Line/Load Tap Questions

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sokbok

Member
Location
Kansas City
Howdy forum.

I define a LINE side tap as on the Utility aka LINE side of the main service disconnect.
I define a LOAD side tap as on the customer aka LOAD side of the main service disconnect, usually back-feeding a breaker on the opposite end of a bus bar and using the 120% rule.

I have a simple situation that is confusing me regarding a LOAD side connection on the customer's side of a 400Amp disconnect. This disconnect then feeds a 200Amp Main Breaker Load Center and there is space to use multi-tap connectors after the main service breaker to tap into the conductors on their way to the breaker panel.

I used to think this was not allowed. I would always require utility shut down and tap on the LINE side of the main service disconnect and treat our Solar AC Disco as a new service disconnect and bond N and G there. I know that if we tap after the main service disconnect, we would not bond N and G in our PV AC Disco.

What code sections are applicable to this scenario? 705.12(1)(b)?
What does the code say about the ampacity rating of the conductors feeding from the LOAD side of the main disconnect to the breaker panel.
What would be the changes if the main breaker was 400A in the breaker panel?

Below is a simple drawing. Option A is LINE side and Option B is LOAD side.

thanks for any insights
tap 2.jpg
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The sections were renumbered each of the last several code cycles. So it's a little daunting to list all the references for different cycles without knowing which one you need. But note the actual content has mostly not changed since the 2014 NEC, just been renumbered.

The line side option is governed by 705.11 in the 2020 NEC, formerly 705.12(A).

The load side option is governed by 705.12(B)(1) in the 2020 NEC. It places additional requirements on the feeder; it is rare you can just tap an existing feeder for solar without doing anything else. This section was (hope I've got this right) 705.12(B)(2)(1) in the 2017 NEC and 705.12(D)(2)(1) in the 2014 NEC. Prior to 2011 your load side tap option was not addressed by the code.

240.21(B) also applies to taps. In your diagram you have a tap. But, by the way, if you just add a backfed breaker to a panelboard on the load side of the service disconnecting means, then it is a load side connection, but it is probably not a tap.
 

PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
A feeder tap for the PV interconnection is definitely allowed. Keep in mind that 705.12(B)(2)(2) (2017 NEC) modifies the tap rules in 240.21. For example, if you are using the 10'/10% tap rule, the minimum size of your tapped PV feeders would be 10% of (400A + 1.25*inverter output current).
 

sokbok

Member
Location
Kansas City
See feeder taps 240.21(B) 1 &2
I have reviewed the Feeder Tap Rules and while helpful, they use the terminology LOAD which is not exactly correct. If under 10' it simply states that the inverter output conductors making the tap are rated for the ampacity of the "load" but in this case its a generating source, but still, yes the tap conductors need to be rated for 125% of inverter output, not OCPD rating FYI.

So I then go to 705.12(B)(2)(2) and while confusing I think it states that if I am not making my tap at the end of the feeder, but somewhere in between then the Main feeder needs to have an ampacity of the Disconnect + 125% of the PV inverter output. Which does not make sense electrically because the PV will reduce the demand on that conductor.

A feeder tap for the PV interconnection is definitely allowed. Keep in mind that 705.12(B)(2)(2) (2017 NEC) modifies the tap rules in 240.21. For example, if you are using the 10'/10% tap rule, the minimum size of your tapped PV feeders would be 10% of (400A + 1.25*inverter output current).
I can not seem to find this 10% rule. I do find a 1/3 mention if the feeder is over a certain length. Any code reference?

Its simple that the Inverter output conductors need to be rated correctly for 125% of inverter output, but does the existing feeder need to be increase to meet code? I should not think so, but I am afraid it does. Does this change on the LINE/LOAD sides of my tap? Also what if the drawing below had a 400A breaker instead of a Main Lug Only Distribution panel?

Thanks for any help on this tedious subject.
 

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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I have reviewed the Feeder Tap Rules and while helpful, they use the terminology LOAD which is not exactly correct. If under 10' it simply states that the inverter output conductors making the tap are rated for the ampacity of the "load" but in this case its a generating source, but still, yes the tap conductors need to be rated for 125% of inverter output, not OCPD rating FYI.

So I then go to 705.12(B)(2)(2) and while confusing I think it states that if I am not making my tap at the end of the feeder, but somewhere in between then the Main feeder needs to have an ampacity of the Disconnect + 125% of the PV inverter output. Which does not make sense electrically because the PV will reduce the demand on that conductor.

The PV does not reduce the demand on the downstream section of the feeder. It has no effect on the demand. However it can defeat the overcurrent protection scheme by supplying more current to the feeder than the main overcurrent device allows.

Example: An existing 100A breaker supplies a feeder to a 100A main-lug panelboard. If I tap the feeder to connect a 32A PV inverter, it is now possible for 132A to flow down the feeder to the panelboard.

That is why the code states that the downstream portion of the feeder needs to be sized for both sources, as you mention, or an additional overcurrent device must be placed between the tap and the downstream load.

I can not seem to find this 10% rule. I do find a 1/3 mention if the feeder is over a certain length. Any code reference?

The 10ft tap rule requires the tap conductors to have at least 1/10th the ampacity of the OCPD protecting the main feeder. 240.21(B)(1)(4). It seems you read this section but just missed the last line.

Its simple that the Inverter output conductors need to be rated correctly for 125% of inverter output, but does the existing feeder need to be increase to meet code? I should not think so, but I am afraid it does.

It matters where the source is connected. If the source is at the opposite end of the feeder, current can only flow in one direction, so the feeder cannot be overloaded (Kirchoffs Law). However if the source is tapped in the middle of the feeder, see above.

Does this change on the LINE/LOAD sides of my tap?

Yes, if you tap a source to the middle of a feeder, only the side of the feeder going to the loads needs to be upsized or given an additional OCPD.

Also what if the drawing below had a 400A breaker instead of a Main Lug Only Distribution panel?

In my opinion that would serve as the additional OCPD needed for the load, and you would not have to upsize the feeder. Some might quibble over whether the breaker needs to be near the tap instead of at the panelboard, but I've done it by installing a main breaker in an existing panelboard.

Thanks for any help on this tedious subject.

You're not the only one who is confused. ;)
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
What is the size of the conductors from the 400A disconnect to the 200A main breaker panel? If those conductors are already a feeder tap, that would preclude Option B, yes?

Cheers, Wayne

If the conductors from the 400A disco to the splice are rated 400A then it would be fine. But it's a good question, as is the question of why there's the 400A and 200A difference.
 
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