Q: I am designing an office complex should I be doing Neher-McGrath calculations?
A: Ever project needs to be analyzed based on its specific operating perimeters, but the simple answer is NO. While anytime current is running through a resistance there is heat produced the standard de-rating factors found in NEC 310.16 are more than adequate to compensate for building with standard load factor, like offices. That being said if there are any areas in the building that have a higher than building average load factor then these specific areas should be analyzed.
Q: How much does it cost?
A: Every solution is unique based on many site specific variables. Some solutions will require only a rearrange of conduits in a specific duct bank; others may require larger conductor size, Added Conduits and or a LowRHO backfill. But keeping your conductors within their temperature parameters is not only a requirement if the NEC but it lowers your overall energy use by illuminating some of your I2R losses. In addition, if the ductbank is run directly under the building slab the heat from the conductors will be transferred to the building. This may be beneficial in some specific environments. But in most environments this is heat that will have to be removed from the building. Based on this information providing the correct Neher-McGrath solution can save money over the life of the project.
Q: Will your LowRHO® backfill really increase ampacity and save me money?
A: Not always, in some situations LowRHO® is not a good solution. There are many variables that need to be considered. In some conditions the best answer is to modify the duct bank configuration or placement. There are spicific configerations where LowRHO® will make the heating issue worse. Ever solution is site specific.
Q: Back as little as 5 years ago these calculations were rarely performed why is an issue now.
A: One reason is as rack density get higher and datacenter become fully loaded this is becoming more if an issue. When a datacenter is built the maximum design load is rarely online day one. Typically we see day one loads in the 25-50% range. Over time these loads are increased. A typical data center may not be operating at design load for years after the project is completed. We have been involved in several projects as expert witness and analytical consultants where this exact thing happened. We have seen this repeated and we know that there are still many data centers out there now that are still operating below their design load. As these datacentes hit and run at full load we anticipate this issue to arise.
Q: How will this process affect my construction schedule?
A: If addressed properly and early there will be no affect on construction schedule. It is only where the issue is not addressed that there can be major effects on the schedule. The key is talk about it early in the design process and determine if this is an issue for your project. Most projects like schools, offices, and general commercial project will not be an issue.
Q: How do I determine if I have an issue?
A: Ask yourself these questions:
1. Is this Project a critical environment?
2. Is the design of this project based on a high load factor model?
3. Will the design call for large duct banks?
If the answer to any of these questions is yes then ask an expert for further review of your project. We can recommend an engineering company that specializes in this type of analysis.
Q: Can I just by some software and do my own calculations?
A: There are abundant software packages sold today that will perform this complex calculation. Some of these systems are very good and some are weak or poorly designed. The package you buy is dependent on the complexity of calculation you need to perform and how much tolerance you have for a steep learning curves and your budget. In addition there are numerous variables that you need to know and understand fully before you can even start the calculation. Let's say you have the software and the required system variable and you do a calculation. Now you have the results and you are half way home. Much like an X-Ray the information is only as good as the Dr. who analyzes it. It takes years to truly understand all of the results and find the best solution. In addition these calculations can only be performed under Engineering Supervision, Per NEC 310.15. We are fixing more and more issues caused by engineers that are not truly qualified to interpret the calculation results. Yes you can do the calculation yourself but with your underground conductors being the life blood of your business, why would you?
Q: Since RHO is relative to moisture content how can I determine the expected moisture content of soil directly around conduits or a ductbank?
A: This is a complicated question. ASTM D5334 - 08 describes the procedure for determining RHO value bases at a given moisture content. However, it provides no direction as to what moisture content will actually exist at the installation location at a future time. Underground heat sources, such as ductbanks, cause drying of the soil around the source. Water evaporates from the warmer soil and condenses in cooler locations. In soil that has good thermal stability an equilibrium condition will developed where the liquid return flow matches the vapor distillation from the heat source. It is important to state that even in this equilibrium condition the moisture directly around the conduit or ductbank will be less than the moister content would be if the heat source did not exist. In soil that has poor thermal stability this equilibrium condition is never achieved and soil continues to dry to 0% moisture or until the moisture is replaced. As such it is a huge but all too common mistake to assume that the RHO value should be based on native moisture at time of install. There are numerous factors that determine how fast moisture will leave a heated area but it is not un-common with load factors above 90%for soil exposed to 75°C-85°C conduit to lose 50% of their moisture with in the first 9000 hours of operation. Three of the most common types of installations are as follows:
Type 1 - Under a structure with a vapor barrier.
Type 2 - Exterior with full exposure to environment.
Type 3 - Exterior covered by road, concrete or other non-pours material.
In a type 1 installation the vapor barrier mitigates some of the moisture loose. You will still loose moisture over time but this loss will be much slower.
Type 2 is a mixed bag. If your installation is in a location where it rains all the time then you are constantly replacing some of the lost moisture. If however you are in a desert environment the soil will continue to dry faster that would be expected in a type 1 location. Additionally in a desert the hottest time if the year is also the driest all this during the time of year you need the most power at the highest load factor.
