1200 amp service conductor derating

[rjn72]

We have a 1200-amp service being installed with 4x parallel 4-inch pvc conduits. We will be installing brown, orange, yellow, gray 600 mcm aluminum conductors with a 250 ground in each conduit. The derating because of 4 current carrying conductors in each conduit is the reason for the 600 sizes.

The question is because the breaker only has lugs for 4x 500 mcm conductors and they do not have a 4-hole lug kit for 600s.

We are feeding into the side of the existing gear from a 2 ft x 4 ft 3r box from underground. It is about 15 ft through the gear to the breaker location, all fairly open. I see a few options.

1. use reducing pins on the 600 conductors to make the pins fit the breaker. This may not work because the pins may interfere with the mounting of the breaker. Like everything right now nothing is here to try it out. Once the breaker arrives, if I can get a couple of pins, I would know for sure, I think. The problem is we are going to have to shut the gear down to install the breaker and this shuts down a facility running 24/7. We don't have the luxury of something not working.

2. use Polaris lugs for each conductor adapting from 600 aluminum to 300 copper or 400 aluminum inside the 2x4 can and run the 300/400 through the open gear to the breaker. This would look like hammered crap in the can, and is it possible to derate the conductors since they are no longer in the conduit and are now running through the gear?

3. use 8-hole Polaris lugs in the 2x4 can and adapt from the 4 x 600 aluminum to 4 x 300 copper conductors or 4 x 400 aluminum conductors and route those through the gear. This would look considerably better, but is it legal? Is the derating only taken into account in the conduit run and I can reduce the wire size once in the gear?

Thak you guys. Any other options I have not thought of besides quitting and going to work elsewhere?

 

[LarryFine]

The derating because of 4 current carrying conductors in each conduit is the reason for the 600 sizes.

Not necessary; the neutral need not be counted as a CCC.

 

[infinity]

Two things stand out, one you likely don't have 4 CCC's in each raceway and two there are no EGC's ahead of the service disconnect.

 

[rjn72]

Two things stand out, one you likely don't have 4 CCC's in each raceway and two there are no EGC's ahead of the service disconnect.

3 hots and 1 neutral are all current carrying conductors last time I calculated. This is a sub feeder from the main service gear to a separate building on site. I'm not sure what you mean by no egc's ahead of the service disconnect.

 

[augie47]

Infinity answered while I was typing.........
It is highly unlikely that you need to count the neutral as a current carrying conductor.
There are almost no installations where the majority of the load is non-linear.

 

[rjn72]

Infinity answered while I was typing.........
It is highly unlikely that you need to count the neutral as a current carrying conductor.

Can you explain why not. If it is carrying the unbalanced portion of the load it has to be calculated. This is a 277 / 480 volt feeder. It will have some unbalanced loading.

 

[infinity]

We have a 1200-amp service being installed with 4x parallel 4-inch pvc conduits. We will be installing brown, orange, yellow, gray 600 mcm aluminum conductors with a 250 ground in each conduit

You said that you are installing EGC's for the service.

 

[electrofelon]

3 hots and 1 neutral are all current carrying conductors last time I calculated. This is a sub feeder from the main service gear to a separate building on site. I'm not sure what you mean by no egc's ahead of the service disconnect.

99.8% of the time the neutral does not count as a CCC. That section talking about nonlinear loads is very outdated and in my opinion should be deleted. Even if it was to apply, note the word "majority", which considering the likelihood of HVAC loads would make it probably not apply.

There are no egcs ahead of the service disconnect. The grounded conductor / neutral is used for bonding. There is a possibility you would need a supply side bonding jumper run with the conductors depending on specifics.

 

[infinity]

Can you explain why not. If it is carrying the unbalanced portion of the load it has to be calculated. This is a 277 / 480 volt feeder. It will have some unbalanced loading.

Look at 310.15(B)(5).

 

[rjn72]

There are nearly no HVAC loads as this is for a welding facility. I am unsure how welders and shears operate when it comes to harmonics and unbalanced loads. This is a design build, but I will reach out to the engineer offsite for clarification.

 

[electrofelon]

There are nearly no HVAC loads as this is for a welding facility. I am unsure how welders and shears operate when it comes to harmonics and unbalanced loads. This is a design build, but I will reach out to the engineer offsite for clarification.

I wouldn't worry about it, just count it as 3CCC.

The key thing to note is that section is about when to count a conductor as current caring for derating purposes. It doesn't have anything to do with unbalanced loading or actual current on the neutral conductor. Imbalance resulting in current on the neutral, will result in less current on one of the current carrying conductors, so there is no net increase in overall heating.

 

[infinity]

I wouldn't worry about it, just count it as 3CCC.

I agree and then you can use 500 kcmil conductors which solves the breaker terminal problem.

 

[winnie]

I agree that it is unlikely that the neutral needs to be counted as a CCC.

Are 8 lug kits available for the breaker? It might be easier to use 8x 250kcmil rather than 4x 600kcmil. This gives the required ampacity even if this is a rare case where the neutral is a CCC.

