Unbalanced/Balanced Loads

[kookyburns1952]

Bear with me on this, I'm a student. I'm using Holt's Study Guide Volulme 1 for the NEC 08, page 100.
To determine conductor sizes for a 200 amp load I've multiplied by 125% for the ungrounded conductors and 100% for the grounded conductor. The study guide then asks the question about sizing the grounded conductor based on the 50 amp unbalanced load. Where do they get the 50 amp unbalanced load from? The only thing I see is the difference between the multiplying for the ungrounded conductor size and the grounded conductor size; 250 - 200=50. Please define the unbalanced load so a student can understand. The textbook gives these examples throughout, but leaves me confused at times. The unbalanced load is used then to determine the minimum grounded conductor size from table 250.122. A def on balanced load would also be helpfull.

 

[charlie b]

Welcome, both to the NEC Forum and to the world of electrical design and installation. There is much to learn, so do not be concerned if things are confusing at first. Things are often confusing to the experienced members as well.

You do have a degree of confusion going on here. Let me point out a few things.

? To start with, not all of us have a copy of the study guide you are using. So we won’t be able to give much help, unless you give us more information about the original question.

? You don’t multiply the “ungrounded” by 125%. The factor of 125% (or as it is sometimes described, adding an additional 25%, the math is the same in either case) comes into play in several places within the NEC. One is in the calculation of a load. We are asked to include 125% of the continuous load, along with 100% of the non-continuous load. The distinction is that a continuous load is expected to be running at full capacity for more than 3 hours without a break. Another place the factor comes into play is that we add 25% to the load of the largest motor powered by the same source, or powered via the same conductors. So without knowing the context of the question, I do not know how the 125% would have come into that situation.

? The “grounded” conductor, often (but not always) the same as the “neutral” conductor, can be sized in accordance with the imbalanced load. The simplest example is a single phase, 120/240 volt panel. It will be fed by two ungrounded conductors (called line 1 and line 2, or called phase A and phase B) and one grounded (or neutral) conductor. If a particular branch circuit powers a receptacle, then current for that circuit will flow away from the panel on phase A, and back to the panel on the neutral, so that the two currents are the same. The same panel might power a 240 volt dryer, and it will not have any neutral current. Thus, the total load seen on a phase conductor and the total load seen on the neutral might not be the same.

? More to the point, consider a 10 amp, single phase load connected to phase A (with that 10 amps returning to the panel via a neutral wire), and a 15 amp, single phase load connected to phase B (with that 15 amps returning to the panel via a separate neutral wire). Suppose at one point in time these are the only two loads on that panel that happened to be turned on. The total load seen by the neutral wire that feeds the panel will only be 5 amps. That is the amount of imbalance between the phase A loads and the phase B loads.

? Table 250.122 has to do with the size of the “equipment grounding conductor.” That is one of many phrases in the NEC that contain some version of the word “ground.” It is not the same wire as the “grounded” or “neutral” wire, so it doesn’t have anything to do with your original question.

Start with that, and tell us where you need to go next.

 

[Smart $]

I don't have the documentation you refer to so I'll have to wing it with my own example...

First, I believe the unbalanced load being referred to is the maximum possible non-fault current on the grounded conductor... i.e. not the difference in calculated load on the different UNgrounded conductors.

Say you had panel loads of the following:

120/240 1? 3w system
60A, 240V
30A, 240V
16A, 120V L1
16A, 120V L2
5A, 120V L1
24A, 120V L2

First off, only line-to-neutral loads contribute to unbalanced current on the grounded neutral conductor... so you have a potential maximum of 16A + 5A = 21A through L1-N loads, and a possible maximum of 16A + 24A = 40A by L2-N loads. In this case, you would have to size your grounded conductor for 40 amps possible through L2-N loads... OR ... the comparable minimum size GEC based on Table 250.66 using the largest phase conductor... whichever is the larger size of the two.