400 amp residential feeders using parallel 1/0s??

[jzadroga]

I was going through the 2014 code book to size a 400 amp underground service when I came to the conclusion that I might be able to use parallel 1/0s. Let explain and maybe someone could chime in when they find my mistake.

First the layout of the service: We are connecting to the utility transformer at the corner of the property. We are installing a 400 amp meter/main next to the transformer. Then the feeder goes 400ft in two 2 inch PVCs buried 4? deep to the foundation. We then go into the basement where there is a 400 amp disconnect. At the meter the feeders emerge in PVC out of the ground 4ft and at the house they extend into the basement in PVC 10ft. The calculated load for the house is 250 amps.

My first step was using 310.15(B)(7)(2). Which allows conductors with an ampacity of 83% of the feeder rating. In my case 322 amps for one conductor or 166 amps for parallel conductors.

Next I found the ambient adjustment correction factor for the ambient temp of the underground run which from what I could find was 55 degrees F. So the adjustment factor is 1.15. Since the amount of conductors that leave the 55 degree portion is 10ft or less at each end I feel 310.15(A)(2)Exception applies.
Next I went to Table 310.15(B)(16) and found that 1/0 THWN copper, after applying the temp correction, can handle 172.5 amps in the 75 degree C column.
Now I have never heard of this before so I?m thinking I have made a mistake by applying the ambient adjustment factor. But I can?t see where in the code book that would not allow me to use the correction factors.
When I asked the inspector he said that using the 83% residential adjustment does not allow me to take any other adjustments.
Am I mistaken or can I use 1/0 in this situation.

 

[augie47]

IMO, 110.14 would prevent you from terminating the 1/0 at greater than it's 75? ampacity of 150 amps, or 300 amps for the parallel system.
As a side note, locally we would not allow 10 ft to the inside disconnect per 225.32 but the allowable distance varies by AHJ.

 

[Smart $]

IMO, 110.14 would prevent you from terminating the 1/0 at greater than it's 75? ampacity of 150 amps, or 300 amps for the parallel system...

I'm inclined to disagree (subject to further thought and/or discussion ).

110.14(C)(1), last part of the general statement second sentence states...

...conductor ampacities used in determining equipment termination
provisions shall be based on Table 310.15(B)(16) as
appropriately modified by 310.15(B)(7).

Wouldn't that mean for 310.15(B)(7) applications we can divide the allowable ampacity by 83%?

150A ? 83% = 181A ---> ? 2 = 362A

And I also have to disagree with the inspector about other adjustments. That contradicts the both Informational Notes.

 

[GoldDigger]

The modification to [2014] that removed the table was in part to clarify that the intent was always to apply adjustments and corrections as appropriate instead of using a single table value.

 

[augie47]

Since we are not using '14 yet I certainly can't say I have a grasp on it.
I guess I'm trying to compare it to the old 310.15(B)(6) when we had a 200 amp service we could use a 2/0 Cu, 150 amp conductor.
With the '14 Code are we using the 83% for the calculation of the conductor for the service size or the load ??? (I'm not clear on that).
If we are apply the 83% to the service size then I see no way we can use conductors rated at 150 amps at 75? for a service greater than 300 amps.
If we use his calculated load of 250 amps then obviously the parallel 1/0s are large enough but I cant see protecting them at 400 amps.

Obviously I'm not co0mfortable with '14 yet. Hopefully I will be more so as this thread progresses.

 

[Dennis Alwon]

The 83% is pretty much the same as the old Table. The difference would be in the ampacity adjustments that may need to be applied.

200 amp service.............83% of 200 = 166 amps. This means we need a conductor that is good for at least 166 amps after any de-rating or adjustments have been made. 2/0 at 75C is good for 175 amps so it is still good, However if it is se cable and you have a situation where you must use 60C then 2/0 would not be usable.

In many cases one could avoid having to use se cable at 60C since we are allowed to use art. 310.15(A)(2) exception

 

[Smart $]

I guess I'm trying to compare it to the old 310.15(B)(6) when we had a 200 amp service we could use a 2/0 Cu, 150 amp conductor.

175A. Under 2014, only has to be 166A for 200A service (83%).

With the '14 Code are we using the 83% for the calculation of the conductor for the service size or the load ??? (I'm not clear on that).

83% of service rating is minimum conductor ampacity. Calculated load is permitted up to service rating, same as 2011.

If we are apply the 83% to the service size then I see no way we can use conductors rated at 150 amps at 75? for a service greater than 300 amps.

Look at it this way: both "normal" ampacity and "normal" terminal limitation can be factored by 120.5%* compared to non-(B)(7) installations. The increase in terminal limitation is in 2011 also. If it wasn't, you couldn't use 2/0 on a 200A service.