Jon

 

[roger]

Can you explain why not. If it is carrying the unbalanced portion of the load it has to be calculated. This is a 277 / 480 volt feeder. It will have some unbalanced loading.

Because the unbalanced portion does not add to the total heating of the conductors.

Here is a single phase example, there is a total of 40 amps on all conductors, the neutral doesn't add any load.




Now load both ungrounded conductors to the same current, 10 amps, and you only have 20 amps on all conductors, the neutral doesn't add any load.

277/ 480 doesn't change anything

 

[winnie]

Expanding on the above example:

Whenever current flows through a conductor it generates some heat. This is true of a neutral even when it isn't _counted_ as a CCC.

The worst case for heating is when the maximum number or conductors are carrying their maximum current. But this can never be all 4 conductors because the neutral is only carrying the unbalanced current.

So the worst case for heating us when the 3 phase conductors are all fully loaded and the neutral is carrying 0 current. In this case the neutral generates zero heat.

If the neutral is carrying some current then a phase conductor must be carrying less than maximum. In this case you have less total heat production. You would have 4 wires, all carrying some current, but producing less heat then the 3 wire worst case. Thus the logic of counting them as 3 CCC.

The above analysis fails if you have lots of line-neutral harmonics, because then you might get significant neutral current even with the phases perfectly balanced.

I think you could also see an issue if the neutral is significantly smaller than the phase conductors, however Code doesn't address this aspect of things.

Jon

 

[synchro]

Infinity answered while I was typing.........
It is highly unlikely that you need to count the neutral as a current carrying conductor.
There are almost no installations where the majority of the load is non-linear.

Can you explain why not. If it is carrying the unbalanced portion of the load it has to be calculated. This is a 277 / 480 volt feeder. It will have some unbalanced loading.

There are nearly no HVAC loads as this is for a welding facility. I am unsure how welders and shears operate when it comes to harmonics and unbalanced loads.

Probably stating the obvious, but L-L or 3-phase loads contribute zero current on the neutral, no matter how much their currents are unbalanced or how much harmonic content they have. So only L-N loads contribute to the neutral current, and only in the amount of unbalance present and the harmonic content at triplen frequencies.

Also, it should be remembered that the amount of heating power is proportional to the square of the current. And so the heat produced in the conductor drops faster and faster as current is reduced from a given level. And so the contribution of a neutral conductor to the heating within a raceway is often negligible compared to that of the other conductors which have 3-phase and L-L loads as well as any L-N loads that might be present.

 

[texie]

3 hots and 1 neutral are all current carrying conductors last time I calculated. This is a sub feeder from the main service gear to a separate building on site. I'm not sure what you mean by no egc's ahead of the service disconnect.

You got infinity's response because you are using NEC terms incorrectly. Your post said "service" when in fact this is a feeder. Proper terms are critical as it often means differing requirements.

 

[charlie b]

The worst case for heating is when the maximum number or conductors are carrying their maximum current. But this can never be all 4 conductors because the neutral is only carrying the unbalanced current.

So the worst case for heating us when the 3 phase conductors are all fully loaded and the neutral is carrying 0 current. In this case the neutral generates zero heat.

If the neutral is carrying some current then a phase conductor must be carrying less than maximum. In this case you have less total heat production. You would have 4 wires, all carrying some current, but producing less heat then the 3 wire worst case. Thus the logic of counting them as 3 CCCCC.

I agree with Winnie here (not an uncommon event). I will add that, a decade or three ago, I did a detailed analysis of the heat generated by all three phases and the neutral under random loading (balanced and not balanced) loading conditions. I proved that if the three phase loads are not equal (i.e., some neutral current), the total heat generated by the four wires will be lower than the heat generated if the three phases alone (i.e., no neutral current).

 

[LarryFine]

I proved that if the three phase loads are not equal (i.e., some neutral current), the total heat generated by the four wires will be lower than the heat generated if the three phases alone (i.e., no neutral current).

So, not only is the presence of a neutral not a heat deficit, it is likely to provide a heat benefit.

 

[don_resqcapt19]

I agree with Winnie here (not an uncommon event). I will add that, a decade or three ago, I did a detailed analysis of the heat generated by all three phases and the neutral under random loading (balanced and not balanced) loading conditions. I proved that if the three phase loads are not equal (i.e., some neutral current), the total heat generated by the four wires will be lower than the heat generated if the three phases alone (i.e., no neutral current).

I am sure you did a detailed calculation, but you can do a very simple one if you don't want to know the actual watts of heat that is produced. If you assume all of the conductors of the circuit are the same size, then the resistance of the conductors is equal. The heat produced in each conductor will be I²R and you just sum the I²R values for all of the circuit conductors based on the current in each conductor. As you said for a three phase circuit the worst case is always when 3 conductors are carrying the same amount of current. That happens with a 3 phase load, or with two equal line to neutral loads on the 3 phase multiwire circuit.