Conduit and Conductor Sizing for Transformer to Service Entry: Cheapest and Best?

[Njohneer]

:? I am an engineering technician learning how to design light commercial (office, retail) electrical systems.

I have some questions for you about specifying service lines, and interpreting 2011 NEC Table 310.15 B 16

My goal is to specify what the electrician will actually buy and use, not some ideal that's too expensive for our clients.

Our jobs are all in the Dallas/Fort Worth Texas area, and the utility provider is Oncor.

Is aluminum that much cheaper than copper? What about "copper-clad" aluminum? Which do you prefer?
Then, which type: THHN, THHW, XHHW ?
Do you calculate the size based on 75dC or 90dC ?
Do you always get service in 200A increments?
What is the cheapest way to run these? for example, should I specify (3) 400A runs to equal the 1200A, or specify larger conductors?
200A
400A
600A
800A
1,000A
1,200A
We size the neutral at full load. Is that correct?

Thank you!

 

[augie47]

Huge number or variables....

:? I am an engineering technician learning how to design light commercial (office, retail) electrical systems.

I have some questions for you about specifying service lines, and interpreting 2011 NEC Table 310.15 B 16

My goal is to specify what the electrician will actually buy and use, not some ideal that's too expensive for our clients.

Our jobs are all in the Dallas/Fort Worth Texas area, and the utility provider is Oncor.

Is aluminum that much cheaper than copper? Last I checked 2 to 1 for equivalent size but that varies by date and lolcation What about "copper-clad" aluminum? Never see it. Which do you prefer? Too many opinions
Then, which type: THHN, THHW, XHHW ? Many conductors are dual rated. Locally AL is normally XHHW, Cu THHN/THWN-2
Do you calculate the size based on 75dC or 90dC ? Termination is almost always based on 75?
90? is normally used for derating calculations
Do you always get service in 200A increments? NO
What is the cheapest way to run these? for example, should I specify (3) 400A runs to equal the 1200A, or specify larger conductors?
200A
400A
600A
800A
1,000A
1,200A
Larger services are normally parallel runs
We size the neutral at full load. Is that correct?
Totally determined by nature of the load... often some reduction is allowed, occasionally upsizing is required for harmonics.

Thank you!

 

[Njohneer]

Thank you @augie47

I just got off the phone with a master electrician. Reviewing NEC's Table 310.15 b 16 he suggested:

Remember I am only concerned with the run from utility transformer to building service entry.

Because 90% of our work is light commercial 208V/3 phase, he recommends using copper only (aluminum requires upsizing, increases conduit size, and loosens at connections over time). The price savings isn't worth it in the long run.

Furthermore, because we cannot say exactly what service equipment will be installed, it is best to calculate conductor size at 90C to ensure everything complies.

Also, he said looking at the 90C copper column for 200A service, 2/0 can be used even though the column reads 195A. Does that 5A not make enough difference to go up to 3/0?

Please advise...

 

[ron]

Light commercial (office, retail) electrical systems usually requires a PE to sign / seal for permit application. Is that not the case for the area you are designing?

90 deg amapacities are too high for the 75 deg terminations that have been tested with the equipment you are using. See 110.14(C)

You can round up from the 301.15.b.16 in certain circumstances such as allowed in 240.4(B)

 

[augie47]

IMO, your electrician is giving you bad advice.
1st you must determine the "service point", that point where the utility wiring connects to the premises wiring. Any wiring on the utility side of that point has noting to do with the NEC.
For conversations sake I will assume you are addressing service wiring installed by the electrician and thus under NEC regulations.
As far as the AL/Cu consideration, overlooking personal preferences, AL is normally less expensive even after taking adjustment into consideration but that might not be true in all cases so you could look at that on a case by case basis.
To me, the real issue is the comment concerning the 90? rating. As referenced in my earlier reply, NEC 110.14 will almost always limit your termination ampacity to the 75? rating maximum.
The 90? rating is primarily used for derating and the end result must not exceed the termination temperature ratings.
2/0 Cu has a 75? amapcity of 175 amps. The load is not allowed to exceed that ampacity.
(for your commercial application).
240.4 allows 'next size up" for your overcurrent device however 175 is a "standard size" (see 240.6) so if you are using 2/0 cu in a non-residential installation, in almost all situations you are limited to a 175 amp ampacity.
Only if you happen to have a situation where the equipment will allow 90? terminations would you be allowed to use 2/0 Cu at 195 amps.

 

[Njohneer]

90 deg amapacities are too high for the 75 deg terminations that have been tested with the equipment you are using. See 110.14(C)
Thank you! That means use the 75C column.

You can round up from the 301.15.b.16 in certain circumstances such as allowed in 240.4(B)
Round up? I want to round down. That is, if I need 400A service, the table says to use 600, but one electrician I know runs 500 for 400A services because 380A is "close enough". Is that too dangerous? Just stick with 600?

