Cable size Calculation for 1600A service - Page 2 - Electrician Talk - Professional Electrical Contractors Forum
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Old 03-30-2019, 10:14 AM   #21
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Is there something in the CEC that says as long as you maintain 12'' between groups that grouping is acceptable?
If I remember right, the 12" guideline came up when the code made the change to wire ampacities with using the IEEE 835 calculations, taking into account the amount of heat, and EMF the surrounding ground would disperse(?). I thought in the early 2000's this came out and instead of recalculating every parallel service, they found 12" to be a minimal distance where one run would not effect the other..

http://www.electricalindustry.ca/lat...2-22nd-edition

Rule 4-004 in Appendix B reviews the calculations..
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Old 04-01-2019, 03:46 PM   #22
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Originally Posted by 908Eng View Post
My suggested configuration for 1200A is:
1200A-3∅ (UNDERGROUND) - 4 SETS OF (4) #500MCM ALUM. IN 4"C
*** INSTALL IN 2 GROUPS OF 2 CONDUITS, MAINTAIN MIN 12" [0.3m] CLEARANCE BETWEEN CONDUIT GROUPS, SEE CEC APPENDIX D, DIAGRAM D11, DETAIL 2


If the local AHJ is not keen to accept this, you always have the option of doing an IEEE 835 calculation in accordance with CEC 4-004 (1) (e). Your wire supplier will likely do this for free. You send the supplier the exact configuration that you want to use. Just keep slightly increasing the conduit spacing until you get 1200A ampacity. Your trench may get a little wide but at least you don't need to stack them. I'd be very surprised if you couldn't use 4 sets of #500 ALUM.

CAVEAT: Information presented in this post does not constitute professional engineering advice.
Thank you for the response. Please forgive me for asking, but by my calculations I can't use 500kcmil AL. The switch gear is rated for 70 degree conductors so wouldn't I have to use the correction factor of .886? That would de-rate 500 AL below what is needed for 1200 amp
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Old 04-01-2019, 05:44 PM   #23
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Thank you for the response. Please forgive me for asking, but by my calculations I can't use 500kcmil AL. The switch gear is rated for 70 degree conductors so wouldn't I have to use the correction factor of .886? That would de-rate 500 AL below what is needed for 1200 amp
Yes, it looks like this calculation would give 1194.3A vs 1200A. In my book that is close enough (99.5% of the required ampacity). If it ever comes up, I would simply do a small temperature correction factor based on actual ground temperature (see CEC appendix B notes on Rule 4-004, 2018 page 579). If the ground temperature is less than 20 deg C, you can apply this correction.

On a side note, you only have to assume that the ground temperature is 19 deg C to give a large enough correction factor to push you over 1200A for 4 sets of #500MCM alum.

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Old 04-02-2019, 05:29 AM   #24
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You would use the ampacities from table D11A for 5 cables per phase. Your equipment is probably rated at 75 degrees so Multiply that by .866 because the ampacities givin in that table are at 90 degrees. (See note 2)
I am trying to follow along but if D11A, if it is Annex D: Examples D 11. That is for Mobile Homes so I'm lost there. And then: Where is .886 coming from? I want to say under Table 310.15 (B)(2)(a) 75 degree C @ .88 adjustment factor, but that is for ambient temp of 96-104 F.
What am I missing here?
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Old 04-02-2019, 07:40 AM   #25
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I am trying to follow along but if D11A, if it is Annex D: Examples D 11. That is for Mobile Homes so I'm lost there. And then: Where is .886 coming from? I want to say under Table 310.15 (B)(2)(a) 75 degree C @ .88 adjustment factor, but that is for ambient temp of 96-104 F.
What am I missing here?
Wrong book brother. This is Canadian code, not the NEC
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Old 04-02-2019, 05:47 PM   #26
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Wrong book brother. This is Canadian code, not the NEC
...well damn. It all makes sense now.
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Old 04-13-2019, 03:38 PM   #27
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I know that this is going to be an unpopular opinion but I have taken a much different approach to the D11 tables since their introduction. After 3 conductors per phase the ampacity drops off far too much. I've never actually seen a contractor stack conduits like in the D11 diagrams either. In some cases the service entrance equipment can't even handle the number of conductors that the D11 tables dictate. No one around here is prepared to pay for copper except for maybe the government.

My solution... go with a wider trench and install in 2 groups of 3 conduits side-by-side (6 per phase total). Each group would have spacing as per the D11 Detail 3. Maintain at least 12" apart between the groups. I then use the 3 conductors per phase column in D11B. It is a little unconventional but it seems to work fine.

For 1600A I usually recommend:
1600A-3∅ (UNDERGROUND) - 6 SETS OF (4) #500MCM ALUM. IN 4"C
*** INSTALL IN 2 GROUPS OF 3 CONDUITS, MAINTAIN MIN 12" [0.3m] CLEARANCE BETWEEN CONDUIT GROUPS, SEE CEC APPENDIX D, DIAGRAM D11, DETAIL 3

Lastly, (and most controversially) I don't agree that the underground soil temperature is 20deg C as noted by default in rule 4-004 notes. There are a few additional temperature correction factor calculations noted in CEC appendix B rule 4-004. Assuming a more realistic ground temperature of 5 deg C gives a correction factor of around 1.1. Applying both the 75 deg derating (0.886) and the temperature correction of 1.1 gives a total correction of 0.975. This means that I interpret the values in the D11 tables at nearly face value.

I've done this on countless projects and have never had it rejected. In situations where the contractor didn't maintain spacing, we sent the parameters to the wire supplier (usually Southwire or similar) and they did a formal IEEE 835 simulation to determine the actual ampacity. Usually the IEEE 835 simulation gives slightly higher ampacities than the D11 tables but not always.

CAVEAT: Information presented in this post does not constitute professional engineering advice.

Hello 908,

I appreciate for your reply.

After calculating by myself based upon your suggestion, detail 4, 5 and 6 ( stacking installation) in the D11 shall not be practical on site. But existing drawing shows a lot of detail 4, 5 and 6. You recommend side by side installation will be more practical. Shall I revise the these design in the future ? This is my dilemma as well. Because code book does not tell thees things..
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Old 04-14-2019, 01:44 AM   #28
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Hello 908,

I appreciate for your reply.

After calculating by myself based upon your suggestion, detail 4, 5 and 6 ( stacking installation) in the D11 shall not be practical on site. But existing drawing shows a lot of detail 4, 5 and 6. You recommend side by side installation will be more practical. Shall I revise the these design in the future ? This is my dilemma as well. Because code book does not tell thees things..
If you deviate from your engineered drawings you run the risk of rejection by the inspector or the engineer. Best to present your alternate methods prior to install.

If the conduit and cable arrangement are at your discretion, two parallel groups is what I would normally recommend. You're right to say that the code does not suggest this arrangement. This does not suggest it is prohibited; it is simply non-standard. Non-standard arrangements are permitted in some situations as long as you get approval. I have found this alternate arrangement to be a necessary step to keep things practical and compatible with the service entrance gear. If your inspector rejects your alternate methods, you may be able to get some help from your friendly neighborhood engineer or wire supplier.

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