Safely Maximize Power from a 100amp 3-Phase Delta Panel

[EricJ]

What is the maximum amount of power that can be safely loaded on a 100amp 3 phase delta panel assuming all loads are 240v?

120 - 120 - 208.

Here's what I was thinking:

120v * 100a * 80% = 9600w
120v * 100a * 80% = 9600w
208v * 100a * 80% = 16640w

Total = 35,840w

How would the loads be distributed?

 

[ActionDave]

This is the second thread that I remember when the OP gave his occupation as EIT.

What is an EIT?

 

[GoldDigger]

This is the second thread that I remember when the OP gave his occupation as EIT.

What is an EIT?

Engineer In Training?

https://www.google.com/search?q=eit+certification&ie=utf-8&oe=utf-8
I suggest that we put it back, approve, and explain the workings of a three phase 4-wire high leg delta.

If it were not for the center tapped neutral, the pro-forma delta and wye energy calculations for balanced loading would give the same result.
But in this case the phase relationships are not right and the current in the two ends of the center-tapped winding are not in phase with the line to neutral voltage.

 

[kwired]

What is the maximum amount of power that can be safely loaded on a 100amp 3 phase delta panel assuming all loads are 240v?

120 - 120 - 208.

Here's what I was thinking:

120v * 100a * 80% = 9600w
120v * 100a * 80% = 9600w
208v * 100a * 80% = 16640w

Total = 35,840w

How would the loads be distributed?

The panel can handle 100 amps per leg regardless of conditions below.

The source however is a different story. Full delta source with three equal sized coils can handle same current per leg.

Open delta or mismatched units in full delta are going to vary in capacity per leg, depending on what is used for the source.

 

[EricJ]

EIT is the certification you receive after college and before becomeing a PE.

I am helping a client explore the feasibility of a project and also want to understand how 3phase delta works a bit better.

The panel can handle 100 amps per leg regardless of conditions below.

The source however is a different story. Full delta source with three equal sized coils can handle same current per leg.

Open delta or mismatched units in full delta are going to vary in capacity per leg, depending on what is used for the source.

Assuming leg c is the delta leg, when a load is placed between a - c does leg c deliver more of the power because it has a higher potential?

 

[gar]

170317-1216 EDT

EricJ:

Look at the question you asked. Then look at your answer. Then ask yourself how does your answer correspond to the question. The proposed answer does not correspond to the question.

What is the maximum amount of power that can be safely loaded on a 100amp 3 phase delta panel assuming all loads are 240v?

Your question stated all 240 V loads. A 3 phase delta panel alone does not define what kind of delta you have. The delta source could be anyone of several types. But this really does not matter relative to the question ask. Note: your proposed answer implied that you were thinking of a high leg delta, but that was not stated in the question and does not matter for the question ask.

That you have specified a current limitation as part of the question, then this means your load must be pure resistance for maximum power.

What does safely mean? Here one must make some assumption. The logical assumption for this question is that the panel and power source can safely tolerate a 100 A load per leg on a continuous basis,

From a visualization perspective or even a practical resistive load, you can consider the load to be a wye. For a balanced resistive wye load the voltage from a line to the mid point of the resistors is 240/1.732 = 139 V. It is easy to see that this voltage is in phase with the line current. Therefore power can be calculated from V*I.

Thus, 1/3 of the total maximum power is 139*100 = 13,900 W, and total power is 41,700 W.

.

 

[EricJ]

Gar

Oops that was a mistake. I am specifically referring to a high leg delta configuration.

By safely I really mean without overloading any breakers or fuses and having enough head room so that plugging in a small appliance or stero or something of the sort doesn't trip a main breaker or fuse.

I think my question was worded poorly. I guess what I'm wondering is if it is possible to distribute 240v loads on a high leg delta configuration in such a way that the current from all three legs are balanced thus utilizing the full power capacity of the panel? Or will the current on the high leg always be less?

 

[GoldDigger]

Gar

Oops that was a mistake. I am specifically referring to a high leg delta configuration.

