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Old 03-23-2019, 10:05 PM   #1
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Default Cable size Calculation for 1600A service

Hi guys

I need some help calculating cable sizes for a main service. I'm preparing by myself for some job, because my senior left this office recently. But I need to prepare cable sizing soon.

Before that, understanding some previous project for calculation, main incoming was required 1600A, the cable size was 5 runs of 4 # 750MCM copper cable via concrete encased duct bank. I really want to how this size come out step by step?

This was cable assembly not individual cable per phase, I use Table 2 instead of D10 or others even if duct bank installation. Each run conductor ampacity would be 1600/5= 320A. After that shall I apply for derating factor T5C ?

Thanks in advance
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Old 03-23-2019, 11:35 PM   #2
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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)
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Old 03-24-2019, 04:17 AM   #3
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Does it matter if you are terminating on bus or a breaker in the Canadian Code?
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Old 03-24-2019, 08:28 AM   #4
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Does it matter if you are terminating on bus or a breaker in the Canadian Code?
Does what matter? The termination temperature?
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Old 03-24-2019, 09:26 AM   #5
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Originally Posted by eddy current View Post
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)
Hi Eddy

Thank for your reply.

According to D10A, 750MCM has 447A ampacity for 90 degree. For 75 degree, 447A x 0.886 will be 386A. But demand current was 1600/5=320A, so that 386A would be far enough?? My idea was 600MCM is big enough, is't it?
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Old 03-24-2019, 09:44 AM   #6
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Hi Eddy

Thank for your reply.

According to D10A, 750MCM has 447A ampacity for 90 degree. For 75 degree, 447A x 0.886 will be 386A. But demand current was 1600/5=320A, so that 386A would be far enough?? My idea was 600MCM is big enough, is't it?
D10A is for direct buried copper wires, you need to use D11A for raceways
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Old 03-24-2019, 12:07 PM   #7
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There is the other item you may want to be aware is derating on distance due I do not know how long the run it will be per OP plan will be.

Just keep in your mind the raceway have different rating than direct burial or multi conductors in the conduit.

I am aware 750 kcm copper is not super common item to use but check 600 Kcm also to see what you can come up and if your location do allow alum conductors I would go that route due the cost factor.
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Old 03-24-2019, 04:12 PM   #8
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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.
james64 and canbug like this.
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Old 03-27-2019, 10:45 AM   #9
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908 Eng, Say hi to Keith for me!
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Old 03-27-2019, 11:10 AM   #10
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908, I like your explanation and agree with all but the soil temp. I would suggest that at least around Calgary it would be around 10C but every region is going to be a little different.


Tim.
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Old 03-27-2019, 11:11 AM   #11
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It would be quite unusual if distance were a factor for such a Service as it is ordinary practice to design away until the Poco's transformer is as close to the distribution equipment as is possible.

The last time I installed 1600A @480Y277 the conductors were at most 25 feet long -- tip to tip.

Copper is hopelessly uneconomic. As posted above, only the government would stump the bill.

Around here, the Poco would entirely decide what is acceptable. They won't stock every size of conductor. They won't even run copper unless it's an 'Adder' -- and you toss in a four-week bump in delivery. Their 'Green Book' has all of this figured out. It's not necessary to re-design the wheel.

Naturally, I took their stuff and compiled a table. To the OP, you ought to do likewise. All of the Big Boys have figured this out. Instead of racking their brains, they just look at the table of specifications. They re-visit these issues but very rarely. No-one has the time to mess around with such basics.
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Old 03-27-2019, 11:44 AM   #12
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Quote:
Originally Posted by canbug View Post
908, I like your explanation and agree with all but the soil temp. I would suggest that at least around Calgary it would be around 10C but every region is going to be a little different.


Tim.
That is a fair point. The few degrees difference only seems to affect the ampacity by a few amps. I usually only stretch this code rule when we are a couple amps short of our service entry equipment. Thanks!
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Old 03-29-2019, 03:29 PM   #13
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Quote:
Originally Posted by 908Eng View Post
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.
Is there something in the CEC that says as long as you maintain 12'' between groups that grouping is acceptable?
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Old 03-29-2019, 03:39 PM   #14
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Short answer: No.

Longer answer: There is no CEC reference that indicates 12" separation between groups. There isn't even a reference that says that you can install in 2 groups! In most jurisdictions the inspection authority would allow the Electrical Engineer to make a judgement call as long as it seems reasonable. In Alberta, at least, we have a STANDATA that says that a main secondary conductor arrangement can be reviewed and approved by an engineer.

12" group separation seems reasonable to me but it is definitely up for debate. I think it is better than stacked conduits.

CAVEAT: Information presented in this post does not constitute professional engineering advice.
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Old 03-29-2019, 07:31 PM   #15
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Thanks 908, at least maybe it is something I can bring up to my local inspection authority, however I am less than optimistic with them given their track record over the last 6-8 months. Let me run by you my situation.

