Sizing Service Conductors Correctly

[dnem]

My building department might operate like almost every other building department, but I'm not going to assume that. So let me explain the background of this thread and how my department operates.

In my building department, we send all apps thru plan review before a permit can be issued. Residential is completely separated from Commercial projects and go thru separate plan reviewers.

Residential plan review does not include any electrical at all. I have yet to see anyone even include any electrical symbols, circuits, or specs on any residential prints. Us ESI inspectors in my department never look for prints on residential projects/houses during an inspection. We go by NEC only. We have no local codes of any kind.

Commercial is totally different. All electrical specs must be included on all submissions and go thru plan review. During service, rough, and final inspections, the approved prints must be on site and the electrical installed according to the approved prints. Any deviation from the approved prints is looked at by us inspectors and considered. We then use our judgment in determining whether or not that deviation is significant enough to be considered a violation that must be corrected.

Because our Commercial plan reviewer has far more plans being submitted to review than he is capable of doing, he subs out review work to a plan review private contractor. Lately this contractor has been approving Commercial plans [which then get a much abbreviated review from our plan reviewer and then approved by our department] that don't include wire sizes. Electrical contractors have been choosing wire sizes themselves and frequently not doing it properly.

I want to run thru the process and see if anybody else has anything to add to it.

For Residential you simply go to Table310.15(B)(6)
For Commercial you can not. You [usually] must use Table310.16. But that table is alot more complicated than the residential table and it's very easy to screw-up.

You need to start by knowing which temperature column to use. 110.14(C) "Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."
Most electrical inspectors know that the lowest rated "weakest link in the chain" is almost always the lug. Lugs for #1 and smaller wire are lab rated for 60?C, unless otherwise labeled. Lugs for #1/0 and larger wire are lab rated for 75?C, unless otherwise labeled.

So now you have the right column in Table310.16. Now to pick the right sized conductor from that column. 240.4(B) "Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected), shall be permitted to be used, provided" ..... blah blah blah ..... and "(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or circuit breaker...". 240.6(A) lists the standard ratings for 800a or less as: 1, 3, 6, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 601, 700, 800.

For example:.....
A 90 amp limit taken from Table310.16 would require a 90 amp or smaller breaker.
A 91 amp limit taken from Table310.16 would require a 100 amp or smaller breaker.

Getting specific:
Your 100 amp service needs a 91 amp or larger rated wire from Table310.16. For #1 or smaller you're looking at the 60?C column. For this decision, it does not matter if the wire is rated 90?C. You still need to stick to the columns dictated by the lug rating. You need a #2. The #4 you get from Table310.15(B)(6) can not be used. Wrong table.

Your 150 amp service needs a 126 amp or larger rated wire from Table310.16. For #1/0 or larger you're looking at the 75?C column. You need a #1/0. The #1 you get from Table310.16 column 75?C can not be used. Wrong column for lugs connected to #1 wire. The #1 you get from Table310.15(B)(6) can not be used. Wrong table.

Your 200 amp service needs a 176 amp or larger rated wire from Table310.16. For #1/0 or larger you're looking at the 75?C column. You need a #3/0. The #2/0 you get from Table310.15(B)(6) can not be used. Wrong table.

As long as I haven't made any mistakes in my references, this is the process required to select service conductors. And every week or so I find a Commercial service wired according to Table310.15(B)(6) which has to be redone.

Any questions or comments ?

David

 

[iwire]

I agree with most of it.

My only real concern is the terminal temp.

None of the equipment I install has 60 C terms regardless of the equipment being less than 100 amps.

The equipment is marked as 75 C.

I have heard that some cheap HVAC disconnects still have 60 c terms.

As far as Table.15(B)(6) I agree with you, that only applies to dwelling units however those dwelling units could be in a 30 story high rise.

All I am saying is that table applies to more than single family homes.

 

[dnem]

I agree with your point about your equipment. From your perspective, your idea is working. But you're dealing with a different situation than an inspector deals with.

None of the equipment I install has 60 C terms regardless of the equipment being less than 100 amps.

The equipment is marked as 75 C.

I have to look at what is available in the marketplace, not just lug temps of common equipment. Especially with a global market, manufacturers are coming in to get business. They have to provide prospective customers with a reason to start using their product. They commonly go for the low price offering. They can get a low price from using third world labor, or higher productivity, or lower material costs.

