Sizing a Cable

[xCod3m4nx]

Bob - k = 12.9 ohm would be for the 75c rating. So even though I'm looking at max running amps being 140A, I still need to size the wire according to it being 200A since it is connected to a 200A CB right? Using that calculator sets me back at the original 600kcmil so I would have to buy a 750kcmil unless I look at parellelling since okonite doesn't carry a 600kcmil.

Rick - The service company is upgrading an existing 200A panel to 400A to accomadate for the new trailer being installed 780ft away from the panel. So we will have to look at cost for a disconnect < 30ft away I'm guessing.

 

[xCod3m4nx]

Bob - Since the cable will be buried, ambient earth temp used is usually 20c I think, might be wrong here.

 

[bob]

Bob - k = 12.9 ohm would be for the 75c rating. So even though I'm looking at max running amps being 140A, I still need to size the wire according to it being 200A since it is connected to a 200A CB right? Using that calculator sets me back at the original 600kcmil so I would have to buy a 750kcmil unless I look at parellelling since okonite doesn't carry a 600kcmil.

K is the volume resistivity in ohms-cmil/ft. It varies with ambient temperature. K = 10.5 20C K = 10.6 25C 12.9 75C for copper conductors. It has nothing to do with the 75C rating of the conductor. 75C relates to the max insulation temp. You need to decide what the max load is going to be and use that figure to get the VD. The main breaker has nothing to do with it.
Use the calculator that Smart$ posted to assist you.

 

[suemarkp]

Bob - k = 12.9 ohm would be for the 75c rating. So even though I'm looking at max running amps being 140A, I still need to size the wire according to it being 200A since it is connected to a 200A CB right? Using that calculator sets me back at the original 600kcmil so I would have to buy a 750kcmil unless I look at parellelling since okonite doesn't carry a 600kcmil.

The wire must have an ampacity equal to or greater than the main disconnect rating, but there are no code rules for voltage drop (but the HVAC units may not be happy with that). So you could run 3/0 copper the whole way. Do you have a specification sheet or other job requirement which limits your voltage drop? If not, you can use your judgement based on the load you expect, or ask you customer what they want to pay for (full 200A capability with 3% voltage drop or something less).

 

[xCod3m4nx]

Bob - I see what you're saying about K. So if I'm looking at 140A running and about 175A starting; 20c, 780ft, 140A, copper, 240v 1-phase, 350kcmil gives me what I'm looking for I believe.

Suemarkp - the only info we have from the company is that the old single trailer used 58A, and another 7A was used by the gate and flood lights. So they were using 65A on a 200A supply from the energy company. They said that the new double wide would need about 130A at least. The energy company said they an up the service to 400A which would be cheaper than dropping a new line just for the double wide which will only be used for 6-8months. We are just trying to find the best way for them to power the new double wide.

All - Thanks for all the help, I greatly appreciate it. Sorry that I'm new to using the NEC codebook and running power calcs, and that some things that should be easy for me to understand I might not get at the moment.

 

[xCod3m4nx]

So I think I've got most of what I need calculated, but correct me if I'm wrong.

Info: Material - Copper; Cable Size - 350kcmil; Length - 780ft; Voltage & phase - 240V 1-phase(120/240V 1-phase); Ambient Temp - 30c (cable is buried but just using 30c instead of 20c); Max Continuously Running Load - 140A; Max Starting Load - 175A

With the 350kcmil, (copper cable, 75c) I'm at allowable 310A which is fine for the 200A breaker it will be branched off of. The VD will be 7.1V or 3% which also meets regulations. If I run a grounding wire with the 350kcmil, 3/C will that account for a grounding means? Or since it it 780ft from the main service panel it is connected will I have to have another service disconnect within 30ft of the trailer? Also according to table 250.122, for 200A a copper 6 AWG grounding wire would work? Also if I used two 3/C 3/0 cables for the run, the VD would be about 1.7%. So two 3/C 3/0 cables with a 6 AWG ground wire each would also do the job and might be cheaper?

 

[ibew501ed]

the EGC #6 would need to be increased proportionally as the ungrounded conductors were.

 

[jumper]

the EGC #6 would need to be increased proportionally as the ungrounded conductors were.

What he said. 250.122(B)

 

[xCod3m4nx]

Not really sure if I'm doing this calculation correct for adjusting the size of the EGC

3/0 cable would be suitable for the 200A breaker and has 167800 CM. 350000 CM is the new 350kcmil cable I'm using to adjust for VD. So 350000/167800 = 2.09; therefore, with the existing ground for the 200A being #6 at 26240 CM the new updated EGC wire would be 2.09 x 26240 = 54842 so this would give me a #2 EGC?

Also, how would I run the calc if I wanted to resize for the two 3/C 3/0 cables in parallel?