I want to build a 1200amp service for an apartment building. So what I have been planning is coming into a 1200a heavy duty disconnect switch, out to a 1200a tap box, then out to my meter stacks etc. So figuring service conductors has been less than fun given the temperature correction factors that need be applied.
As you have mentioned above, no one will swing for copper conductors other than government, for which I agree, so I am left with AL conductors if I even want to think about being awarded the job.
The switch is supplied with lugs for 750kcmill max. So its looking like I am stuck running 4 runs of 750kcmill which comes in at 1226 amps. The issue I have with this is that I do not know if my switch has the capacity to terminate that amount of conductors per lug not to mention that I don'y really want to stack my conduit pipes like they have in the detail provided in D11.
Care to provide any insight into the situation that I may have missed?
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Old 03-29-2019, 07:49 PM   #16
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A disconnect switch at that size is a monster - and will be custom built for your order. They don't keep them on the shelf. Because of this, just order a switch with the lugs you want. They will give it to you.

Consequently, why would you need a tap-box? Just how many meter stacks are you planning for? You could easily come off such a disconnect with four conductors per phase. Two of which would feed an entire 'six-stack' of apartments w/ 600A -- a hundred a pop.

If this feed was short or underground, you've got the tap-rules on your side.

For us, this would be installed as a NEMA3R line up, with power being brought up from below and then passing left to right, say, to twin meter stacks... perhaps with an extra 1/2 stack ( 3 ) set so that a house panel could be separately metered.

My Poco has already determined what they want. ( EUSERC ) So for us, such designs are 'canned designs.' But, I'll admit we don't face snow.

Do you get a big price break for your scheme?
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Old 03-29-2019, 08:06 PM   #17
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The switch is all built sitting on a shelf in Ontario ready to ship. It is the largest Siemens makes that isn't special ordered apparently. The spec sheet that I was given has the lug size (750) but not the amount of termination spaces.
As for the tap box, it is needed in between the disconnect and the meter stacks. Out of the load side of the switch, into the tap box, which is mounted directly to the meter stacks. This is where it is converted to bus bars which carry on to the rest of the meter stacks. I think I have 8 stacks all together
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Old 03-30-2019, 12:24 AM   #18
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Quote:
Originally Posted by jusmac View Post
Thanks 908, at least maybe it is something I can bring up to my local inspection authority, however I am less than optimistic with them given their track record over the last 6-8 months. Let me run by you my situation.

I want to build a 1200amp service for an apartment building. So what I have been planning is coming into a 1200a heavy duty disconnect switch, out to a 1200a tap box, then out to my meter stacks etc. So figuring service conductors has been less than fun given the temperature correction factors that need be applied.
As you have mentioned above, no one will swing for copper conductors other than government, for which I agree, so I am left with AL conductors if I even want to think about being awarded the job.
The switch is supplied with lugs for 750kcmill max. So its looking like I am stuck running 4 runs of 750kcmill which comes in at 1226 amps. The issue I have with this is that I do not know if my switch has the capacity to terminate that amount of conductors per lug not to mention that I don'y really want to stack my conduit pipes like they have in the detail provided in D11.
Care to provide any insight into the situation that I may have missed?
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.
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Old 03-30-2019, 01:00 AM   #19
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The switch is all built sitting on a shelf in Ontario ready to ship. It is the largest Siemens makes that isn't special ordered apparently. The spec sheet that I was given has the lug size (750) but not the amount of termination spaces.
As for the tap box, it is needed in between the disconnect and the meter stacks. Out of the load side of the switch, into the tap box, which is mounted directly to the meter stacks. This is where it is converted to bus bars which carry on to the rest of the meter stacks. I think I have 8 stacks all together
Don't be shocked if it's a two-up lug set-up. I'd pick up the phone and nail this down.

Since fuses are usually expected to be 125% or more of the current, don't be surprised if Siemens didn't expect that you'd be pulling ~1000A across this disco.

You're intending to take the load all the way up the scale.

(2) @ 750kCMIL (Cu-THHN) 475 x 2 = 950A

Temperature correction (1.05) would get you to 997.5 -- which might then be protected by 1200A fuses... on the basis that you don't want them to blow because of a short-term surge... like everyone coming home at the same time.

Yes, I'm using Ugly's tables... I'm sure Canada's are somehow different.

Are you sure you can load your fuses that tightly to the projected load?

Does that fly up north?

Last edited by telsa; 03-30-2019 at 01:05 AM.
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Old 03-30-2019, 01:07 AM   #20
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BTW, are the 750kCMIL lugs dual-rated -- that is can they take (2) 500kCMIL as well? ( per lug hole so four per phase )

Then that would make perfect sense.
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