Testing labs, such as UL, will accept a piece of equipment with a lug that is built to function at no more than 60?C as long as it will accept a conductor no larger than #1. If it passes all tests and meets all criteria, it is approved and listed by the lab. It hits the market with a listing approval and a 60?C rated lug.

As an inspector, I have to be aware that these products are being approved and then inspect accordingly. So I have to assume that the product is built to minimum standards unless labeled otherwise.

David

 

[iwire]

All I was trying to convey is we can't assume 60 or 75 C, we have to check the equipment.

Most times I can use 3 AWG copper for 100 amp feeders.

 

[peter]

I like this.
Why not have a new Code table similar to 315b6...for commercial?
For commersial sevices
use #2 for 100 amps.
use #1/0 for 150 amps
use #3/0 for 200 amps
and so on.
~Peter

 

[dnem]

When I make various contractors aware of this process of selecting a conductor size, the most common reaction is: "Then why do the manufacturers make 90?C wire if it can't be used ?"

The answer is correction factors.
Here are 2 of them:

1) There is an ambient temperature correction factor chart at the bottom of Table310.16. Most of the time, a high ambient temp is only an issue for a section of the conduit run and doesn't affect the area that contains the panels. In that situation, the ambient correction is a number that is multiplied by the wire temp rating. The wire insulation is able to withstand less amperage when the ambient temp is higher. But when you decrease the allowable amps on the wire that has a higher amperage rating because the insulation can withstand 90?C without damage, you still might be above the amps allowed on the lug.

The end result may be that no change is required even with the higher ambient.

2) There is a conduit wirefill adjustment factor in Table310.15(B)(2)(a).
[This should not be confused with max allowable conduit wirefill in Annex C: Tables]
Wirefill deduction only applies to conduits and not panels, so it is only an issue for conduit runs and not the lugs in the panels. When you put more than 3 wires in a pipe, the adjustment factor is a percentage number that is multiplied by the wire temp rating. When multiple wires are confined together, their ability to cool themselves is reduced. The wire insulation is able to withstand less amperage when their cooling ability is less. But when you decrease the allowable amps on the wire that has a higher amperage rating because the insulation can withstand 90?C with out damage, you still might be above the amps allowed on the lug.

The end result may be that no change is required even with more than 3 wires in a pipe.

David

 

[jim dungar]

David

It is very important to understand the temperature limitation is based on the termination not the lug. This is important because most lugs are actually rated at 90C, and there are many contractors that feel they can simply replace any manufacturer's lug in order to use a higher rating.

 

[radiopet]

Jim,

I agree.......what they don't consider is the 90 degree is for derating factors....and as bob said...most of the terminals I deal with are if anything 75 degree minimum.

 

[electrofelon]

I like this.
Why not have a new Code table similar to 315b6...for commercial?
For commersial sevices
use #2 for 100 amps.
use #1/0 for 150 amps
use #3/0 for 200 amps

Peter,

In most cases wouldnt it be:

#3 for 100 A
#1 for 150 A
#3/0 for 200 A

since these are under 800 A?

 

[dnem]

I like this.
Why not have a new Code table similar to 315b6...for commercial?
For commersial sevices
use #2 for 100 amps.
use #1/0 for 150 amps
use #3/0 for 200 amps

Peter,

In most cases wouldnt it be:

#3 for 100 A
#1 for 150 A
#3/0 for 200 A

since these are under 800 A?

No, Electrofelon, your wire sizes are wrong.
See my original post above. I talk about the under 800 amp rule of 240.4(B)(2).

I work thru it one step at a time.

David

 

[dnem]

David

It is very important to understand the temperature limitation is based on the termination not the lug. This is important because most lugs are actually rated at 90C, and there are many contractors that feel they can simply replace any manufacturer's lug in order to use a higher rating.

Jim,

Can you post the difference between termination and lug ?
What specific part of the termination is the weakest link ?

David

 

[George Stolz]

David, I'd say if the commercial electricians are left to size their own conductors, they'd better be able to do so. Things may be a little different in FL, but the NEC is still the law, and you're just the enforcer. If these guys keep wasting money buying the wrong wire, that's no skin off your nose, right?

