Tap sub feeders

[deadshort]

We are doing a load-side tap of 80A PV (protected by 100A fused disconnect) into sub-feeders between a 200A main breaker and 200A remote sub panel (main feeds directly into lugs in sub, no "main" breaker in sub panel).

Question is: Do we need to install a new, additional 200A breaker in the sub panel since we are tapping in between the main breaker and sub? All the conductors are rated for 200A and since we're not landing PV in the sub I don't see how it could be overloaded. I agree I would be more comfortable with some protection in the sub but am wondering if this would be a waste of time/money or if it's even necessary. Thanks.

 

[pv_n00b]

A 200A service only will allow a 40A back feed if you put a main breaker in the sub, no back feed if you don't. This is probably going to have to be a supply side interconnection.

 

[deadshort]

A 200A service only will allow a 40A back feed if you put a main breaker in the sub, no back feed if you don't. This is probably going to have to be a supply side interconnection.

I'm not sure I follow what you're saying. No backfeed? What is you're interpretation of a supply-side connection? Would that be in between the meter and main breaker or between the meter and the utility lines?

Also, we're not landing a PV breaker in the sub. We are interconnecting (with piercing connectors) directly into the feeder wires that lead from the main breaker (in the meter panel) to the lugs of a 200A "sub" panel (which is actually more of a main load center).

Again, the tap will have a fused 100A disconnect between it and the PV.

Edit* I should add that there is no provision for a tap between the meter and main breaker. It is solid bussing directly attached to the meter lugs.

 

[ggunn]

I'm not sure I follow what you're saying. No backfeed? What is you're interpretation of a supply-side connection? Would that be in between the meter and main breaker or between the meter and the utility lines?

Also, we're not landing a PV breaker in the sub. We are interconnecting (with piercing connectors) directly into the feeder wires that lead from the main breaker (in the meter panel) to the lugs of a 200A "sub" panel (which is actually more of a main load center).

Again, the tap will have a fused 100A disconnect between it and the PV.

Edit* I should add that there is no provision for a tap between the meter and main breaker. It is solid bussing directly attached to the meter lugs.

At the very least you will have to install a main breaker in the sub. Otherwise, it can draw 280A (200A from the main plus 80A from the PV) before anything trips. That would be true no matter how much PV you are adding because the feed from the service and the feed from the PV are both on the same end of the sub's bus. But I believe that pv_noob is correct; the most you can connect at the point you propose is 40A. Your MDP is fed from one end by 200A from the service; the most you can feed it from the other end is 20% of that, i.e., 40A.

 

[NYSolar]

Is there another 200amp breaker in MDP? How are you protecting 2 panels with one OCPD? Yes, when using piercing taps on feeders you need OCPD on upstream and downstream of tap. not sure on siting code. maybe someone else will chime in.

 

[ggunn]

Is there another 200amp breaker in MDP? How are you protecting 2 panels with one OCPD?

We see it pretty often where an MDP is connected to the service though a breaker and a sub is fed either through a load breaker in the MDP or directly from feed through lugs on the opposite end of the MDP bus from the main breaker. In the first case the sub is protected by the load breaker feeding it but in the second case the main breaker in the MDP is its only protection. In either case, though, if you tap the conductors between the MDP and the sub with PV, most of the time you have to add a main breaker to the sub. The exception would be if the protection on the sub and the PV breaker summed together are equal to or less less than the rating of the bus in the sub.

 

[jaggedben]

We are doing a load-side tap of 80A PV (protected by 100A fused disconnect) into sub-feeders between a 200A main breaker and 200A remote sub panel (main feeds directly into lugs in sub, no "main" breaker in sub panel).

Question is: Do we need to install a new, additional 200A breaker in the sub panel since we are tapping in between the main breaker and sub? All the conductors are rated for 200A and since we're not landing PV in the sub I don't see how it could be overloaded. I agree I would be more comfortable with some protection in the sub but am wondering if this would be a waste of time/money or if it's even necessary. Thanks.