 

[augie47]

90 deg amapacities are too high for the 75 deg terminations that have been tested with the equipment you are using. See 110.14(C)
Thank you! That means use the 75C column.

You can round up from the 301.15.b.16 in certain circumstances such as allowed in 240.4(B)
Round up? I want to round down. That is, if I need 400A service, the table says to use 600, but one electrician I know runs 500 for 400A services because 380A is "close enough". Is that too dangerous? Just stick with 600?

You are going to have to decide which horse you wish to ride
In your original post you stated:
My goal is to specify what the electrician will actually buy and use, not some ideal that's too expensive for our clients.
On a 400 amp service, if the calculated load does not exceed 380 amps, you will find very few electricians who will use 600kcmil Cu unless required by specs to do so

 

[Njohneer]

On a 400 amp service, if the calculated load does not exceed 380 amps, you will find very few electricians who will use 600kcmil Cu unless required by specs to do so

Now that is what I was looking for! They run the line based on the calculated load, not the nominal load. :happyyes:

 

[augie47]

Keep in mind the NEC is the minimum standard designed to protect property and persons.
It is not a design manual nor is it written to take into account what is best for you client in either the short or long term.

 

[Njohneer]

Keep in mind the NEC is the minimum standard designed to protect property and persons.
It is not a design manual nor is it written to take into account what is best for you client in either the short or long term.

Very good point. Nevertheless, for our clientele the GC or electrician makes the final decision of what to actually buy and install, and the minimum is all they are willing to pay for. I'm grateful for building inspectors, simply because they keep uninformed owners and cheap/lazy electricians in check.

I can specify, and our PE can seal, whatever we want, but when it's all said and done, the city plan inspectors aren't the same people as the building code inspectors, and building code inspectors inspect according to the NEC, not to the PE's sealed plans.

I'm cynical, I know, but I believe that is the reality. Am I wrong? :dunce:

 

[don_resqcapt19]

...
I can specify, and our PE can seal, whatever we want, but when it's all said and done, the city plan inspectors aren't the same people as the building code inspectors, and building code inspectors inspect according to the NEC, not to the PE's sealed plans. ...

While that is how I think it should be as far as the city inspections, a contractor who provides a fixed cost to the owner based on the sealed plans and specs and then installs something different without a written change order should be sued in civil court for fraud, and maybe even charged with fraud in criminal court.

 

[Njohneer]

Wire and Conduit Sizing Refernce Table

Wire and Conduit Sizing Refernce Table

Is this a viable :? table to refer to for basic utility transformer to 208Y/120V-3Ph-4W building service entry wire and conduit sizing?

From NEC Table 310.15(B)(16) at 75C CU:

	Load A	Wire AWG/MCM	Conduit Inch (4 wires)
	100	3	1.25
	115	2	1.25
	130	1	1.5
	150	1/0	1.5
	175	2/0	2
	200	3/0	2
	230	4/0	2
	255	250	2.5
	285	300	2.5
	310	350	3
	335	400	3
	380	500	3
Load Over 380A	Load A	Wire AWG/MCM	Conduits Inch
400	2x200A	3/0	2x2"
600	2x310A	350	2x3"
800	3x285A	300	3x2.5"
1000	3x335A	400	3x3"
1200	4x310A	350	4x3"

 

[electrofelon]

:

Is aluminum that much cheaper than copper? Which do you prefer?
Then, which type: THHN, THHW, XHHW ?

Price out a few thousand feet of copper vs alumium and there is pretty good chance you will lean toward the aluminum hmy: I almost always use aluminum for 200 amp + stuff and often for 100 amp feeders. You may have a job spec that requires copper. Generally single conductor aluminum is type XHHW. I have only seen aluminum THHN a few times outside of a cable assembly. There are geographical differences as to what is available and stocked.

Do you always get service in 200A increments?

Often but not always. A common practice for other than small services is to use a service rated MLO panelboard with 2-6 breakers serving as your disconnecting means. In that case there is no single main and the service size is the ampacity of the service conductors which is usually an odd number.

What is the cheapest way to run these? for example, should I specify (3) 400A runs to equal the 1200A, or specify larger conductors?
200A
400A
600A
800A
1,000A
1,200A

The ampacity per unit of conductor area goes down as the conductor gets larger so it is more efficient in terms of amount of conductor material required to run several sets of parallel smaller conductors. For example a 250 Kcmil aluminum conductor gives you 205 amps while a 750 Kcmil conductor gives you 385, so the 750 has three times the conductor area but less than twice the ampacity. Of course there are more conduits so you have to weigh out the right balance for the given situation. Generally as a rough rule, I stick to 200-250 amp sets.

We size the neutral at full load. Is that correct?

Generally for your run of the mill typical three phase 120/208 service you would just skip the load calc and run a full size neutral. certain situations would require further investigation.