Parallel conductors

[ggunn]

I had always thought that when performing a line side tap onto the service conductors feeding an MDP that I needed to tap all the conductors when there was a parallel set, but I recently spoke to another designer who told me that he only taps one conductor for each phase as long as the ampacity of the single conductor is more than the maximum output (X 1.25) of the inverter(s). It seems logical to me that when interconnected this way nothing the PV system could ever do would cause an increase in current on the untapped conductors, but it also seems that the rules covering parallel sets are written so that the conductors are all identically treated and this way of tapping them would make them not identical.

Whaddya think? I haven't found any language in the NEC that addresses this issue.

 

[kwired]

I had always thought that when performing a line side tap onto the service conductors feeding an MDP that I needed to tap all the conductors when there was a parallel set, but I recently spoke to another designer who told me that he only taps one conductor for each phase as long as the ampacity of the single conductor is more than the maximum output (X 1.25) of the inverter(s). It seems logical to me that when interconnected this way nothing the PV system could ever do would cause an increase in current on the untapped conductors, but it also seems that the rules covering parallel sets are written so that the conductors are all identically treated and this way of tapping them would make them not identical.

Whaddya think? I haven't found any language in the NEC that addresses this issue.

I agree with the fact that individul conductors connected in parallel are supposed to be identical and the added tap to just one conductor of a set throws off being identical
.
Many instances you may be able to get away with such an install never developing any troubles, but IMO that is not the intent of the rules of keeping parallel conductors all identical. A small change in impedance of one conductor of the set will make a conductor carry a different net current and the idea is to keep them equal to balance current across all conductors of the set.

 

[ggunn]

I agree with the fact that individul conductors connected in parallel are supposed to be identical and the added tap to just one conductor of a set throws off being identical
.
Many instances you may be able to get away with such an install never developing any troubles, but IMO that is not the intent of the rules of keeping parallel conductors all identical. A small change in impedance of one conductor of the set will make a conductor carry a different net current and the idea is to keep them equal to balance current across all conductors of the set.

I understand all that, but is it addressed in the NEC? The designer I spoke with claims to have multiple PV systems in place (and successfully inspected) with single conductors tapped.

 

[GoldDigger]

I understand all that, but is it addressed in the NEC? The designer I spoke with claims to have multiple PV systems in place (and successfully inspected) with single conductors tapped.

All that the NEC says is that parallel conductors have to be identical in several ways:

310.10(H)(3) [2014]:

(2) Conductor and Installation Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, equipment grounding conductor, or equipment bonding jumper shall comply with all of the following:
(1) Be the same length.
(2) Consist of the same conductor material.
(3) Be the same size in circular mil area.
(4) Have the same insulation type.
(5) Be terminated in the same manner.

You could argue that either the "same length" provision or the "terminated in the same manner" provision would prohibit a tap, even if the conductor is not broken and spliced in the process (e.g split bolt).
But that is not a strong argument to a strict constructionist.
The motivation behind the parallel rules, that of having an even distribution of current among the parallel paths, would argue strongly against tapping one set, either for loads or for PV feed, but it is not there in so many words, AFAICS.

 

[petersonra]

take a look at 300.3 (B)(1)

It would appear that at the very least the tapped conductor would have to be run in the same raceway as the conductor it was tapped from.

 

[GoldDigger]

take a look at 300.3 (B)(1)

It would appear that at the very least the tapped conductor would have to be run in the same raceway as the conductor it was tapped from.

I do not see it saying that, since the tapped conductor is arguably not part of the paralleled installation.
But I recognize your right to think so.

Now if the PV feed were tapped equally to all conductors of the paralleled set, I think 300.3 would apply.
And in any case if multiple ungrounded conductors were tapped (in one set), then all of the tap conductors would have to occupy the same raceway, but not necessarily the same as the feeder set being tapped.

A general argument that all conductors of a single circuit, rather than just those portions which form a parallel group, seems uncalled for. It would mean that you could not separate circuit conductors to separate raceways beyond the point where the parallel group is terminated.

 

[ggunn]

The motivation behind the parallel rules, that of having an even distribution of current among the parallel paths, would argue strongly against tapping one set, either for loads or for PV feed, but it is not there in so many words, AFAICS.

The counter argument to that would be that the intent of the identical nature requirement of the code is to ensure that none of the parallel conductors could be made to carry more current than its ampacity would allow. I don't see how a PV tap on one conductor in a parallel set could cause any of the other conductors to carry more current than they would otherwise.

 

[GoldDigger]

The counter argument to that would be that the intent of the identical nature requirement of the code is to ensure that none of the parallel conductors could be made to carry more current than its ampacity would allow. I don't see how a PV tap on one conductor in a parallel set could cause any of the other conductors to carry more current than they would otherwise.

Only to the (perhaps trivial) extent that the GTI and accessories attached to that circuit may at some point constitute a small load.
In the event of a fault in the inverter, there would be mismatched current until the OCPD activated, and that OCPD might be sized to the parallel set, not to a single conductor.
If the GTI is protected by OCPD that is small enough to protect a single conductor, there is still the argument that until that OCPD opens (if it does) that feeder conductor is seeing a larger current than the others in the group.

