NEC Table 310.60(C)(77) and related Figure 310.60(C)(3) 1

[milwaukeebob]

Hello,
I have a question related to NEC Table 310.60(C)(77) “Ampacities of Three Single-Insulated Copper Conductors in Underground Electrical Ducts (Three Conductors per Electrical Duct) Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure 310.60(C)(3)..."

I’m involved with a project that has two (2) large 3 phase xfmrs installed. Each xfmrs primary side cabling is installed in underground (UG) ductbank and consists of three (3) 15kV, MV-90 rated cables per phase (gauge unimportant for this question). This UG ductbank for both xfmrs for the majority of the run is arranged in a configuration “similar” to detail 3 of NEC Figure 310.60(C)(3) – six (6) electrical ducts in a 2Hx3W arrangement. One additional important not is that proper spacing of the ducts was not adhered to during the installation.

When both xfmrs are energized and loaded, it's clear to me that NEC Table 310.60(C)(77) “Six Circuits” section is applicable for determining the ampacity of all the installed cables. The “Six Circuits” section shows a lower ampacity than the “Three Circuits” section which make sense because there’s a further derating required because of the impact of the extra heat from the cables in the added ducts.

My question is related to the scenario when only one of the xfmrs is energized and loaded with the other one out of service. In this scenario and with the configuration of the plant, the remaining xfmr will carry the full load and the other xfmr primary side circuit/cabling will be de-energized and not seeing any current flow. How does this get evaluated with regard to Table 310.60(C)(77)? It’s understood the ductbank is still six (6) electrical ducts in a 2Hx3W arrangement but now only one set of cables in three (3) of the ducts is flowing current and giving off heat. In this case, would the ampacity for these cables now be taken from the “Three Circuits” section of Table 310.60(C)(77) or does the Code still consider this a “Six Circuit” installation?

Thanks for any help and guidance on this.

 

[waross]

The tables are based on rated full load current ratings of the transformers.
There is no reduction in cable ampacity allowed based on part loading.
There are exceptions.
I don' think that any exceptions apply in your instance.
If there is a possibility that a transformer may be converted from ONAN to ONAF sometime in the future, you should base the cable ampacity on the full load ONAF current.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[milwaukeebob]

Waross, I'm honored by your response. Thank you.

Honestly, my question was from the perspective of someone else who I'm likely going to be having a disagreement with. I agree with what you've said. The new installation I've gotten involved with in the field has two (2) smaller GSU xfmrs with an ONAF rating of 40 MVA each. At 13.8kV on the X side of the GSU's, that's 1,673.5 amperes/phase full load capability. For each of these GSU's, the EPC contractor installed three (3) 15kV, 1000 kcmil, MV-90 cables/phase from their respective 2000A MV GCB and arranged the UG ductbanks for these runs to the GSU's in a six (6) circuit arrangement (as I mentioned the duct arrangement is not in accordance with Code and normal industrial engineering practices). When I first saw it, my initial response was "why didn't you guys run above-ground segregated phase busduct from the MV gear to the GSU's"? Having not dealt with any UG cabling above 750kcmil in my career and having not done alot of new design work in the past 10 years, I didn't realize there was this much of an ampacity shortage...that was until I went to the applicable NEC Tables. Based on my calcs, they're short slightly more than one (1)1000 kcmil/phase
The engineer of record and the EPC contractor is likely going to argue that in normal plant configuration (both GSU xfmrs in service), the xfmrs will share the plant capability and loading and not be anywhere near their full load rating. The biggest problem I see is when one of the xfmrs is out of service for whatever reason and the plant generators are at full capability. I've calculated ~1746.6 amperes/phase at the X side of the remaining GSU which is ~73amperes/phase above the ONAF rating of the xfmr.

 

[davidbeach]

If you are governed by the NEC you have your answer. If you're a utility and the NESC governs you might have some opportunities for engineering judgement.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[jghrist]

With only one transformer in service, the situation is more like Detail 2 (three circuits). You don't have heat from the unloaded cables.

 

[milwaukeebob]

Thanks davidbeach. I appreciate your response. Getting responses from you and Waross is more than I could ever ask. This new design and installation are governed by NEC. Although I've worked for "utilities" and IPP's in the past, this isn't one of those cases. I was brought in to mainly help the owner navigate some of the challenges and witness/coordinate some of the on-site testing of the new equipment. I didn't anticipate finding some of the major design issues I am.

 

[bacon4life]

NEC Table 310.60(C)(77) is just about calculating the ampacity of the cables. Other sections dictate what the required capacity actually is.

As an NESC based person, it seems totally reasonable to me to calculate the ampacity both ways. Just bring CT's from both sets of cable into a microprocessor based relay to protect the cabling under both capacity constraints. We rely on this kind of calculation all the time for circuits leaving medium voltage substations.