By safely I really mean without overloading any breakers or fuses and having enough head room so that plugging in a small appliance or stero or something of the sort doesn't trip a main breaker or fuse.

I think my question was worded poorly. I guess what I'm wondering is if it is possible to distribute 240v loads on a high leg delta configuration in such a way that the current from all three legs are balanced thus utilizing the full power capacity of the panel? Or will the current on the high leg always be less?

If you balance the line to line loads evenly across all three phases, it will result in equal currents in all three ungrounded wires.
The fact that you are looking at a high leg delta does not change that in the least.

Your problem is that if you put balanced line to line resistive loads on a wye source you can calculate the total power in two different ways:

You can multiply the load (single line to line) currents by the line to line voltage and add up the three terms.
Or you can multiply the line current (resulting from two line to line loads, out of phase) by the line to neutral voltage and add them up.

The two results must be exactly the same, since the source cannot tell whether the line currents result from a balanced delta load or a balanced wye load.

Your problem is that you are trying to make a similar calculation using the line to neutral voltages in a high leg delta. That simple current times line to neutral voltage calculation does NOT work in this case since the current in the A and C wires is NOT in phase with the A or C to N voltage.

The fact that the B leg is carrying the same current as the A and C legs but has a higher L-N voltage does NOT mean that the B leg is somehow supplying more power. If you do the calculation correctly, the sum of the three voltage x current x power factor products will be equal to the delta calculation of the simple application of the three phase power formula.

As an example of why your calculation does not work, look for a moment at a corner grounded delta with a grounded B leg.
It is not true that all of a sudden the B leg does not provide any power. And the line currents in A and C are NOT in phase with the A-B or A-C voltages.

 

[tsamples]

The question is still fuzzy. If all of the loads are 240v, then you are not utilizing the grounded center-tap. I assume, then, that you are picturing the normal result of a 12/240 high-leg panel where every third breaker space is skipped, because the loads are 120v. That wouldn't occur with 240v loading, so this wouldn't be an issue.

 

[gar]

170317-1957 EDT

EricJ:

You need to try to figure out what question you really want to ask and precisely define the conditions. So far your comments make no sense.

Possibly you may want to ask why there are single phase systems with a center tapped secondary that have a single added transformer that turns the system into a wild leg three phase system? We have many of these in our area. The answer has nothing to do with panel and breaker ratings.

Viewed from a transformer capability perspective you generally do not have equal line current capability. But when you get to an individual panel where there are many customers on the same two transformers, then it is possible to view the panel capability of a single panel as having equal current on each leg. This can not be true for all customers simultaneously, or the total load of all customers has to be limited to the current capability of the smaller transformer (logically the one supplying the wild leg).

As your question was originally proposed we have to assume that when you speak of a maximum current that this means a precise value for calculation purposes. Below that value is OK, and above the value is an overload. But in the real world for actual breaker panels, and transformers there is no precise threshold. Rather the limits are defined by ratings, temperature, materials, and expected lifetime. Thus, your small added loads are not significant until they become larger.

A very important part of your future education is learning how to define and ask the correct question. A project or customer may not send you in the correct direction.

.

 

[EricJ]

Thanks guys for all of the advice. I'm sorry if my question isn't entirely clear. I know I have a lot to learn and that is why I am here asking questions.

I will try one more time to ask the question in a clear manner that has one descrete answer and isn't so fuzzy.

A building is equipped with a 100amp panel powered with 3-phase delta high leg electricity with AC voltages of 120 - 120 - 208 on each leg when measured w respect to ground. Each leg is restricted by a 100 amp breaker and the transformer supplying the power does not present any limitations. The building is to be equipped with 240v AC resistive loads. Assuming the breakers will trip at 80% capacity, what is that maximum amount of power one can draw with this configuration without tripping the breakers? How would the loads be wired such that they take full advantage of the available power? Assume no voltage drop and a power factor of 1.

 

[iwire]

Assuming the breakers will trip at 80% capacity,

Why are we assuming that? It is not reality.

I also want to ask is your question purely about electrical theory or do you want to meet the NEC code requirements as well?