 

[kwired]

What is the point of tapping into the conductor if you are not going to either :
A - inject current into the system

B- take current from the system

besides some device whose primary function is only to monitor voltage, current, frequency ...?

If you are taking or injecting current, are you not going to make the impedance of that one conductor different then the rest of the conductors of the set, assuming they were all the same before you made the tap?

 

[jaggedben]

Maybe I'm thinking about this too simply, but it seems to me that doing this guarantees that the current in the two sets of conductors will be unequal at some point in time and space.

I also tend to think that 'terminated in the same manner' is broad enough to prohibit a tap on only one set. You could tap both sets and then parallel the taps to one set of conductors that heads to the inverter, though.

 

[ggunn]

Maybe I'm thinking about this too simply, but it seems to me that doing this guarantees that the current in the two sets of conductors will be unequal at some point in time and space.

I also tend to think that 'terminated in the same manner' is broad enough to prohibit a tap on only one set. You could tap both sets and then parallel the taps to one set of conductors that heads to the inverter, though.

In a past job I worked with folks who did EMS (energy management systems), and when I asked them why they didn't just put one CT on one of a set of paralleled conductors and multiply their reading by the number of conductors, they told me that they don't do that because they had seen as much as 15% difference between the current in paralleled conductors. I don't think anyone cares if the current isn't exactly the same, only that no conductor gets overloaded.

 

[Carultch]

I had always thought that when performing a line side tap onto the service conductors feeding an MDP that I needed to tap all the conductors when there was a parallel set, but I recently spoke to another designer who told me that he only taps one conductor for each phase as long as the ampacity of the single conductor is more than the maximum output (X 1.25) of the inverter(s). It seems logical to me that when interconnected this way nothing the PV system could ever do would cause an increase in current on the untapped conductors, but it also seems that the rules covering parallel sets are written so that the conductors are all identically treated and this way of tapping them would make them not identical.

Whaddya think? I haven't found any language in the NEC that addresses this issue.

If you want to avoid trouble, you should make your tap onto all conductors in the system. Do everything possible to avoid breaking the symmetry, any time there are multiple sets.

The "same length" part of the rules for parallel conductors means that the length from terminal-to-terminal needs to be the same.
The "terminated in the same manner" part of the rules for parallel conductors means that one set cannot run from equipment to equipment, while another set is terminated at a separately installed connector.
I do not read anything in the code that would prohibit making the parallel conductors non-identical, by interrupting one of them for the tap without terminating it (via a split bolt, or insulation piercer). This is a gap in the language of the code that didn't consider this possibility.

It is true that it is impossible to get all conductors exactly identical so that the current is identical. And in order to minimize any variation, your design on paper should not have any differences among the conductors that would cause currents to differ.

As an example, suppose you are tapping 400A of PV (2 sets of 3/0) onto 1000A of service (3 sets of 400).

This means that both of the 2 sets of 3/0 need to connect to all 3 sets of 400. And you'd likely do this with an 8-terminal connector (3 in for service supply, 2 in for PV, 3 out for service loads). Like any busbar, I'd group each source on opposite ends of the connector with the loads in the center, to diminish the current to zero within the connector.

 

[ggunn]

If you want to avoid trouble, you should make your tap onto all conductors in the system. Do everything possible to avoid breaking the symmetry, any time there are multiple sets.

The "same length" part of the rules for parallel conductors means that the length from terminal-to-terminal needs to be the same.


As an example, suppose you are tapping 400A of PV (2 sets of 3/0) onto 1000A of service (3 sets of 400).

This means that both of the 2 sets of 3/0 need to connect to all 3 sets of 400. And you'd likely do this with an 8-terminal connector (3 in for service supply, 2 in for PV, 3 out for service loads). Like any busbar, I'd group each source on opposite ends of the connector with the loads in the center, to diminish the current to zero within the connector.

And that is the way I have done it as well, but a designer I respect tells me that he has tapped single conductors in parallel sets and passed inspections. We can go round and round in here (as we frequently do) about what we think should and should not be the way to do this, but unless there is a code reference that addresses it directly it's just conjecture on our part.

 

[kwired]

In a past job I worked with folks who did EMS (energy management systems), and when I asked them why they didn't just put one CT on one of a set of paralleled conductors and multiply their reading by the number of conductors, they told me that they don't do that because they had seen as much as 15% difference between the current in paralleled conductors. I don't think anyone cares if the current isn't exactly the same, only that no conductor gets overloaded.

I have never measured same current on any set of parallel conductors myself - all the more reason to not intentionally introduce any imbalance IMO.

And that is the way I have done it as well, but a designer I respect tells me that he has tapped single conductors in parallel sets and passed inspections. We can go round and round in here (as we frequently do) about what we think should and should not be the way to do this, but unless there is a code reference that addresses it directly it's just conjecture on our part.

Your designer has found a loophole in the words of the NEC but there is no loophole in the intention -JMO. So maybe a change is needed in this area - and may happen if this becomes a common practice.

