Calculating Wire Size

[eeee]

I have a calculated design load of 155.6KVA feeding a 225KVA, 3 phase, 208delta/120 volt, 12,460 volt primary, delta-wye pad mounted transformer and a 57KVA calculated design load feeding a 75KVA, 3 phase, 208delta/120 volt, 12,460 volt primary, delta-wye pad mounted transformer.

I have been looking at NEC table 310.16 and using an engineering calc calculator. The engineering calc calculator gave me a wire size of all 0's on one calculation and does not match the NEC for the other calculation.

I want to use XLP, 133% concentric nuetral primary underground cable and underground secondary cable for this project.

I get 155.6KVA/(208v)(sq 3)=431.9amps for the first secondary load. This translates to a 225KV transformer, the next size up. The primary amps calculate to 7.2amps.

I get 57KVA/(208)(sq 3)=158.2 amps for the second secondary load. This translates to a 75KVA transformer, the next size up. The primary amps calculate to 2.6amps.

The NEC has various cable types that must be selected from. I am using 60 degrees centigrade which is 140 degrees farenheit) and assuming THHN cable for the secondary, XLP, 133% concentric nuetral for the primary.

I am getting 600AWG secondary cable for the first calc and 1/0 secondary cable for the second calc. using THHN cable and referencing the NEC, although my engineerinc calc calculator gives me 900AWG and all 0s for the next calc (although no option is permitted for type of cable that I see when using the eng. calc calculator).

For primary cable, I can not find 133% XLP concentric nuetral cable in the NEC. to figure out my AWG.

Does someone know how to do the table look up in the NEC for my situation for both primary and secondary cable??

 

[bob]

Re: Calculating Wire Size

I have a calculated design load of 155.6KVA feeding a 225KVA, 3 phase, 208delta/120 volt, 12,460 volt primary, delta-wye pad mounted transformer and a 57KVA calculated design load feeding a 75KVA, 3 phase, 208delta/120 volt, 12,460 volt primary, delta-wye pad mounted transformer.

The transformers you are describing should be
3 phase, 208/120 wye not delta.

I am using 60 degrees centigrade which is 140 degrees farenheit and assuming THHN cable for the secondary, XLP, 133% concentric material for the primary.

Why use the 60C rating?

I am getting 600AWG secondary cable for the first calc and 1/0 secondary cable for the second calc. using THHN cable and referencing the NEC.
Does someone know how to do the table look up in the NEC for my situation for both primary and secondary cable??

I am wondering why someone who is supposed to be engineering this complex job does not know how to look in table 310.16 and get a conductors rating.

I am getting 600AWG secondary cable for the first calc and 1/0 secondary cable for the second calc. using THHN cable

You can not use THHN in an underground installation.

I get 155.6KVA/(208v)(sq 3)=431.9amps for the first secondary load. This translates to a 225KV transformer, the next size up. The primary amps calculate to 7.2amps. I am getting 600AWG secondary cable for the first calc and 1/0 secondary cable for the second calc. using THHN cable and referencing the NEC

600 kcm at 60C is not the correct size for 432 amps and 1/0 is not adequate for for?158 amps.
I think your should use conductors for the full load of the transformer unless the customer will never add load in the future.

If you are the consultant on this job, I think the forum members should get their cut. They have guided you through the whole job.

[ January 31, 2006, 05:22 PM: Message edited by: bob ]

 

[eee]

Re: Calculating Wire Size

Bob,

You really do deserve a cut and I'd give you won if I knew you. I hated to ask for the information I asked for, but feel I have a jump start now for future calculations. I could have called a cable manufacturer to find out THHN does not go underground, etc.

NEC 310.8 (c) says I can not use THHN in a wet location for underground cables. I have a high water table, so I consider myslef to be wet. NEC 310.8 (b) says you can use THHN underground for a dry or damp location.

The southwest is where I lie, but not in the desert, so I figure 60 degrees centigrade (140 degrees Farenheit) is in the ball park. Above 140 degress Farenheit is getting a little warm for the upper range in climate I assume. Anything above 140 degress must be reserved for the real hot areas in the world I would assume.

Yep, my 3 phase transformer is 12,460 primary Delta and 208Y/120 Wye connected on the secondary. I did not characterize that correctly and I knew better.

I took a design class in Denver with a room full of EEs and I was tought in class that the wire is sized for the design load as is the next size up for the transformer, but the wire is not sized for the transformer ampacity. Fuses and breakers are sized for the transformer ampacity.

That did not pass my common sense test. I am glad you say I should size my wire for the transformer ampacity. That way I can fully use my transformer and I will do that. May cost a little more money, but cable is comparitably cheap compared to the cost of a transformer.

NEC table 310.5 says I can use copper number 2 underground as a minimum for a 12,460 volt primary. I will therefore use a 15KV, 133% XLP concentric nuetral, #2 underground cable for both secondaries in this job. Since my primary amps are between 2 and 11 amps, I assume #2 should work for both. I would probably upsize my cable if my primary amps were large. Interesting that 12,460 volt primary cable does not have an ampacity table like the 0 to 2000 volt cables do in NEC table 310.16.

For my secondaries, I will size them for the 3 phase transformer ampacities. 225KVA/(sq 3)(208)=624amps. Using THHW cable at 60 degree Celcius, that is 1500AWG cable I will use if I am doing my table look-up correctly in the NEC.

