3000 A conductor sizing for service entrance

[17Steve69]

I am sizing conductors for a 3000A service, 480V, 3phase, 4 wire system. I plan to use 90 deg wire, THHN. I have not decided on what type of cable yet but for these calculations I used SE cable in raceway, underground. The ambient temp is earth temp, 20 deg C. This wire comes from the service transformer to metering then to a transfer switch (service entrance rated with 3000A 100% rated breaker)(all outdoors). So I didn't derate for higher ambient temperature but did uprate for 20 deg C by 1.08 per table 310.15(B)(3)(a).
I broke the 3000A down into 3 different raceways or 1000A per raceway. I did calculations based on 500, 600, 700 and 750 kcmil wire with a neutral ran with each raceway and it is a current carrying conductor. Here is the number of conductors I came up with:

Total Current	Ambient Correction Factor	Number of CCC derate
3000	100%	50%
Number of Parallel Runs	3.00
Current per Run	1000.00
Corrected Ambient Current	1000
Corrected CCC (10-20 CCC's)	2000
	# of conductors per phase (90 deg wire)		Number of CCC (exclude neutral)	# of CCC's with neutral
500kcmil	4.306632214	5	15	20
600kcmil	3.898635478	4	12	16
700kcmil	3.561253561	4	12	16
750kcmil	3.461405331	4	12	16

Now if I figure out conduit fill, I am pushing into greater than 6" RMC conduit for each run (3 runs). Did I do something wrong? I have been beating this to death but keep coming up with the same problem. Should I be looking at a different cable type like MI or MC?
Does 310.15(B)(3)(3) allow nonderating if using USE cable in a trench (says "adjustment factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of RMC, IMC, rigid PVC or RTRC having a length not exceeding 10' ")?
Any help or confirmation of my calculations would be appreciated. Thanks.

 

[ron]

Because these are underground, you need to consider the dirt or concrete that will not allow the heat to release as if it were above ground.

What orientation will the conduit be in?

I would start from the Table B.310.15(B)(2)(7) for single conductors.

 

[electrofelon]

If I am correctly understanding what you are proposing, I would say that I and most people would not do it that way. I would do more parallel runs in separate pipes to avoid such a severe derating. In fact I would probably only have one set per conduit, and definitely not more than two.

 

[petersonra]

If I am correctly understanding what you are proposing, I would say that I and most people would not do it that way. I would do more parallel runs in separate pipes to avoid such a severe derating. In fact I would probably only have one set per conduit, and definitely not more than two.

is he seriously talking about pulling 15 or 20 large conductors through a single underground raceway? or am I misunderstanding what his chart is trying to say?

ETA: In any case THHN cannot be used underground. I don't think SE cable can be used underground either.

 

[iwire]

I have installed and worked on a a number of 3,000 amp services and everyone of them used eight sets of 500 copper in eight raceways.

That results in 3,040 amps of conductor.

500 Kcmil copper @ 75C is rated 380 amps

8x380=3,040

 

[17Steve69]

is he seriously talking about pulling 15 or 20 large conductors through a single underground raceway? or am I misunderstanding what his chart is trying to say?

ETA: In any case THHN cannot be used underground. I don't think SE cable can be used underground either.

No, I was trying to use only 3 or 4 raceways but that would be a pain to and would violate any fill requirements, as I found out. I think SE cable can be used underground in a raceway, not direct buried. I am not sure about the THHN but fairly certain on the SE cable. I just haven't got that far in my design. I was using 90 deg wire to see if I could get past the conduit fill issues with 3 or 4 conduits.

 

[17Steve69]

Because these are underground, you need to consider the dirt or concrete that will not allow the heat to release as if it were above ground.

What orientation will the conduit be in?

I would start from the Table B.310.15(B)(2)(7) for single conductors.

They are in a horizontal run (buried in earth).

 

[17Steve69]

I have installed and worked on a a number of 3,000 amp services and everyone of them used eight sets of 500 copper in eight raceways.

That results in 3,040 amps of conductor.

500 Kcmil copper @ 75C is rated 380 amps

8x380=3,040

I was trying to avoid so many raceways. In addition, I have a CCC in my neutral so have to derate every set anyway by 80% so 8 sets conductors in 8 raceways would not be adequate (would need 10 sets of 500kcmil 75 deg). I guess I am stuck using more raceways than I want too no matter what I do. The electricians will love me no matter what I do. :rant:
I can uprate based on 20 deg ambient (1.08 factor) though since less than 10'/10% of run.

 

[petersonra]

I'm pretty sure you cannot use THHN Underground as underground is considered a wet area and THHN is not rated for it. However most versions of THHN are also rated is thwn which can be used in wet areas. I noticed in your original post you said something about using MI cable. I can't even imagine that would be appropriate.

In another part of the original post you said something about USE cable. You can certainly use use cable Underground.