A type 3 allows moisture to leave but none to reenter. Under a roadway or covered canopy the soil will dry out like in the type 2 but because rain water cannot be absorbed in to ground the moisture cannot be replenished at all.
There are test that can predict thermal stability including a "2 zone test" and a "Time to dryout test" but it is important to note that neither of these tests are recognized by ASTM or IEEE.
Q: The NEC uses 90RHO for earth Is this a safe number to use?
A: The simple answer is No. The NEC arrived at these values from AIEE Paper 57-660, The Calculation of the Temperature Rise and Load Capability of Cable Systems, by J. H. Neher and M. H. McGrath that stated the average soil in the US was 90RHO in-situ. Average RHO in the US, while an interesting statistic, is a useless piece of information when it comes to designing a specific site. Imagine engineering a site based on the average voltage in the US or designing a drainage system in Seattle based on the average rainfall in the US. But it gets worst, even if we test, and the soil on a specific site is in fact 90RHO In-situ we know that once we install a heat source i.e. conduit system the soil will dry out. As engineers we are interested in the RHO of the soil when we install the conduit system only as a reference point. What we really need to know is what will the soil RHO value be around my conduits after the soil has been exposed to 90°C for 10 years or 20 years.
If you design based on 90RHO and the actual RHO is higher, your system is at risk. If your site has a lower then 90RHO then you are throwing money away by installing too many conductors.
Q: Why is moisture content and compaction so critical?
A: Moisture content and compaction is critical because it helps to provide an unbroken path vie thermal bridging for heat escape. The accepted RHO value for water is about 165. Air on the other hand has a RHO value of about 4500. If soil is not compacted correctly then there is more air between the backfill partials. This air gap causes breaks in the thermal bridges and increase the overall heat resistance of the full. It is important to note that if the soil is not compacted correctly then the maximum RHO of a given sample tested per IEEE-442 at 0% moisture can be greatly exceeded. See illustration below for an example.
Q: What is thermal capping?
A: Thermal capping is a situation that can exist when the soil is stratified at the installation location. When you look at a soils report you want to know the RHO value at the depth the conduit or duct bank installation depth. But you also need to look at all the soil above the installation depth. If the soil is stratified then there could be layers of soil above the conduit that are at a higher RHO value then the soil at the installation depth. This is known as the thermal capping or thermal blanketing effect. The engineer must take the soil above the conduct into account and not just use the RHO value at the installation. This effect is difficult to quantify and is another reason that the calculations should only be performed by an engineer that is highly experienced in the specifics of Neher-McGrath.
Q: What other issues are there with stratified soil?
A: Another issue that we need to be aware of with stratified soil is spoils mixing. When a trench is excavated the spoils from the trench are typically dumped on the side of the trench. Typically the most cost effective way of digging a trench is vertical. In the trench is dug to its greatest depth in a location then the backhoe is moved and the next portion of the trench is dug to its greatest depth this process is repeated for the length of the trench. If the soil is not stratified then this may not be an issue however in stratified soil where there are varying values for RHO the spoils from differing stratified layers are dump in the same pile on the side of the trench. If this material is intended to be used for back full above the ductbank then we need to be aware that their will be some RHO averaging in the spoils pile. So once again we cannot just look at the RHO values from the level that the ductbank is installed.
Q: Is the Neher-Mcgrath method the only way to calculate underground ampacity?
A: No. Neher-Mcgrath method is the industry standard calculation but there are situation where the Neher-Mcgrath method falls short. The Neher-Mcgrath method is based in ampacity that is static over a given time period. In a Dynamic current situation the Neher-Mcgrath method is a bit conservative. A Dynamic current analysis is best done using the KEMA model. This model is designed for transmission grade power and is applicable to medium voltage and high voltage cables where the operator (typically a utility company) needs to know how much current they can push through a given cable over the next 24 hours.
The model can be used to predict cable temperatures in the future (based on an expected loading) or to calculate the maximum ampacity for the next hours or days. By using these mathematical techniques in a control room setting, for example, the following can be calculated:
The current the cable can transport over the next 24 hours.
The length of time the cable can transport a given ampacity.
The KEMA dynamic thermal model has been comprehensively verified in many situations and in cable types, including oil-filled and polymeric cables in the voltage range from 10 to 150 kV, and installed in various cable environments. Thus, the model is well-suited for use in a cable ampacity management system used to determine on-line circuit loading.
Q: How do I ask a question?
A: Email Us.Info@neher-mcgrath.com
.: Professional Engineering
Neher-McGrath are specialized and complicated calculations. We highly suggest that all results and calculation be made under the guidance of and licence professional engineer that specializes in this type of work.
NEC 310.15 (C) Engineering Supervision, indicates that these calculations should be performed under engineering supervision.
There are several software packages available that will calculate the Neher-McGrath equations. Each of these packages has its pros and cons. Please feel free to email us with your particular project requirements and we can help you find the software package that best suits your needs. Email Us
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.: RHOMON® Conduit Thermal Couplings:
RHOMON® Conduit Thermal Couplings are specially engineered pieces of conduit that are placed in your duct bank during installation. These units monitor temperature and can be set up in a self contained system or tied into the building BMS system