Under the 2014 NEC (not in force in California until next month), you can do this only if you add the new 200A breaker in the subpanel as you described. Under the current code you really cannot do a load side tap that large, and you'd have to ask your AHJ if they allow load side taps at all. I'm assuming that the meter/main you're describing contains only the 200A breaker as service disconnect and that it is not a panelboard with branch circuits.

 

[deadshort]

Under the 2014 NEC (not in force in California until next month), you can do this only if you add the new 200A breaker in the subpanel as you described. Under the current code you really cannot do a load side tap that large, and you'd have to ask your AHJ if they allow load side taps at all. I'm assuming that the meter/main you're describing contains only the 200A breaker as service disconnect and that it is not a panelboard with branch circuits.

Correct. The meter/main has a 200A main breaker bolted directly to the meter lugs. No breaker provisions. The 200A main feeds a remote 200A panel with 4/0 AL conductors.

Wouldn't the limiting factor be the ampacity of the feeders? (4/0 = 205A) More than adequate to handle the 80A of PV. I can see why a new 200A breaker should be placed in the sub so it doesn't overload the bussing there.

 

[GoldDigger]

Under earlier NEC the feeder from main to sub would be subject to the 120% rule.
If no main in sub, the sub bus would be subject to the same 120% rule based on amps of main breaker.
The only time the 120% rule would not apply would be for a supply side tap, which has no OCPD at all on the POCO side.

Sent from my XT1585 using Tapatalk

 

[Carultch]

Correct. The meter/main has a 200A main breaker bolted directly to the meter lugs. No breaker provisions. The 200A main feeds a remote 200A panel with 4/0 AL conductors.

Wouldn't the limiting factor be the ampacity of the feeders? (4/0 = 205A) More than adequate to handle the 80A of PV. I can see why a new 200A breaker should be placed in the sub so it doesn't overload the bussing there.

The wire from the interconnection point to the house loads panelboard, is potentially subject to 280A. Suppose the utility supplies 200A, and the PV supplies its full 80A, and there is a fault. That section of conductor from the interconnection point to the house loads panel, is subject to 280A. If it is not rated for 280A, it is overloaded. Therefore, if greater than a tap-rule's length, it is not acceptable. Either a wire needs to be protected at its ampacity from the maximum possible supply, or it needs to be limited in length to one of the tap rules, with all context rules followed accordingly.

Put a 200A breaker immediately downstream of the interconnection point, and then you'll be all set.

 

[deadshort]

The wire from the interconnection point to the house loads panelboard, is potentially subject to 280A. Suppose the utility supplies 200A, and the PV supplies its full 80A, and there is a fault. That section of conductor from the interconnection point to the house loads panel, is subject to 280A. If it is not rated for 280A, it is overloaded. Therefore, if greater than a tap-rule's length, it is not acceptable. Either a wire needs to be protected at its ampacity from the maximum possible supply, or it needs to be limited in length to one of the tap rules, with all context rules followed accordingly.

Put a 200A breaker immediately downstream of the interconnection point, and then you'll be all set.

Yep, after thinking it through this is exactly what we came up with. Thanks.

 

[ggunn]

If no main in sub, the sub bus would be subject to the same 120% rule based on amps of main breaker.

Except that the sub gets the feed from the service and the feed from the PV on the same end of the bus, which makes it subject to the 100% rule, not the 120% rule. No matter how much PV he puts into the tap, he has to put a 200A main breaker on the sub since at present it's a 200A bus protected by the 200A breaker in the MDP.

 

[jaggedben]

Except that the sub gets the feed from the service and the feed from the PV on the same end of the bus, which makes it subject to the 100% rule, not the 120% rule. No matter how much PV he puts into the tap, he has to put a 200A main breaker on the sub since at present it's a 200A bus protected by the 200A breaker in the MDP.

Well, the 2011 code just says that the 'busbar or conductor' can be loaded to 120%. Then it says if you have an inverter connection in a panelboard it has to be opposite end of the main feed if the sum exceeds 100%. So at least in that interpretation the sub is not subject to a "100% rule." However in this case 80A exceeds 120% so the conclusion is the same, albiet reached a different way.