Somewhere in the NEC there is a provision for consideration of separate heating and cooling loads. When the heating and cooling systems are interlocked, the NEC only considers the larger equipment rather than the sum of the equipment sizes. Perhaps you could find a way to apply the same logic to the generator capacity.

Most of the generation plants I have toured do not have full redundancy for all equipment. Having a 10% reduction in plant capacity for the loss of a GSU might be a reasonable design for a plant the often runs at less than 90% output. I assume that in your case it appears the EPC is cheating the owner, rather than the owner choosing a cheaper alternative than 100% redundancy. Hopefully the EPC is used to dealing the the NEC rather than the NESC.

As a utility engineer, I also can use actual ambient air temperatures for calculating the capacity of transformer banks. For certain kinds of power plants the peak capacity of the plant is reduced at higher ambient temperatures.

 

[7anoter4]

Usually, if the transformers are parallel connected, each one is 75% of total required load rated [in my opinion].
Let's say it is about a 15 kV [secondary] transformer 15 MVA. Then the total required load will be 15/0.75=20 MVA.
So, if one of the transformers is out of order the remaining other will support 100% [1.3333*15=20 MVA]
If a supplementary cooling system-start ventilation, oil-pumps or else is not available not more than 1 hour it is permissible in order to maintain the transformer life expectancy.
In this time we have to reduce the load.
Let's say the transformer is equipped with such a possibility and it could withstand continuously the total load.
In this case the cables will get 33.33% more 343 A instead of 257A [According to NEC Table 310.60(C)(77) 3*750 mcm copper].
If the cable is indeed 750 mcm it is o.k.But, if it is only 500 mcm [290 A ampacity] you have 342.7 A in 3*3 500 mcm.
It will be 370 A permissible for only three parallel cable per phase [according to Detail 2] and if the thermal resistance of the concrete will be the same [55] and earth will be 90 then all is ok.
It will be a slightly increased ampacity as the concrete block is bigger and could be cooler.

 

[7anoter4]

I did not remark the actual data of transformers and cables.
I don't think the cables run in a duct bank underground, if are only 3 x 1000 mcm copper conductors per phase.
According to NEC Table 310.60(C)(77) Detail 3 [6 ducts] ampacity of 1000 mcm it is only 390 A. That means 3*390=1170 A and you need 1673.5 A [558 A per conductor].
As I calculated it has to be 55 inches the distance between ducts [vertical and horizontal] for 6 ducts with 3*1000 mcm each if the ampacity is 558 A/cable.
However, will be only 653 A per conductor [3*653=1959 A total ] when 3 ducts will be loaded only. If the remaining total load is 1746.6 A then will be o.k.
The shield will be grounded only at one end and the duct will be of aluminum not p.v.c.
On the other hand ,my calculation is based on Neher and McGrath theory which contains some approximations. Usually, the distances are from 7.5 up to 12", so for 55" I am not sure how accurate it will be.

 

[milwaukeebob]

Bacon4life and 7anoter4, thanks for your valuable input as well. Your time and expertise are truly appreciated and respected.

This facility is unique in that there is considerable redundancy not only for the main GSU's, but also the UAT's, and associated MVSG CB scheme. While it's a small plant in my experiences, multiple failures of CB or xfmrs or just equipment being isolate for routine service will not inhibit the owners ability to produce full output capability of the plant.

When I first started looking at some of the details, my initial head-scratcher moment was when I realized they didn't size the low-side cabling to the ONAF rating of the xfmr (as waross had suggested). The EPC Contract calls for two (2) redundant 30/40MVA (ONAN/ONAF)xfmrs but the limiting factor is now the cabling. The upstream MVSG CB is rated 2000A, the ONAF rating of the xfmr is 40MVA, but the cabling, even under the best-case duct-bank and associated derating scenario, is rated ~35MVA. It's pretty clear to me the EPC Contractor and the EOR missed something. There will be a discussion with them next week and I'm not looking forward to it.

Thanks again

 

[waross]

Please let us know how it goes.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[milwaukeebob]

Hello waross. For various reasons, our meeting with the EPC contractor hasn't happened yet. You, Davidbeach, jghrist, bacon4life, and 7anoter4 have given me much to consider as I prepare for the eventual discussions. In the meantime, I have an important follow-up question for you. I specifically want to focus on your initial response where you indicated the "NEC tables are based on rated full-load current ratings of the xfmrs and there is no reduction in cable ampacity based on part loading".

I know I've probably been clear on the situation but, just to remove any doubt, here is a summary of the situation again - the project I'm currently involved has two(2) 100% redundant GSU xfmrs rated 30/40MVA (ONAN/ONAF) and separate 2000A upstream MV feeder CB's each with its own discrete protection and monitoring devices. Each feeder is comprised of three (3) 15kV, 1000kcmil, MV-90 cables/phase run in under-ground (UG) conduit/concrete encased duct-bank. Both three (3) electrical duct arrangements for each xfmr are run stacked over-under each other thus comprising a six(6) UG electrical duct arrangement (similar to what's shown in Figure NEC 310.60(C)(3) Detail 3. As you know, I've questioned the UG duct-bank design, MV feeder cable sizing, and associated deratings applied by the EPC contractor. It's important to note here that at the ONAF rating, the GSU xfmrs are sized a minimum of 5% above the maximum output capability of the plant. While I agree with you the cabling (with the correct ampacity deratings) should have been designed for the maximum ONAF rating of the xfmr, that's not what happened. At this point, I just want to make sure the max plant output is covered in consideration of all NEC required ampacity deratings.