What are you suggesting as a solution for the problem? Insisting on your reviewers providing a one-line if it's missing? Rejecting the plans without a one-line?

It sounds to me as though the installers have a problem, it's your (blessedly third-party) job to report it and demand it's correction. If you feel so inclined, then offer to help after hours, or something, I would say.

(On the latest bit, I think the lug is where it bolts to the busses, and the termination is where the conductor is bolted down. That's my guess.)

 

[iwire]

Can you post the difference between termination and lug ?

David I believe what Jim is pointing out is that if you have breaker that is rated 75 C simply changing the lugs to 90 C does not change the 75 C rating of the breaker.

 

[iwire]

In most cases wouldnt it be:

#3 for 100 A
#1 for 150 A
#3/0 for 200 A

since these are under 800 A?

No, Electrofelon, your wire sizes are wrong.
See my original post above. I talk about the under 800 amp rule of 240.4(B)(2).

I work thru it one step at a time.

David

Where is Electrofelon wrong?

David this is exact problem of making a chart.

In many cases it is perfectly code compliant to use 3 AWG for a 100 amp feeder or service.

310.16 List the ratings as follows

3 AWG 100 amps @ 75 C and most equipment even below 100 amps is in fact marked as suitable for 75 C use.

1 AWG 130 amps @ 75 C and can be used with 150 amp Overcurrent protection assuming the load is less than 130 amps.

3/0 200 amps @ 75 C.

I tend to agree with George here, if the contractors install the wrong size simply fail the job and move on, it is not your job to design the job. After getting caught a few times these ECs will learn how to size conductors.

Say you make a chart that says a 100 amp feeder requires 3 AWG.

This feeder happens to run through a hot area now it is not large enough.

Bottom line is if the ECs take a job that they have to design they had darn well learn the rules and follow them.

 

[tshea]

I like this.
Why not have a new Code table similar to 315b6...for commercial?
For commersial sevices
use #2 for 100 amps.
use #1/0 for 150 amps
use #3/0 for 200 amps

Peter,

In most cases wouldnt it be:

#3 for 100 A
#1 for 150 A
#3/0 for 200 A

since these are under 800 A?

No, Electrofelon, your wire sizes are wrong.
See my original post above. I talk about the under 800 amp rule of 240.4(B)(2).

I work thru it one step at a time.

David

#3=100Amp Table 310.16 75 degree column. Go by wire type Don't use any TW.
110.14 A lug suitable for #1 is rated 75 degree. Most (but not all) lugs that will accept #3 will also accept #1. This will need to be field verified.

I agree 310.15(B)(6) is residential 120/240 1ph 3w only

 

[dnem]

Can you post the difference between termination and lug ?

David I believe what Jim is pointing out is that if you have breaker that is rated 75 C simply changing the lugs to 90 C does not change the 75 C rating of the breaker.

I wasn't talking about changing the lugs. The lab temp rating is based on the weakest link in the panel, which usually is the lug. But if you change the lug you just end up with a panel with no UL listing. What if the lug wasn't the only reason that the panel had the lower temp rating ? Replacing the lug only leaves a question mark.

Now as far as the subject of the breaker rating.
If we're talking about a main breaker panel, the lug is part of the breaker. The lug is the termination on the breaker that allows you to connect wire to the breaker. That's why I don't understand saying that the lug is different from the termination. Since the lug is part of the breaker, you can't change the lugs on the breaker, you can only replace the breaker. The breaker temp rating of the breaker applies to the whole breaker including the lugs. There is no separation between the breaker and the lugs on the breaker.

David

 

[dnem]

In most cases wouldnt it be:

#3 for 100 A
#1 for 150 A
#3/0 for 200 A

since these are under 800 A?

No, Electrofelon, your wire sizes are wrong.
See my original post above. I talk about the under 800 amp rule of 240.4(B)(2).

I work thru it one step at a time.

David

Where is Electrofelon wrong?

David this is exact problem of making a chart.

Exactly !
You posted before I could. I wanted to say the same thing. You can't make a Commercial chart. There are too many variables. I started this thread to highlight the need to go thru the correct process. I posted examples of conclusions from that process to show that the answers you get are different than the residential Table310(B)(6).

