480V underground feeders vs Medium Voltage

[badgers]

As I read the NEC 2014 version the required derating of Medium Voltage feeders run underground doesn't apply to 480V feeders.

Is this how others read this part of the NEC?
If so, why would they be treated differently?

 

[jumper]

I am confused on your question.

Are you trying to apply MV or conductors over 1000V rules to 480V conductors?

 

[badgers]

In my past experience I have derated underground MV feeders from what I read in the NEC 2014.

I told the EC we have hired that they are running to small of a feeder wire because they didn't derate their 480V feed to this 1200A panel.

The EC's position is that the section that covers 480V feeders doesn't require the same deration factors.
310.15(B)(3) adjustment factors doesn't specifically list duct banks as one of the reasons for adjustments.
310.60 which covers the MV feeders is where the tables and duct bank configurations are located.
They reason that those section do not apply to the 480V feeders and only the MV feeders.

I didn't want to influence the answers by giving to much specifics.

At a high level, I am asking do people generally read that the 480V adjustment factors are limited to everything listed before 310.60?
I reasoned that if MV cables get hot, why not 480V, but I have been told by their foreman that there is a history to the code. That at one time 480V feeders were treated the same and industry pushed back and restructured the code to have them treated differently.

 

[petersonra]

The code has sections that apply to everything.

it also has sections that apply only to above 600V, and other sections that apply only to below 600V.

 

[ron]

We either use the prescriptive tables found in the code for times when the duct bank is one of the standard configurations, OR we do Neher McGrath Calcs for everything non-standard.

Everything underground gets evaluated for mutual heating and possible derate

 

[badgers]

do you use amp calc for that work?

what code reference could a person cite to enforce the practice of "everything" is evaluated for mutual heating.

that is what I felt, but I am getting push back as that its not and NEC required practice.
Mutual heating does not equal ambient temperature when looking at 310.15

 

[luckylerado]

do you use amp calc for that work?

what code reference could a person cite to enforce the practice of "everything" is evaluated for mutual heating.

that is what I felt, but I am getting push back as that its not and NEC required practice.
Mutual heating does not equal ambient temperature when looking at 310.15

480V feeders are subject to derating unless:

" (3) Adjustment factors shall not apply to underground conductors
entering or leaving an outdoor trench if those
conductors have physical protection in the form of rigid
metal conduit, intermediate metal conduit, rigid polyvinyl
chloride conduit (PVC), or reinforced thermosetting resin
conduit (RTRC) having a length not exceeding 3.05 m
(10 ft), and if the number of conductors does not exceed
four."

310.60 will not apply to cables under 2000V. 480 is not medium voltage.

What are they putting in??

 

[ron]

do you use amp calc for that work?

Yes, we use Amp Calc.

I'm not understanding what code reference you are looking for LV or MV.

For LV, 310.15 gives the choice of using tables or Engineering Supervision. The tables only include amapacities for certain ambient temperature conditions.

Underground raceway will have different ambient temperatures than the regular tables in the NEC due to mutual heating. They do have some prescriptive amended tables for underground, but for those configurations non- standard, you need something like AMP Calc or the formula shown in 310.15(B)(2)

 

[badgers]

I am looking for code references that *REQUIRE* a 480V feeder to have adjustments made when run underground.

1200A feeder with (4) sets of 350 kcmil THWN which would seem to have 1240 amps of capacity except they are run 20-25 feet underground.

Ron, the code in 310.15 does talk about ambient temperature, but that isn't the same thing as self induced heating. That is one of the major points here. These are not equal.

310.60(C)(77) has tables for specific duct banks that even indicate in their heading based on "Ambient Earth Temp of 20°C" NEC recognizes that its not the ambient temperature, but is the lack of thermal conductivity.

Everything discussed up to this point has skirted the core question.
Is there a code reference that clearly requires derating of a 480V feeder running underground?

The comment by luckylerado has been the core of the EC position: its not a MV feeder so they don't have to increase wire size.
I did see that comment about under 10 feet, but I am at least 20 feet.

I think it might be good engineering practice to upsize the feeder, but my opinion doesn't carry much sway.
Based on our 10 hr/day usage I don't expect anywhere near 100% load factor, but we don't even get to that point in the discussion.

Thank you all, and I welcome any input you have.

 

[ron]

I am looking for code references that *REQUIRE* a 480V feeder to have adjustments made when run underground.

1200A feeder with (4) sets of 350 kcmil THWN which would seem to have 1240 amps of capacity except they are run 20-25 feet underground.

Ron, the code in 310.15 does talk about ambient temperature, but that isn't the same thing as self induced heating. That is one of the major points here. These are not equal.

310.60(C)(77) has tables for specific duct banks that even indicate in their heading based on "Ambient Earth Temp of 20°C" NEC recognizes that its not the ambient temperature, but is the lack of thermal conductivity.

Everything discussed up to this point has skirted the core question.
Is there a code reference that clearly requires derating of a 480V feeder running underground?

The comment by luckylerado has been the core of the EC position: its not a MV feeder so they don't have to increase wire size.
I did see that comment about under 10 feet, but I am at least 20 feet.

I think it might be good engineering practice to upsize the feeder, but my opinion doesn't carry much sway.
Based on our 10 hr/day usage I don't expect anywhere near 100% load factor, but we don't even get to that point in the discussion.

Thank you all, and I welcome any input you have.

It does not say you must in the way you are looking for. The ambient temperature of a duct bank is dependent on lots of things, including the surrounding materials, adjacent ducts, etc. The standard tables can only be used for the ambient they are listed. The ambient is not likely 30 deg C (as required in the table) if the conduit itself and nearby is putting off a lot of heat.

At least if you do the ampcalc, your load factor will be ~50% (10 hours per day)

 

[badgers]

thank you Ron, I calculate load factor as
Load Factor= Average Load / Maximum Demand
over 24 hrs.
I bet I am below 50%, as real world is never going to install a 1200A panel and expect 1200A on it.
equipment is shut down (but idle) during lunch and there is a 30 min safety check in the morning, and 15 min after lunch to double check nothing has gone wonky.
machines stop shaking the floor about 15 min before the end of the work day as the shop guys clean up before leaving.
I assumed 10 hrs just to be conservative because the machines idle(air compressors keep the receivers at pressure, etc)

 

[luckylerado]

...self induced heating....

The calculation to find this is not required if you are using the tables to select ampacity. Only under engineering supervision.

310.15 requires ambient temp correction and and adjustment of these feeders. If there are 3 phases and a Neutral, you are down to 992 amps and therefore must increase the conductor size.

 

[don_resqcapt19]

The rules requiring that were in for one code cycle. As long as your conductors are sized by the rules in Article 220, you probably won't have a problem, but if you do some "real world" load calculations and are using an underground duct bank without derating, your conductors will fail from overheating.