Under two(2) GSU xfmr configuration, the plant output is split between both GSU xfmrs and the 15kV feeder cables from the MV CB to the 'X' side of the xfmrs are clearly in a six(6) electrical duct arrangement. With current flowing in all 6 ducts, the 15kV cable ampacity and associated deratings based on NEC Table 310.60(C)(77) are quite logical and clear to me. Under one(1) GSU xfmr configuration, the max plant output and associated current is isolated to just that one xfmr. This means that only three (3) of the UG electrical ducts will have 15kV 1000kcmil cables generating heat and the other set of three(3) will be de-energized. As you know, this is where my initial dilemma and mental "gymnastics" started.

If I'm understanding and extrapolating yours and davidbeach's response correctly, since the plant is governed by NEC, the ampacity of the 15kV cables shall be based on the six(6) electrical duct arrangement under all loading scenario's even when only one(1) of the xfmr's is energized and loaded? Is this correct? If so, is there explicit language in NEC to this effect? I realize I may be asking strange questions but what I'm dealing with at this site is not normal. If I had been involved with the design, these questions short-falls and concerns wouldn't exist.

Also, in support of your initial response, I see Article 215.3(B)(1) states "Feeders Supplying Transformers. The ampacity of feeder conductors shall not be less than the sum of the nameplate ratings
of the transformers supplied when only transformers are supplied" BUT Article 215.3(B)(3) "Supervised Installation" appears to provide an exception. Unfortunately, I'm seeing very little "engineering Supervision" on this job.

 

[jghrist]

I don't see why ampacity has to be based on six circuits (Detail 3) while the plant is served by a single transformer and three circuits. Why can't Detail 2 be used for the case where only three circuits are energized?

 

[waross]

The first answer is that you must size the cables based on all cables being fully loaded.
But, there may be an out.
I am more familiar with the Canadian code but I anticipate a similar rule in the NEC.
When sizing feeders, if it can be shown that not all equipment will ever be in use, simultaneously, the feeders may be reduced.
The most common use of this rule is for heating and cooling.
Feeders need only be sized on the greater of the heating load or the A/C load.
I suggest a chat with the AHJ.
The AHJ may allow an exception.
We can't hazard a guess as we don't know he details of your plant and how you can assure the AHJ that the load will never be increased.
I have been bit by a customer who promised that the load would never be increased and then added only one small load, about twice the capacity of the circuit.
Others here will have similar stories.
Another option is to use Kirk Keyed Interlocks so that only one transformer may feed the load at any one time.
If you get jammed up on this, the Kirk Interlocks are a whole lot cheaper than changing out HV cables.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

If the distance between ducts will be 16" and the duct bank depth will be 18" [minimum recommended in NEC Table 300.50 Minimum Cover Requirements column 2] then 450 A will be ampacity for cable in 6 ducts [55 concrete,90 earth- W/K.cm-20oC Earth,90oC XLPE copper] and 560 A for only 3 ducts filled.

 

[milwaukeebob]

Thanks jghrist, waross, and 7anoter4. As you can see, this issue is troubling me. If I had been involved in the initial design (or for that matter any of you), we wouldn't be involved in this conundrum. The "wild-card" in this whole situation is that the duct-banks were NOT installed in accordance with the EoR drawings AND the minimum separation requirements as defined by the NEC. I won't get into the details but, trust me when I say whether utilizing Detail 2 (3 circuit) or Detail 3 (6 circuit), there are ampacity issues. The details provided by everyone in this thread(and another one by 7anoter4) provided me with the confirmation I needed. I just wanted to see how bad it will get whether we need to utilize the 3 or 6 circuit scenario.

I appreciate all you guys. Although this is my 29th year in the industry, it's been about 15 years since I was a focused design engineer for new plants. The last 15 years, I've been in plant Operations and Maintenance with a small amount of new design work on upgrades to generator excitation systems.

 

[jghrist]

If the duct banks were not installed in accordance with NEC detail conduit spacing, then you will have to calculate the ampacity using the general Neher-McGrath heat transfer formula under Engineering Supervision. Most cable ampacity software will use the Neher-McGrath method. The ductbank and cables are already installed, and the calculations do not show adequate ampacity, you may consider thermal backfill with lower Rho to increase the heat transfer.

If this is a generation plant, it shouldn't be hard to show that both transformers cannot carry the full generator output at the same time.