It's not even clear the 2011 code allows the 200A breaker in the sub to resolve the situation. I think what can be definitively stated is that the 2011 code doesn't explicitly address feeder taps, and any interpretation of the 2011 rules for load side taps is just that ... an interpretation. The 2014 code addresses taps explicitly.

 

[ggunn]

Well, the 2011 code just says that the 'busbar or conductor' can be loaded to 120%. Then it says if you have an inverter connection in a panelboard it has to be opposite end of the main feed if the sum exceeds 100%. So at least in that interpretation the sub is not subject to a "100% rule." However in this case 80A exceeds 120% so the conclusion is the same, albiet reached a different way.

I was taking a bit of liberty by calling it a "100% rule", of course, but that's what in essence it is. If you place a backfed breaker anywhere else other than at the other end of the bus from the feed from the service, its rating (or 125% of the inverter max current starting in 2014) plus the rating of the bus OCPD must be 100% or less than the bus rating. Calling it the "100% rule" was just my shorthand way of saying all that.

 

[pv_n00b]

I'm not sure I follow what you're saying. No backfeed? What is you're interpretation of a supply-side connection? Would that be in between the meter and main breaker or between the meter and the utility lines?

Also, we're not landing a PV breaker in the sub. We are interconnecting (with piercing connectors) directly into the feeder wires that lead from the main breaker (in the meter panel) to the lugs of a 200A "sub" panel (which is actually more of a main load center).

Again, the tap will have a fused 100A disconnect between it and the PV.

Edit* I should add that there is no provision for a tap between the meter and main breaker. It is solid bussing directly attached to the meter lugs.

You said you were tapping in between a 200A main breaker and a sub-panel, this is not a supply side interconnection. A supply side interconnection is on the utility side of the main breaker after the meter.

For 2011 NEC work:
So you have a load side interconnection with a 200A main breaker and I assume a feeder rating for 200A. If that is the case a load side interconnection using the 120% rule would allow you to back feed 40A (1.2*200-200) if you put a 200A main breaker in the sub-panel.

If there is no provision for an interconnection between the meter and the main breaker then you are limited to a load side interconnection. That being the case here is what you can do.

Put a main breaker in the sub-panel to get it out of the way.
You need 80A of back feed so that means you you need a conductor ampacity of 234A or
Reduce the main breaker rating to less than or equal to 160A or
Some combination of the two that gives you 80A of back feed room.

For 2014 NEC work:

705.12(D)(2)(2) added instructions for doing taps, but it's not all that clear.

(2) Taps. In systems where inverter output connections are​

made at feeders, any taps shall be sized based on the​

sum of 125 percent of the inverter(s) output circuit​

current and the rating of the overcurrent device protecting​

the feeder conductors as calculated in 240.21(B).​

If you tap the conductor then the section of the conductor between the PV tap and the sub panel has to meet the requirements of the tap rules in 240.21(B) assuming the source is the main breaker plus 125% of the inverter output. It might work with the existing setup or it might not. Depends mainly on where it will fall in the tap rules.


Good luck

 

[jaggedben]

I was taking a bit of liberty by calling it a "100% rule", of course, but that's what in essence it is. If you place a backfed breaker anywhere else other than at the other end of the bus from the feed from the service, its rating (or 125% of the inverter max current starting in 2014) plus the rating of the bus OCPD must be 100% or less than the bus rating. Calling it the "100% rule" was just my shorthand way of saying all that.

I'm taking issue with the substance, not your shorthand. The red part is debatable under the 2011 code, if the inverter connection is not actually a breaker installed in the panelboard whose rating is under discussion. 705.12(D)(7) in the 2011 code says "...a connection in a panelboard shall be positioned at the opposite end..." If the connection is a feeder tap then it isn't a connection in a panelboard and that section does not address the situation. And one can fall back (although not in the OP's case) to 705.12(D)(2) which allows a busbar or conductor to be loaded to 120% of its rating.

