Simple Panel Loading Question 1

[hanksmith]

I am having issues,

Please see attached, I am trying to find out the total load per phase for some heat tracing, I was told the heat tracing would use about 30A, 208V, Single phase. 30 X 208 +3600W

We are planning on using a 208/120V 3 phase panel, connect the single phase load across 2 phases. The question come up with how to display this on my panel schedule, again see attached, wasn't sure if I should be putting 3600W per phase or divide this by 2 to put 1800W per phase.

Hope I was clear in this if not I will try to clear up any questions that come up.

Thanks

 

[rbulsara]

Firstly, 30A*208V = 6240W. If you balance them perfectly between 3 phases(A-B, B-C and C-A) , you would have a balanced three phase load.

That will give you per phase amps=6240W/(SQRT 3*208V)=17.3A per phase.

Or 6240/3=2080W per phase, devided by per phase voltage i.e. 2080W/120V=17.3A per phase.

 

[rbulsara]

Ahh, I assumed you have multiple heat trace totalling 30A.

Ignore above reponse if it is single and ONLY load on the 3 phase system, it will show up as 30A on EACH of the two phases (lines) you connect to and 0 on the third one.

 

[rbulsara]

To complete my second post, the watts per used phases would be 6240/2=3120W each and 0 on the unused one.

If you look at per phase quantities, the voltage per phase is 120V,but 30 degree out of phase from the current (which is in phase with line to line voltage). So watts per phase would be 30A*120V*cos(30)=3117 (or 3120)W. The 3W differrence is because of rounding off of numbers.

 

[alehman]

I think the branch circuits on the schedule look ok.

If the heat trace would be considered continuous load (likely to be on >3hrs), then the maximum load allowed on a circuit is 80% of rated capacity (24A for a 30A breaker). If it's not continuous this is still poor design and will likely experience nuisance trips.

The load totals show >100% load, which is not be allowed. The same 80% rule applies for the main.

 

[waross]

How's this.

Heat Trace CCT #1 6240 W. Brk #1,
Brk #3

Heat Trace CCT #2 6240 W. Brk #5,
Brk #7

Heat Trace CCT #3 6240 W. Brk #9,
Brk #11

Heat Trace CCT #4 6240 W. Brk #13,
Brk #15

Heat Trace CCT #5 6240 W. Brk #17,
Brk #19

Heat Trace CCT #6 6240 W. Brk #21
Brk 23

Heat Trace CCT #7 6240 W. Brk #25,
Brk #27

Total load, 7 Ccts X 6240 = 43680 W

The loads are not alanced on all thre phases.
Round up to 9 Ccts x 6240 W = 56160 W
(We are adding enough imaginary load to balance the phase loading. This is by far the easiest way to account for the more heavily loaded phases.)

200 amp panel x 120 volts x 3 phases = 72000 W
7200 W x 80% = 57600 W usable.

150 amp panel x 120 volts x 3 phases = 5400 W
5400 W x 80% = 4320 W
A 150 amp panel is too small. Use a 200 amp panel.

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

 

[Compositepro]

Obviously I'm not a electrical engineer but isn't there a beter way to distinguish between a "phase", which is between two wires, and "a wire which one of the phase currents pass through"? There have been a number of threads where people have been confused by this. I understand that this is often just shorthand to speed communication but it does lead to confusion, particularly for new engineers or electricians.

 

[waross]

All circuits have at least two conductors. The conductors are usually but not always wires. Some circuits have more than two wires, such as three wire single phase circuits and three phase circuits.
A "phase" may be from a line to neutral, or from line to line.
Thus, on a 120/208volt panel, "A" phase may be a 120 volt circuit from Line or Phase "A" to neutral, or
"A" phase may be a 208 volt circuit from "A" phase to "B" phase.
There are also circuits that use "A" phase, "B" phase and the neutral. Because of the connection to the neutral, this type of circuit may be referred to as an "A" and "B" phase circuit.
The context or additional information (such as the rated voltage) usually indicates which type of circuit is being discussed.
And, to add to your confusion, The current on "B" phase resulting from a load on "A" and "B" will not be in phase with the current from a load from "B" to "C", and will not be in phase with a current from "B" to neutral. These currents may not be added directly but must be added vectorily. Hence my advice to balance a panel with "phantom" loads to determine sizing.

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

 

[Compositepro]

Thanks, Bill. That is just my point. The term "A-phase" has muliple meanings and can even be used two different ways in one sentence. That just doesn't make much sense but that's life.

 

[waross]

Strive for balance and perspective in life. Think of this issue as just one "Phase" of life.

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