Most times load calculations start with the loads not with the supply.

In other words you add up all your loads and see what the total is and then you make sure the supply exceeds that.

 

[ptonsparky]

Thanks guys for all of the advice. I'm sorry if my question isn't entirely clear. I know I have a lot to learn and that is why I am here asking questions.

I will try one more time to ask the question in a clear manner that has one descrete answer and isn't so fuzzy.

A building is equipped with a 100amp panel powered with 3-phase delta high leg electricity with AC voltages of 120 - 120 - 208 on each leg when measured w respect to ground. Each leg is restricted by a 100 amp breaker and the transformer supplying the power does not present any limitations. The building is to be equipped with 240v AC resistive loads. Assuming the breakers will trip at 80% capacity, what is that maximum amount of power one can draw with this configuration without tripping the breakers? How would the loads be wired such that they take full advantage of the available power? Assume no voltage drop and a power factor of 1.

If your loads are 240v and the transformer supplies 240V L-L. We don't care what the L-G voltages are, nor does the load.

Oh, and it will be 80% of what the 100 amp breaker could handle if it were continuous rated and would not tolerate anything more.

 

[iwire]

Oh, and it will be 80% of what the 100 amp breaker could handle if it were continuous rated and would not tolerate anything more.

Hold on, lets be clear only the part of the load that may be continuous gets calculated at 125%.

So in the case of the main breaker you would have to look at every circuit leaving it and determine which must be calculated at 125%

A standard breaker will (supposed to) hold 100% load for at least 3 hours.

And we still do not know if the OP wants to consider the code or just the theory.

 

[ptonsparky]

Hold on, lets be clear only the part of the load that may be continuous gets calculated at 125%.

So in the case of the main breaker you would have to look at every circuit leaving it and determine which must be calculated at 125%

A standard breaker will (supposed to) hold 100% load for at least 3 hours.

And we still do not know if the OP wants to consider the code or just the theory.

Code, schmode. He is an EIT. I don't think he cares about the NEC yet. Well, not very much anyway.

 

[iwire]

Code, schmode. He is an EIT. I don't think he cares about the NEC yet. Well, not very much anyway.

In that case there is no 80% / 125% to worry about is there?

 

[ptonsparky]

L-L, and L-G or L-N only tell us that you have a Delta configuration. It does not tell us if it is Open or Closed Delta. In this area the Open Deltas were used primarily to supply single phase with a light 3 phase load. The second transformer being smaller than the first. Increased 3 PH load might get the POCO to add the third transformer to make it a Closed. When or if they ever make them all the same size, IDK. Pole mounted transformers for the most part in this area.

 

[ptonsparky]

In that case there is no 80% / 125% to worry about is there?

Nope.

Only that 80 amps = 80% of 100 amps and that his theoretical CB will magically trip @ exactly 80+ amp. No more, no less.

 

[iwire]

Nope.

Only that 80 amps = 80% of 100 amps and that his theoretical CB will magically trip @ exactly 80+ amp. No more, no less.

Why are you saying a 100 amp breaker will trip at 80?

What am I missing?:huh:

 

[kwired]

If you have a 15kVA balanced 240 volt load you draw 36 amps per line.

You also figure that as 5 kVA per line, but not at 240 per line, not at 120 per line, not at 208 on the high leg, but at 138 volts per line, that is what the mid point of the delta would be. 208 is not the midpoint, it is a measurement of the distance from one point from the opposite side of the triangle. 120 is only a mid point of one side of the triangle, but does apply for unbalanced 120 volt load calculating.


But again as OP question is asked a 100 amp three pole main breaker can be loaded to 100 amps per line, but that don't mean you are as likely to have equal current ability per line from the source as you typically have from a wye source with a high leg delta system. Open delta systems don't have equal line current ability, period. Sometimes you do run into a full delta system where they expect higher load on the 120/240 side and put in a bigger transformer for that side, but you can not load the high leg to the same capacity as the other two transformers are not large enough to do so, the 100 amp main will not know any better though.