*Inverse of 83% is 120.5%.

 

[electrofelon]

I was going through the 2014 code book to size a 400 amp underground service when I came to the conclusion that I might be able to use parallel 1/0s. Let explain and maybe someone could chime in when they find my mistake.

First the layout of the service: We are connecting to the utility transformer at the corner of the property. We are installing a 400 amp meter/main next to the transformer. Then the feeder goes 400ft in two 2 inch PVCs buried 4? deep to the foundation. We then go into the basement where there is a 400 amp disconnect. At the meter the feeders emerge in PVC out of the ground 4ft and at the house they extend into the basement in PVC 10ft. The calculated load for the house is 250 amps.

My first step was using 310.15(B)(7)(2). Which allows conductors with an ampacity of 83% of the feeder rating. In my case 322 amps for one conductor or 166 amps for parallel conductors.

Next I found the ambient adjustment correction factor for the ambient temp of the underground run which from what I could find was 55 degrees F. So the adjustment factor is 1.15. Since the amount of conductors that leave the 55 degree portion is 10ft or less at each end I feel 310.15(A)(2)Exception applies.
Next I went to Table 310.15(B)(16) and found that 1/0 THWN copper, after applying the temp correction, can handle 172.5 amps in the 75 degree C column.
Now I have never heard of this before so I?m thinking I have made a mistake by applying the ambient adjustment factor. But I can?t see where in the code book that would not allow me to use the correction factors.
When I asked the inspector he said that using the 83% residential adjustment does not allow me to take any other adjustments.
Am I mistaken or can I use 1/0 in this situation.

I think the code issues of this design have been covered, but I will just offer some suggestions of alternative designs. I generally find the simplest and least expensive way to do a 400 amp service is to use 230.40 Ex 2 and bring two 200 amp sets from the meter to two 200A main breaker panels. The potential problem with this is you would be bringing service conductors to the building so you would be restricted on how far in you could bring the conductors before hitting your service disconnects. Another option is to use the "loophole" of being able to size service conductors serving several service disconnects to be sized to the load, in other words bring in one set of service conductors sized to the load and tap off to each 200 amp main breaker panel. You would still have the service entrance conductors length restriction of course. Just trying to think of ways to avoid 2 400 amp disconnects....

 

[kwired]

I think the code issues of this design have been covered, but I will just offer some suggestions of alternative designs. I generally find the simplest and least expensive way to do a 400 amp service is to use 230.40 Ex 2 and bring two 200 amp sets from the meter to two 200A main breaker panels. The potential problem with this is you would be bringing service conductors to the building so you would be restricted on how far in you could bring the conductors before hitting your service disconnects. Another option is to use the "loophole" of being able to size service conductors serving several service disconnects to be sized to the load, in other words bring in one set of service conductors sized to the load and tap off to each 200 amp main breaker panel. You would still have the service entrance conductors length restriction of course. Just trying to think of ways to avoid 2 400 amp disconnects....

One requirement that must be met before you can use provisions of 310.15(B)(7) (and has been a requirement for some time) is the conductor must be supplying the entire dwelling unit load. If you run two feeds to two 200 amp panels, neither one is supplying the entire dwelling unit load and you can not utilize any provisions in 310.15(B)(7).

You can however use parallel conductors and reduce their size per 310.15(B)(7) up to the meter or some other splice/tap point and then feed two 200 amp panels with full size 200 amp conductors to each panel - this is because the paralleled portion is supplying the entire dwelling unit load.

 

[augie47]

175A. Under 2014, only has to be 166A for 200A service (83%).


83% of service rating is minimum conductor ampacity. Calculated load is permitted up to service rating, same as 2011.


Look at it this way: both "normal" ampacity and "normal" terminal limitation can be factored by 120.5%* compared to non-(B)(7) installations. The increase in terminal limitation is in 2011 also. If it wasn't, you couldn't use 2/0 on a 200A service.

*Inverse of 83% is 120.5%.

OK.. little slow tonight so bear with me. 400 amp service 2014 Code 83%
332 amp required. (for the moment let's ignore the adjustments.)
Parallel conductors 332/2 = 166 amp conductor
Do we not need to take 110.14 into account and terminate with a conductor with a 75? rating of at least 166 amps.

 

[GoldDigger]

....
Do we not need to take 110.14 into account and terminate with a conductor with a 75? rating of at least 166 amps.

I believe we do.

 

[Smart $]

OK.. little slow tonight so bear with me. 400 amp service 2014 Code 83%
332 amp required. (for the moment let's ignore the adjustments.)
Parallel conductors 332/2 = 166 amp conductor
Do we not need to take 110.14 into account and terminate with a conductor with a 75? rating of at least 166 amps.

