208V 1phase loads

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Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
Yep!
Larry%20Fine.jpg


I resemble that remark! :)

Funny how I can remember things like that and can't remember a code ref. that I just read!:D
 

mull982

Senior Member
I'm still confused as to wheather or not the correct answer is 75A or 86.5A for all six loads distributed across phases A and B in the OP.

My understanding is that loads in a panel shcedule were divided between the phases and added on each phase due to the fact that you cannot add the currents in each phase directly due to the phasor relationships. So since you cant add the currents in each phase linearly then we divide the loads kVA between the number of poles and use these values to come up with the "Average" kVA in each phase. This number when used to determine the current howerver wont be an exact number for each phase and is only an average suitable for sizing purposes.

Since the case of the OP has all of the loads across only two phases then the phasor relationships for each phase can be addes lineraly and thus give a value of 86.5A. So I am in the camp that says the correct answer is 86.5A.

Is my line of thinking here correct or is it way off?
 

Finite10

Senior Member
Location
Great NW
I'm still confused as to wheather or not the correct answer is 75A or 86.5A for all six loads distributed across phases A and B in the OP.


Since the case of the OP has all of the loads across only two phases then the phasor relationships for each phase can be addes lineraly and thus give a value of 86.5A. So I am in the camp that says the correct answer is 86.5A.

Is my line of thinking here correct or is it way off?

I'm in the camp with you and LarryFine;

Say we have a 208V/120 3phase panelboard with a main breaker. Say we connect to it 6 heating loads, each 3kva and 208V single phase. Say these loads are ALL across phase A and B.

So for EACH load, you have 3000/208=14.4A. Thus, on the panel, you have 14.4*6=86.5A on EACH leg, and thus 87A is the smallest it can be.

What is the minimum size the MCB can be? Looks like the OP wants min. size Main Ckt Breaker in the end.

86.5 x 1.25 continuous [424.3(B)] = 108.125 so 110A MCB [240.6(A)]
 

al hildenbrand

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Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
So what would the load be if all six heaters were distributed evenly across all three phases?
Six single phase 208 Volt 3 kVA heaters connected in a panel. . . . well that implies six two pole circuit breakers for simplicity's sake. Any one bus will have one side of FOUR two pole circuit breakers attached.

From this one bus bar, there will be six kVA (two heaters) to one phase and six kVA (two heaters) to the other phase. Ignore the remaining two heaters as they don't contribute to the current on this one bus bar.

6 kVA / 208 V = 28.85 A in a phase to phase load.

This one bus bar has two phase to phase loads connected, each totalling 6 kVA (two 3 kVA heaters per phase to phase load), but the currents in each load are 120? apart from each other so they partially cancel out, hence the division by the square root of 3.

(28.85 + 28.85) / 1.73 = 33.35 A current on phase A, or phase B or phase C when the heaters are installed in a balanced manner.
 

al hildenbrand

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Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Al you are forgetting the 3 phase in your last part. 28.5+28.5+28.5/1.73= 50 amps
No, I'm not.

Think about it.

How does the third current (phase B to phase C) of 28.85 add to the current in the conductor supplying the panel's phase A?

It doesn't.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Here, this may be part of the confusion.

The 18 kVA load is connected to a 3 ? 208 V supply in a DELTA. The is no neutral involved.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Well it does if a DP is connected from C to A. I am confused. :confused:
There are two DPs from C to A, and two DPs from B to A. Yes.

Putting DPs from B to C does not draw current from A. The 6 kVA of heaters between B and C will work at 100% whether heaters are connected from C to A or A to B.
 
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