You are calling this a service. Is it really a service? Or is it a giant feeder? Are you planning to run an EGC with it?

 

[electrofelon]

No you cannot use SE cable underground even if in a raceway. Why would you want to anyway?

 

[17Steve69]

I'm pretty sure you cannot use THHN Underground as underground is considered a wet area and THHN is not rated for it. However most versions of THHN are also rated is thwn which can be used in wet areas. I noticed in your original post you said something about using MI cable. I can't even imagine that would be appropriate.

In another part of the original post you said something about USE cable. You can certainly use use cable Underground.

You are calling this a service. Is it really a service? Or is it a giant feeder? Are you planning to run an EGC with it?

They wanted to use USE cable but then balked at having to protect it from grade to 18" underground and with bushings or other water egress prevention. So they said no USE cable since would have to have a 'partial' raceway anyway. EGC will be ran as well. This is definitely a service. To make it more interesting, they are installing two services since utility doesn't support one service above 3000A with one transformer. They will not be tied in parallel any where within the facility. I just through out the MI cable comment cause they have different derating, if at all, than other cables ran directly underground.

 

[Cow]

They wanted to use USE cable but then balked at having to protect it from grade to 18" underground and with bushings or other water egress prevention. So they said no USE cable since would have to have a 'partial' raceway anyway. EGC will be ran as well. This is definitely a service. To make it more interesting, they are installing two services since utility doesn't support one service above 3000A with one transformer. They will not be tied in parallel any where within the facility. I just through out the MI cable comment cause they have different derating, if at all, than other cables ran directly underground.

I thought the only one that direct buries service conductors between the POCO transformer and the service are utility companies? We have always installed raceways for as long as I can remember. Why are the electricians balking at installing conduit?

I also don't understand why you think you need an EGC ran with service conductors?

 

[electrofelon]

I thought the only one that direct buries service conductors between the POCO transformer and the service are utility companies? We have always installed raceways for as long as I can remember. Why are the electricians balking at installing conduit?

Around here in central NY with the soils (read: rocks) we have, I pretty much never direct bury. The utilities will direct bury but they spec a sand cover, which to me is more of a hassle and overall expense than just using pipe. I have worked on some jobs in Georgia clay that were direct buried and I am comfortable with it there in conjunction with megger testing.

I also don't understand why you think you need an EGC ran with service conductors?

I agree. Many odd things about this. Sounds like some of the people involved do not know what they are doing, or we are getting incorrect information.

 

[iwire]

I was trying to avoid so many raceways.

In my experiance less raceways generally makes it harder

In addition, I have a CCC in my neutral so have to derate

I seriously doubt that the majority of the load is non-linear.

The electricians will love me no matter what

You make me drag multiple sets in one raceway and I will hate you.

 

[ron]

They are in a horizontal run (buried in earth).

Which means you cannot start with ampacities in Table 310.15(B)(16) since it is conductors in raceway underground.

 

[petersonra]

Many odd things about this. Sounds like some of the people involved do not know what they are doing, or we are getting incorrect information.

I agree. I don't know what I am doing with respect to this kind of install and a lot of things seemed off to me. Especially the off hand comment about using MI cable.

I also wonder about the comment on having to run the SE at least 18" below grade. I think you have to run it a lot lower than that if it is not in a raceway. And if it is in a raceway, why would you use SE.

 

[augie47]

No you cannot use SE cable underground even if in a raceway. Why would you want to anyway?

Just second guessing but in his original post he mentioned USE so I assume later posts were actually relating to USE.

 

[augie47]

I was trying to avoid so many raceways. In addition, I have a CCC in my neutral so have to derate every set anyway by 80% so 8 sets conductors in 8 raceways would not be adequate (would need 10 sets of 500kcmil 75 deg). I guess I am stuck using more raceways than I want too no matter what I do. The electricians will love me no matter what I do. :rant:
I can uprate based on 20 deg ambient (1.08 factor) though since less than 10'/10% of run.

Unusual in this area to find a 3000 amp service where the neutral counts as current-carrying in regard to derating. Curious as to the nature of the load (requiring derating)???

 

[electrofelon]

Just second guessing but in his original post he mentioned USE so I assume later posts were actually relating to USE.

Yeah was thinking that too.

 

[meternerd]

I thought the only one that direct buries service conductors between the POCO transformer and the service are utility companies? We have always installed raceways for as long as I can remember. Why are the electricians balking at installing conduit?

I also don't understand why you think you need an EGC ran with service conductors?

Surprising to hear that some utilities still direct bury service conductors. Makes it extremely time consuming and costly to repair/replace if it fails. We as a utility (both CA and Alaska) did away with any new direct bury and also in CA, replaced (at our cost) older existing direct burial with conduit. Agree on EGC. Utility will never (AFIK) install or require any grounds in a service drop.