 

[ggunn]

I have never measured same current on any set of parallel conductors myself - all the more reason to not intentionally introduce any imbalance IMO.


Your designer has found a loophole in the words of the NEC but there is no loophole in the intention -JMO. So maybe a change is needed in this area - and may happen if this becomes a common practice.

I don't see imbalance between the current in parallel conductors as being a problem as long as no conductor becomes overloaded. The addition of PV to a conductor in a parallel set can only decrease the current in that conductor and will not increase the current in the other(s). What's the problem?

 

[kwired]

I don't see imbalance between the current in parallel conductors as being a problem as long as no conductor becomes overloaded. The addition of PV to a conductor in a parallel set can only decrease the current in that conductor and will not increase the current in the other(s). What's the problem?

Why does it decrease current?

I understand it will lessen demand from the utility, or if premises load is low you can actually have an increase in current moving toward utility side of the tap, but by injecting current you are still messing with overall impedance of the elements of the parallel conductor aren't you? Change impedance of one element and current in other elements will change as well. How much will it change? Wouldn't that depend on size of conductor you are tapping into, the existing load without consideration of what you are introducing with the PV, the change of current on your element after you make the tap, whether or not that current is flowing towards the premises wiring or toward the utility... possibly several other factors?

Maybe the chances of overloading any element of the set is not that great, IDK, but I am sure you will effect balance of current in all elements, just like making one element a different length, size, etc. will also disturb the balance of current in the entire set.

PV is something I have not worked with, maybe I am just enough behind because of that and am missing something. I feel I have a good understanting of electrical theory, so maybe you can tell me why current decreases here, I don't see it decreasing, just seeing some of it being displaced or diverted from where it normally may flow.

 

[jaggedben]

Your designer has found a loophole in the words of the NEC but there is no loophole in the intention -JMO. So maybe a change is needed in this area - and may happen if this becomes a common practice.

The intention of the rules, I believe, is to prevent any individual conductor from experiencing a meaningful overcurrent due to having a lower impedance than its counterpart(s) when the load is at full draw. If following the 'loophole' doesn't possibly result in contributing to such an overcurrent, then it's not really going against the intention. With that said, I'm withholding judgment on said possibility, feeling not very qualified to answer.

Why does it decrease current?

...
PV is something I have not worked with, maybe I am just enough behind because of that and am missing something. I feel I have a good understanting of electrical theory, so maybe you can tell me why current decreases here, I don't see it decreasing, just seeing some of it being displaced or diverted from where it normally may flow.

I think ggunn has something in mind like this...

Let's say you've got two sets of paralleled conductors rated 500A, together carrying a full load of 1000 to a facility.
PV guy comes along and taps one set of them, but the output will not be more than 400A (by code.)
PV system puts out full output, so now you have 400A flowing into the tapped conductors.
Facility still uses 1000A, so now it draws 500A utility power on one set, and 400A PV plus 100A utility on the other.
Therefore...
The current on the utility end of the tapped conductor set decreased from 500A to 100A.

I realize the actual physics of current flow may not be this simple, but if there's a scenario where there's a meaningful increase in current over the rating of either set of conductors, I certainly couldn't explain how that'd happen. You can seemingly come up with many scenarios where the current is wildly different in the two sets of conductors (e.g. full PV output, zero load), but at first glance they seem to all involve facility draws below what the service would be rated for, and thus currents well below what the conductors would be rated for.

 

[TommyO]

Why does it decrease current?

I don't see it decreasing, just seeing some of it being displaced or diverted from where it normally may flow.

Assuming there's already current flowing to a load, the current to that load would now come partly from the solar and partly from the grid.

TO put some numbers on it, lets say 100A flowing normally, adding in a tap with 40A coming in from solar, and it's tapped at the midpoint.
Now 100A is still flowing to the load, but 40 is coming from the solar and 60 from the grid. So half of the length now has much less current flowing in it.

Is it just that the current is displaced/diverted? Yes, I think that describes it.

But if you look at just a portion of the wire, you can also say that the current has been reduced.


I think the point ggunn was trying to make is that it doesn't make the situation any worse for the tapped wire. Arguably it makes it better. And doesn't impact the other wires. (Reasonable argument IMO)


Here's a thought exercise- what if you have 3 parallel'd conductors, and you *did* tap all 3? now, think about what happens if one of the endpoints fails... Now the current goes from 3 wires down to 2. And ~1/3 of the power is going through the conductor that bridges those 3 taps.

Of course that's picking one failure mode that makes it worse when tapping all 3 - But I can't think offhand of a failure mode that's worse when tapping only one

 

[jaggedben]

I'm also curious if inductive heating is a potential issue here. Say, if you tapped a 3-phase system with an unbalanced PV output. I'm not qualified to do that math though. :ashamed1:

 

[GoldDigger]

I'm also curious if inductive heating is a potential issue here. Say, if you tapped a 3-phase system with an unbalanced PV output. I'm not qualified to do that math though. :ashamed1:

As long as you keep the two or three PV lines or the PV line(s) and neutral in the same raceway from inverter to tap, you should not see any inductive heating.
All the ungrounded conductors of the parallel feeder set should already be the same raceway.