrates importance of balancing the loads, when all loads are not balanced 3 phase.



Rafiq Bulsara

http://www.srengineersct.com

 

[plangetee]

rbulsara thanks for the response. I attached what my phasing diagram looks like.

I'm being asked if the Max Three-Phase Current is for the "Connected Load". I'm not sure what this means. Any thoughts?

 

[rbulsara]

That has written "homework" all over it. You may want to read the this site policy rules.

You may also want to think if it is not for connected load, what else could it be for? You have all the information you would need for the calculations. (to calculate current in each phase). Do the exercise and see what numbers you get.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Actually you may be missing some information. If your loads have differing power factors, you cannot add them directly.
Another joker is the HP/kW without knowing the efficiency or power factor.
Sounds like homework. Not a real world application.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[plangetee]

Lol its for work. I finished school 2 yrs ago, never learned this stuff there, learning it now. I am assuming pf and efficiency are both around .85.

Anyways sounds like no one can actually answer my original question, I will ask chief electrical guy tomorrow.

 

[rbulsara]

This website is not a substitute for schooling or lack of it. This is meant for professionals.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

And yes, learning from your seniors at your work place is a much better way to learn and it will also be pertinent to your work.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Consider your loads from A to B. If the power factor of the two loads is identical the total load will be 58.9 KVA x 2 = 117.8 KVA. However if one load has a power factor of close to 0% and the other has a power factor of close to unity, the total load may be in the range of 1.414.x 58.9 = 83 KVA.
Although it may not apply in your situation, if one load has a PF close to zero lagging and the other has a load of close to zero leading, the combined KVA may be close to zero and in any event much less than 58.9 KVA.
PF matters.
The maximum phase current is usually the maximum current that may flow in the most heavily loaded conductor under any condition of connected load. This does not mean all loads connected. If some loads are PF correction caps, maximum current may occur with the caps disconnected.
Phase loading may be phase to phase or phase to neutral. Maximum phase current may refer to maximum phase conductor current. This is used for sizing switch gear, conductors and protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Connected load simply means you are assuming all loads are connected and running at full capacity. No load cycling or diversity.