 

[jumper]

Not really sure if I'm doing this calculation correct for adjusting the size of the EGC

3/0 cable would be suitable for the 200A breaker and has 167800 CM. 350000 CM is the new 350kcmil cable I'm using to adjust for VD. So 350000/167800 = 2.09; therefore, with the existing ground for the 200A being #6 at 26240 CM the new updated EGC wire would be 2.09 x 26240 = 54842 so this would give me a #2 EGC?

That is what I got also.

Also, how would I run the calc if I wanted to resize for the two 3/C 3/0 cables in parallel?

250.122(F) and 250.122(F)(1). A full sized EGC according to T250.122, breaker size, in each conduit/cable.

 

[jumper]

Also, how would I run the calc if I wanted to resize for the two 3/C 3/0 cables in parallel?

The notes in the handbook says that the EGC in some cables will be undersized for some parallel installs and you would have to comply with 250.122(F)(2). I have never paralleled cables, so I have no experience with this article. I have always installed in conduit and pulled conductors.

 

[xCod3m4nx]

jumper - thanks for all the help. It seems though that if I used the 2 3/0 3/C wires each would still have a #6 EGC since the breaker they will be connected to is 200A. Off the top of your head would you think the one 350kcmil wire or the 2 3/0 wires would cost less for installation/labor?

 

[Smart $]

...will I have to have another service disconnect within 30ft of the trailer? ...

Yes. 550.32(A)

 

[bob]

It seems though that if I used the 2 3/0 3/C wires each would still have a #6 EGC since the breaker they will be connected to is 200A.

You would think this but the ole NEC does not believe that is the way to go.
Read 250.122(B). So if you increase the conductor size by 2, as you did, the EGC cmils must also be increased by 2. From table 250.124, 1 ckt of 3/0 requires a #6 C. The cmils #6 = 26240 cmils. 26240 cmils x 2 = 52480 cmils = #3 cu. Now you need 2 ckts of 3/0 + 1 #3 cu in each sonduit.
Check this discussion http://forums.mikeholt.com/showthread.php?t=117577&highlight=conductor

 

[xCod3m4nx]

So I've been redoing my calculations and I've come up with some new info.

Running 2 3/C wires in parallel, I come up with
CM = (2 x 12.9 x 140/2 x 780) / 7.2 = 195650
So this gives me two 4/0 3/C wires running the 780ft from main breaker to service disconnect <30ft from double wide trailer. The two 4/0 cables also sets me at a 2.8% VD so I'm good there. Now I have to figure out what cables I'm running from the disconnect to the trailer.

 

[xCod3m4nx]

Also, since the main service is 200A until the electrical company upgrades it to 400A. Does the NEC codebook state to only use 80% of that 200 or is that just to account for inrush? For example, if I need to use 190A of the 200A service until it gets upgraded, can I do that? Or should I reduce the loads connected until the service gets upgraded?

 

[Dennis Alwon]

Also, since the main service is 200A until the electrical company upgrades it to 400A. Does the NEC codebook state to only use 80% of that 200 or is that just to account for inrush? For example, if I need to use 190A of the 200A service until it gets upgraded, can I do that? Or should I reduce the loads connected until the service gets upgraded?

Most breakers are rated 100% for non continuous load and 80% for continuous load. Thus if you have a 200 amp breaker and 190 amps of load (non continuous) then you are good t go.

 

[xCod3m4nx]

So I should be fine if: transformer to main CB breaker box which contains (200A main CB, 100A CB, 15A CB, 15A CB, 30A CB, and a 20A CB) right now it only pulls a continuous of 65A load to run everything. Looking to install another 100A breaker into it to account for a continuous load of no more than 80A until the service is upped to 400A. Then the breaker box will be: 400A main CB, 100A CB, 200A CB, 15A CB, 15A CB, 30A CB, and a 20A CB. The continuous load would then jump up from less than 80A to around 130A continuous which would be 195A continuous which is less than 50% of the new 400A.

So having the existing 65A continuous + 80A continuous = 145A continuous which would be good for the 200A x 80% = 160A.

 

[xCod3m4nx]

Been running calcs for the trailer loads at its disconnect:
Main CB = 200A
Lights #1 = 15A 1-pole; Lights #2 = 15A 1-pole; AC Unit #1 = 60A 2-pole; AC Unit #2 = 60A 2-pole; Bathroom #1 = 20A 1-pole; Bathroom #2 = 20A 1-pole; 10 receptacles = 20A 1-pole; Microwave - 20A 1-pole; Coffee Pot - 20A 1-pole; Mini Fridge = 20A 1-pole

Does this sound right for a double wide trailer that I think is 40ft x 16ft, 2 10KW AC units, 2 bathrooms, that will use 4-6 laptops + pc & monitor + 1 printer, other minor loads?

 

[xCod3m4nx]

Question on breakers in main panel and second disconnect. Power comes from transformer to main panel with 400A CB main which will have a CB that then runs from that panel 780ft to a disconnect panel for the new trailer which will have a main 200A CB. Question is, can the CB in the 400A CB panel be a 200A CB also or should I make is a 250A CB.