Single Phase load in three phase system.

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Md Shafiyar Rahman

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Hi,
Good day,
Suppose, I am working in a project e I found, Red=2KW, Yellow=1.9KW & B= 1.87 KW 1phase load and a 5KW motor load. I selected individual protection devices for them. But when these loads will be combined in 3phase breaker in same DB. What will be the total load for the DB?
As per my previous work, the total load will be 5+2+1.9+1.87=10.77KW.

But my confusion is why should like that? 3phase & 1phae load are never same. 3phase needs less current. 1phase need less currents. Why we are not dividing the 3phase loads by 3?

Thanks & Regards,
Shafiyar
 
If you deposit $2,000 into one bank account, $1,900 into a second account, and $1,870 into a third account, how much was your total deposit? Would you want the bank to give you a receipt for one third of that total?

As seen by the upstream power source, KW plus KW equals KW, no matter whether the loads are single phase or threCurrent.

Current does not behave the same way. You need to first add all KW, divide that result by the phase-to-phase voltage, and divide again by the square root of three, in order to calculate current. That is because current leaving on one phase will return to the source on another phase. The two don't add, as they are the same current.
 
Not referring to anyone in particular, it is surprising to me when people sometimes seem to think that in three phase systems a fused disconnect with three 100A fuses means that the total current available is 300A when they wouldn't think that in a single phase system a fused disco with two 100A fuses the total current is 200A.
 
ggunn said:
Not referring to anyone in particular, it is surprising to me when people sometimes seem to think that in three phase systems a fused disconnect with three 100A fuses means that the total current available is 300A when they wouldn't think that in a single phase system a fused disco with two 100A fuses the total current is 200A.
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many have posted exactly that idea on this and other forums
 
ggunn said:
Not referring to anyone in particular, it is surprising to me when people sometimes seem to think that in three phase systems a fused disconnect with three 100A fuses means that the total current available is 300A when they wouldn't think that in a single phase system a fused disco with two 100A fuses the total current is 200A.
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In my experience, those who believe one believe both.
 
Yea this always been confusing.

Suppose you had a 3 phase 30KVA 480/240 Volt Delta/ Y transformer

The primary current is rated 30,000 VA / 480 V x 1.732 = 36 i amperes

Secondary current is 30,000 VA / 240 V x 1.732 = 72 i amperes

However a 3 phase system in a Y system is simply 3 single differential phases sharing a common neutral point but is the secondary output of 72 amperes for each leg to neutral common point or the total accumulated sum of amperes for all 3 legs and neutral current?
 
Jpflex said:
However a 3 phase system in a Y system is simply 3 single differential phases sharing a common neutral point but is the secondary output of 72 amperes for each leg to neutral common point or the total accumulated sum of amperes for all 3 legs and neutral current?
Click to expand...
Consider a MWBC; if there is a perfect balance, there need not be any neutral current between the load center and receptacles. Power factor aside (consider resistive loads for this thought exercise), the current on any one wire will not be much more than the imbalance. That is why code considers three current carrying conductors in a four wire three phase circuit.
 
Jpflex said:
Yea this always been confusing.

Suppose you had a 3 phase 30KVA 480/240 Volt Delta/ Y transformer

The primary current is rated 30,000 VA / 480 V x 1.732 = 36 i amperes

Secondary current is 30,000 VA / 240 V x 1.732 = 72 i amperes

However a 3 phase system in a Y system is simply 3 single differential phases sharing a common neutral point but is the secondary output of 72 amperes for each leg to neutral common point or the total accumulated sum of amperes for all 3 legs and neutral current?
Click to expand...
First the secondary voltage of a Wye connection will typically be 208Y/120 although some data centers have been using 240Y/138V.

Yes, the secondary would look like it is made of three transformers, each with the Line-Neutral voltage and 1/3 the total KVA. Because this is three phase the currents need to be added 'vectorily' at the common junction point.

So the secondary current can be determined as you did 30,000/(240x1.732) or by the line to neutral voltage alone; 10,000/138.
 
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Jpflex said:
However a 3 phase system in a Y system is simply 3 single differential phases sharing a common neutral point but is the secondary output of 72 amperes for each leg to neutral common point or the total accumulated sum of amperes for all 3 legs and neutral current?
Click to expand...
It won't be 72a in the wye configuration.
 
Md Shafiyar Rahman said:
Hi,
Good day,
Suppose, I am working in a project e I found, Red=2KW, Yellow=1.9KW & B= 1.87 KW 1phase load and a 5KW motor load. I selected individual protection devices for them. But when these loads will be combined in 3phase breaker in same DB. What will be the total load for the DB?
As per my previous work, the total load will be 5+2+1.9+1.87=10.77KW.

But my confusion is why should like that? 3phase & 1phae load are never same. 3phase needs less current. 1phase need less currents. Why we are not dividing the 3phase loads by 3?

Thanks & Regards,
Shafiyar
Click to expand...

For the sake of this discussion, we'll assume they're constant power loads. Real power is always simple addition no matter how they're fed. Amperage (or volt-amps) on the other hand do not necessarily add up.
 
I get 83.3a at 208v for 30Kva.

You either multiply the voltage or the current by 1.732, but the result is the same.
 
Jpflex said:
Top gone says line to neutral would be 16 amperes but I’m not sure where he gets that figure
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That would be the maximum line amps for the 3, single-phase loads. Adding the line amps of the motor load will give you the total current of the total load.
 
topgone said:
Short answer: Maximum line amps = 16.2 amperes. (Vector math).
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That's for 3-phase 208V L-L and the 3 single phase loads of 2 kVA, 1.9 kVA, and 1.87 kVA each connected L-L in a balanced fashion, if you assume that each single phase current has the same phase offset from the applied voltage.

Since the load values are all very close to each other, just adding them up and dividing by 3*120V (or sqrt(3) * 208V if you prefer) gives a very close approximation, 16.0 A.

Cheers, Wayne
 
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