As I said earlier, my disagreement is subject to further thought and/or discussion. As you present the matter, it seems like a clear cut, straight forward assessment. But when we evaluate for terminal temperature limitation for non-(B)(7) installations, the determination is based on calculated load, not OCPD rating, correct?

Let's say for a non-(B)(7) feeder we have a 200A OCPD rating with a calculated non-continuous load of 150A. What is the minimum size conductor we can use on this circuit based solely on terminal temperature limitation? 1/0 Cu. It's plainly evident minimum size for terminal temperature limitation is not affected by OCPD rating. However, we can not use 1/0 under typical conditions of installation because it would not be protected by the OCPD. However, could we use a 1/0 75?C conductor if installed in a 50?C ambient, and no other adjustment? The answer is yes... 1/0@150A ? 1.20 ambient correction = 180A... which is protected by a 200A OCPD per 240.4(B).

Now the only difference between the preceding example and the OP's scenario is 310.15(B)(7) and the ambient correction factor...

(I have to give my brain a break :happyyes

 

[jzadroga]

Do we not need to take 110.14 into account and terminate with a conductor with a 75? rating of at least 166 amps.

Doesn't 110.14(C)(1) also say:

Equipment Provisions. The determination of terminationprovisions of equipment shall be based on 110.14(C)(1)(a) or
(C)(1)(b). Unless the equipment is listed and marked otherwise,
conductor ampacities used in determining equipment
termination provisions shall be based on Table 310.15(B)(16)
as appropriately modified by 310.15(B)(7).

I believe this allows the listed ampacity on table 310.15(B)(16) to be modified by 310.15(B)(7) for terminations.

 

[kwired]

Both (B)(7) parts (1) and (2) end with : shall be permitted to have an ampacity not less than 83 percent of the (service or feeder) rating.

Before when they just had the table you didn't have to worry about termination ratings, you just found the value in the table that applied to you and went with that size conductor no matter what the terminal rating was or what type conductor insulation you were using.

I think think they either dropped the ball and forgot about termination temp here or they intended it to not apply. Be nice if they did reference 110.14 either way to clarify their intention.

Kind of going on what was done in the past, I'd say not to pay too much attention to termination temp until the NEC clarifies it's intent here, but you also need to see how your inspector is going to interpret this, preferably before you install it or you could be doing it over.

 

[augie47]

I have to join Smart$ in the brain resting but, on second and third look, since 110.14 addresses 310.15(B)(7), I tend to agree but as kwired points out, be prepare for a lengthy discussion with your inspector

 

[jzadroga]

I have to join Smart$ in the brain resting but, on second and third look, since 110.14 addresses 310.15(B)(7), I tend to agree but as kwired points out, be prepare for a lengthy discussion with your inspector

You agree that 1/0 could be used?

 

[augie47]

You agree that 1/0 could be used?

Get back to you on that. I had concetrated on the termination... didn't approach any other issues.

 

[kwired]

I just realized that 110.14(C)(1) does mention 310.15(B)(7). After looking at that - I think termination temp doesn't matter when using 310.15(B)(7). This doesn't appear to be any change either from 2011 as far as 110.14 (C)(1) goes, so if the CMP intention was to have change as it relates to term temp, they didn't change enough to get it done.

 

[Smart $]

Doesn't 110.14(C)(1) also say:



I believe this allows the listed ampacity on table 310.15(B)(16) to be modified by 310.15(B)(7) for terminations.

I agree... but the question is modified how?

2011 is automatic as kwired mentioned.

Under 2014, I believe it allows us to factor the non-(B)(7) limit by 83%. For example, the allowable ampacity must be not less than the noncontinuous load plus 125% of theccontinuous load times 83% for determining the minimum conductor SIZE.

Under 2011, the factor varies, but for the 200A service example, 2/0 Cu is compliant and otherwise rated 175A.

175A/200A = 87.5%

 

[kwired]

I agree... but the question is modified how?

2011 is automatic as kwired mentioned.

Under 2014, I believe it allows us to factor the non-(B)(7) limit by 83%. For example, the allowable ampacity must be not less than the noncontinuous load plus 125% of theccontinuous load times 83% for determining the minimum conductor SIZE.

Under 2011, the factor varies, but for the 200A service example, 2/0 Cu is compliant and otherwise rated 175A.

175A/200A = 87.5%

If you go through the table published in the past though, and apply 83% factor and round up to next larger conductor if it doesn't hit a 310.15(16) value (in the 75C column) you mostly get same results that were published in the table, so I have to assume the table was originally based on 83%.