Table B.310.7 is provided in annex B of the NEC. Why is information provided if it is not required for Electrical Ducts? In the begining of the annex it states that the annex is not part of the requirements of the NEC. Take for example 3-400 kcmils are worth 1005 amperes if you use table 310.16, but using B.310.7, with a RHO of 120 for the soil and three electrical ducts, which is the case for this environment, 3-400 kcmils are only worth 840 amperes. This is a huge difference, especially in my case where I'm doing a coordination study. Does someone have some comments on this?
Table B.310.7
- Thread starter selmer
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- Lockport, IL
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Re: Table B.310.7
You are permitted, but not required, to use the information in the Annex. In this case, you would not want to use the information in the Annex. You may have an alternative, or you may not, depending on two things. "Thing One" is whatever your AHJ will accept. "Thing Two" is the question of whether the basis for selecting a conductor size is an Article 220 Service Calculation.
This is a tricky, and controversial topic. It has been discussed several times on this Forum. You can do a word search and find some of the earlier discussions.
The short answer is that if a Service Calculation gives you a result of, let us say, 1000 amps, it is commonly known and understood that the actual load is likely to be much lower. In fact, it is common for the serving utility to give you a service transformer rated as low as 40% of the calculated service load. Because of this built-in conservatism, many AsHJ will allow you to use 310.16 to select the conductors. Please note that Table 310.16 does not apply to an underground installation, but that it is being used because of the available conservatism in the load calculation.
On the other hand, if the load you plan to handle with the conductors was determined in any other way, such as actual measurements (e.g., for a service upgrade of an existing facility) or a summation of the known connected load (for a new facility), then there is no conservatism built into that determination. You can't use 310.16 in that case. But there is no other "required" method available to you. So you have two remaining choices. One choice is to find your planned configuration in B.310, and use whatever ampacity the tables give you. Yes, they will be lower than 310.16, but that is unavoidable. The other choice is to hire a PE to perform a Neher-McGrath calculation. Let me advise you, however, that I predict the results that the PE would give you will also be well below the numbers shown in 310.16.
You are permitted, but not required, to use the information in the Annex. In this case, you would not want to use the information in the Annex. You may have an alternative, or you may not, depending on two things. "Thing One" is whatever your AHJ will accept. "Thing Two" is the question of whether the basis for selecting a conductor size is an Article 220 Service Calculation.
This is a tricky, and controversial topic. It has been discussed several times on this Forum. You can do a word search and find some of the earlier discussions.
The short answer is that if a Service Calculation gives you a result of, let us say, 1000 amps, it is commonly known and understood that the actual load is likely to be much lower. In fact, it is common for the serving utility to give you a service transformer rated as low as 40% of the calculated service load. Because of this built-in conservatism, many AsHJ will allow you to use 310.16 to select the conductors. Please note that Table 310.16 does not apply to an underground installation, but that it is being used because of the available conservatism in the load calculation.
On the other hand, if the load you plan to handle with the conductors was determined in any other way, such as actual measurements (e.g., for a service upgrade of an existing facility) or a summation of the known connected load (for a new facility), then there is no conservatism built into that determination. You can't use 310.16 in that case. But there is no other "required" method available to you. So you have two remaining choices. One choice is to find your planned configuration in B.310, and use whatever ampacity the tables give you. Yes, they will be lower than 310.16, but that is unavoidable. The other choice is to hire a PE to perform a Neher-McGrath calculation. Let me advise you, however, that I predict the results that the PE would give you will also be well below the numbers shown in 310.16.
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Re: Table B.310.7
If you are doing a project where the loads were calculated using Article 220, you should be ok using 310.16. If this is an application where the loads were calculated by a more precise method(the calculations in Article 220 are very conservative) you really need to look at the Annex if you don't want to burn your feeders up.
Don
If you are doing a project where the loads were calculated using Article 220, you should be ok using 310.16. If this is an application where the loads were calculated by a more precise method(the calculations in Article 220 are very conservative) you really need to look at the Annex if you don't want to burn your feeders up.
Don
wirenut1980
Senior Member
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- Plainfield, IN
Re: Table B.310.7
Don't get me wrong, I agree that appendix B needs to be taken into consideration when calculating ampacities of conductors in an underground ductbank. But I think the code allows me to use Table 310.16 for ampacity calculations underground in a ductbank without hiring a PE for Neher-McGrath calculations.
Why does it not apply? If I look at the title of Table 310.16 it includes direct burial in Earth.
310.15(B) has a FPN that states what you are saying, but FPN's are not required to be followed.
Don't get me wrong, I agree that appendix B needs to be taken into consideration when calculating ampacities of conductors in an underground ductbank. But I think the code allows me to use Table 310.16 for ampacity calculations underground in a ductbank without hiring a PE for Neher-McGrath calculations.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Table B.310.7
It would apply to three conductors surrounded by dirt. It would not apply to nine conductors in sets of three, with each set surrounded by air, then conduit, then dirt.
john m. caloggero
Senior Member
Re: Table B.310.7
I did a quick Neher-McGrath calculation based on 3 sets of 400kcmil, cu conductors in schedule 80 PVC at a 30 inch burial depth, with earth thermal resistance of 120. The result was 840 amperes.
I did a quick Neher-McGrath calculation based on 3 sets of 400kcmil, cu conductors in schedule 80 PVC at a 30 inch burial depth, with earth thermal resistance of 120. The result was 840 amperes.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Table B.310.7
Kinda tells you how the table was developed, doesn't it?
Kinda tells you how the table was developed, doesn't it?
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