For the 75KVA transformer,75KVA/(sq 3)(208)=208 amps. Using THHW cable for a wet location at 60 degrees celsius,that yields 4/0 cable that I will use for this secondary; again hoping my table look-up procedure is accurate.

 

[bob]

Re: Calculating Wire Size

The southwest is where I lie, but not in the desert, so I figure 60 degrees centigrade (140 degrees Farenheit) is in the ball park. Above 140 degress Farenheit is getting a little warm for the upper range in climate I assume. Anything above 140 degress must be reserved for the real hot areas in the world I would assume.

eeee
I think you are over your head. The temperature
values shown in table 310.16 are the temp rating for the insulation not the area you live in. God forbid that you live in an area with a temp of
140 degrees. The conductors that are used today are rated at 90C. However due to restrictions on equipment you may only use the 75C col.
Suggest you use parallel runs of a smaller cable rather that 1500 kcm.(2 - 400 kcm 75C)

[ January 31, 2006, 02:23 PM: Message edited by: bob ]

 

[eeee]

Re: Calculating Wire Size

I just got off the phone with Greybar. They told me that most cable is dual rated in recent times. THHN means the cable has a nylon coating and that is what the N is for. Fortunately in the NEC, Table 310.16, THHN and THHW and THW-2 are all in the same column for 90 degrees Centigrade.

I appears from Greybar that you don't spec out a THHW, THW-2, or THHN cable anymore, since they are now dual rated. You just spec out the dual rated cable when requesting it. That must mean then that the dual rated cable has the THHW quality of going underground in conduit and duct when the water table is very high, i.e. wet conditions as specified in the NEC, table 310.16.

 

[pierre]

Re: Calculating Wire Size

Where did you get your schooling?

What company are you working for? It would seem you have no experienced help.

As has been posted, you are in way over your head, and I suggest that you either pass this job up, or get professional EE help.

If you have and NEC, take a look at the definition: Location, Wet in Art 100.

[ January 30, 2006, 04:48 PM: Message edited by: pierre ]

 

[tom baker]

Re: Calculating Wire Size

"NEC 310.8 (b) says you can use THHN underground for a dry or damp location.
Any underground location is a wet location.
This is in Art 100 of the NEC, definitions. Definitions are important so they are listed first.
I'm going to have our chief moderator, charlie b, who is knowledgeable about these issues and is a EE, take a look at this post.

 

[charlie b]

Re: Calculating Wire Size

At Tom's suggestion and request, I have reviewed this thread. My principal comment is to echo and rephrase Bob's comment: You are in over your head. I don't know your employment situation (i.e., how many people in the company, how many are licensed PEs, what level of supervision and what level of training you are given, etc.). But you need to understand that we are here to assist, not to teach. We can help with specific questions, but we are not here to teach you the profession. That should come from your employer.

I will address a few of your comments and questions.

Originally posted by eee:The southwest is where I lie, but not in the desert, so I figure 60 degrees centigrade (140 degrees Fahrenheit) is in the ball park.

The column headings in Table 310.16 (i.e., 60, 75, and 90C) refer to the temperature rise that the conductor can safely sustain, assuming an ambient temperature of 30C. The current values in the table tell us how much current we can pass through a given conductor, in an ambient environment of 30C, without risk that the overall temperature (ambient plus heating due to current flow) will not exceed the conductor's limit. For example, you could safely pass 200 amps through a 3/0 THHW, and be confident that the temperature will not exceed 105C (i.e., 30C ambient plus 75C rise due to current flow).

Originally posted by eee: I am glad you say I should size my wire for the transformer ampacity. That way I can fully use my transformer and I will do that. May cost a little more money, but cable is comparatively cheap compared to the cost of a transformer.

First, the word "ampacity" applies only to conductors. Transformers do not have "ampacity," they have "rated current."

Secondly, what you were taught in your class is correct. You only need to size the conductors for the calculated load. But it is often a good idea to size them for the entire rating of the transformer. That would be a design choice, and not an NEC requirement.

Originally posted by eee: NEC table 310.5 says I can use copper number 2 underground as a minimum for a 12,460 volt primary. I will therefore use a 15KV, 133% XLP concentric neutral, #2 underground cable for both SECONDARIES in this job. Since my primary amps are between 2 and 11 amps, I assume #2 should work for both.

I added emphasis to the word "secondaries." I think you meant "primaries."

Yes, a #2 is the smallest you can use, and yes it will have an adequate ampacity.

Originally posted by eee: Interesting that 12,460 volt primary cable does not have an ampacity table like the 0 to 2000 volt cables do in NEC table 310.16.

Yes it does. I believe Table 310.77 applies to your situation.

Originally posted by eee: 225KVA/(sq 3)(208)=624amps. Using THHW cable at 60 degree Celsius, that is 1500AWG cable I will use if I am doing my table look-up correctly in the NEC.

You would do better with a pair of 500 MCM or three sets of 250. Using the 60C column, they would give ampacities of 640 (for a pair of 500 MCM) or 645 (for three sets of 250 MCM).

Originally posted by eee: 75KVA/(sq 3)(208)=208 amps. Using THHW cable for a wet location at 60 degrees Celsius, that yields 4/0 cable that I will use for this secondary; again hoping my table look-up procedure is accurate.

Please look again. At 60C, a 4/0 is only good for 195 amps. That is less than the transformer's rated 208 amps, but above your calculated load. So it would meet code, but it would not meet your chosen design criterion (of sizing for the transformer's rating).