In many cases it is perfectly code compliant to use 3 AWG for a 100 amp feeder or service.

310.16 List the ratings as follows

3 AWG 100 amps @ 75 C and most equipment even below 100 amps is in fact marked as suitable for 75 C use.

1 AWG 130 amps @ 75 C and can be used with 150 amp Overcurrent protection assuming the load is less than 130 amps.

3/0 200 amps @ 75 C.

You?re proving your own point here.

You?re posting that #3 is good for 100a service and #1 is good for 150a service. Those figures are only good if the gear is labeled as being good for more than the minimum required by the testing labs. Testing labs do not require lugs than can only accept wire sizes of #1 or smaller to be rated for 75?C. If you?re going to say #3 for 100a or #1 for 150a you have to add, if equipment is labeled 75?C. If it?s not labeled, then the inspector will assume listing minimum. As I posted before, Lugs for #1 and smaller wire are lab rated for 60?C, unless otherwise labeled. Lugs for #1/0 and larger wire are lab rated for 75?C, unless otherwise labeled.

I tend to agree with George here, if the contractors install the wrong size simply fail the job and move on, it is not your job to design the job. After getting caught a few times these ECs will learn how to size conductors.

Say you make a chart that says a 100 amp feeder requires 3 AWG.

This feeder happens to run through a hot area now it is not large enough.

Bottom line is if the ECs take a job that they have to design they had darn well learn the rules and follow them.

I agree that it?s the contractors obligation to know what their doing. But I don?t get any pleasure in failing installations especially when I can see the amount of work and expense that will be required to redo the install. I would much rather alert contractors to the issue before they make the mistake. It?s their responsibility to go thru the procedure of selecting a conductor. I?m not trying to post a new chart here, just inform the contractor of the process.

I didn?t post a chart. I wrote a long post about a process of using different sections of the code book. The end result is based on the info plugged into the process.

David

 

[iwire]

I wasn't talking about changing the lugs. The lab temp rating is based on the weakest link in the panel, which usually is the lug. But if you change the lug you just end up with a panel with no UL listing.

No not necessarily.

I can change lugs on many panel boards, a common reason to change lugs is from single barrel lugs to stacker lugs.

Like these

What if the lug wasn't the only reason that the panel had the lower temp rating ? Replacing the lug only leaves a question mark.

Exactly what Jim was pointing out, installing higher temp rated lugs will not raise the temp rating of the equipment.

Since the lug is part of the breaker, you can't change the lugs on the breaker, you can only replace the breaker.

You don't do much commercial work do you?

Yes you can change the lugs on the breaker.

The reason this has to be done sometimes is to get the right lugs for the conductors being used.

Say I have a 400 amp breaker, it may have arrived with lugs rated for one 600 Kcmil each. However I might chose to run three sets of 1/0 copper. To do that I have to get lugs to accept that combination.

The breaker temp rating of the breaker applies to the whole breaker including the lugs.

It may or may not.

There is no separation between the breaker and the lugs on the breaker.

Often there is a separation there is a breaker with a rating and a lug or terminal with a rating.

Keep in mind that a breaker may be used without lugs at all, it may be bolted right to bus bars at one or both ends.

 

[iwire]

You?re proving your own point here.

You?re posting that #3 is good for 100a service and #1 is good for 150a service. Those figures are only good if the gear is labeled as being good for more than the minimum required by the testing labs.

Saying 3 AWG is good for 100 is no more 'wrong' or 'right' than saying it's not.

You said electrofelon posted wrong info, I disagree.

Commercial equipment is rated 75 C in almost all instances, of course if it is not marked that way than it is 60 C.

Same problem with NM, if they used 3 AWG NM for 100 amps it would be a violation, switch to MC and its OK. 8)

Good luck to you, I think your trying to train ECs and I just don't see that as your job.

I don't expect the inspector to lead me by the hand, I learn the rules and other ECs should as well.

 

[dnem]

Good luck to you, I think your trying to train ECs and I just don't see that as your job.

I don't expect the inspector to lead me by the hand, I learn the rules and other ECs should as well.

So if I post a piece of information that might be valuable to you, you think you're being "lead by the hand".

How about if I come to your job after you install it incorrectly and red tag your work ?
Do you think that's a better situation ?

David