I'm not saying the 2014 code isn't a lot more sensible. I've been following 2014 rules on some of these cases even though it's not in force here yet, precisely because AHJs tend to look at it and approve it based on common sense, and I can avoid having this very discussion. The cost of a breaker is often less than another truck roll with a revised planset that follows on a confused back-and-forth phone conversation with the plan checker. :happyyes:

 

[SolarPro]

I'm taking issue with the substance, not your shorthand. The red part is debatable under the 2011 code, if the inverter connection is not actually a breaker installed in the panelboard whose rating is under discussion. 705.12(D)(7) in the 2011 code says "...a connection in a panelboard shall be positioned at the opposite end..." If the connection is a feeder tap then it isn't a connection in a panelboard and that section does not address the situation. And one can fall back (although not in the OP's case) to 705.12(D)(2) which allows a busbar or conductor to be loaded to 120% of its rating.

I'm not saying the 2014 code isn't a lot more sensible. I've been following 2014 rules on some of these cases even though it's not in force here yet, precisely because AHJs tend to look at it and approve it based on common sense, and I can avoid having this very discussion. The cost of a breaker is often less than another truck roll with a revised planset that follows on a confused back-and-forth phone conversation with the plan checker. :happyyes:

Totally. To my mind, the NEC 2014 and 2017 revisions are more about clarifying the pre-existing rules and allowances than changing them. Most AHJs will appreciate the additional level of detail, as it makes everyone's job a little easier.

 

[ggunn]

I'm taking issue with the substance, not your shorthand. The red part is debatable under the 2011 code, if the inverter connection is not actually a breaker installed in the panelboard whose rating is under discussion. 705.12(D)(7) in the 2011 code says "...a connection in a panelboard shall be positioned at the opposite end..." If the connection is a feeder tap then it isn't a connection in a panelboard and that section does not address the situation. And one can fall back (although not in the OP's case) to 705.12(D)(2) which allows a busbar or conductor to be loaded to 120% of its rating.

I'm not saying the 2014 code isn't a lot more sensible. I've been following 2014 rules on some of these cases even though it's not in force here yet, precisely because AHJs tend to look at it and approve it based on common sense, and I can avoid having this very discussion. The cost of a breaker is often less than another truck roll with a revised planset that follows on a confused back-and-forth phone conversation with the plan checker. :happyyes:

Nevertheless, under 2011 or 2014 I would (and did) do the same thing for the same reason. A connection is a connection whether it is a direct landing or through a conductor.

 

[PWDickerson]

Some of the information regarding the 2014 NEC rules outlined in 705.12(D)(2) in this thread is incorrect. For reference, see slides 4-17 in the following link.

http://www.slideshare.net/solpowerpeople/interconnection-rule

705.12(D)(2)(1) concerns feeders, where a PV system interconnection point is made at a feeder conductor. Note that this is not a tap, and the tap rules don't apply.

705.12(D)(2)(2) talks about how to treat a non-PV related tap in a feeder that also has a PV system interconnection (not considered at tap) made elsewhere on the feeder.

The following reference to 705.12(D)(2)(2) is from the Apr/May 2014 edition of SolarPro:

"Pre-exiting taps or taps added to a feeder with a PV supply source must be sized per 705.12(D)(2)(2) and 240.21(B), based on the sum of 125% of the inverter(s) output-circuit current plus the feeder OCPD rating. See Figure 3 for an example tap conductor calculation. Note that inverter output-circuit conductors, which must not have an ampacity less than the rating of the circuit OCPD, cannot meet the definition of a tap conductor."

I don't believe the 120% rule applies under the NEC 2014 in the OP's case. With the addition of a 200A breaker protecting the portion of the feeder conductors between the interconnection point and the sub-panel, I believe the PV system output current can be up to 160A protected by a 200 amp fused disconnect. The meter main has no provision for supplying branch circuits, and so is not limited by the 120% rule. This situation is the only one I am aware of where a system in excess of the 120% rule limitation is allowed on a load-side connected system.

 

[Carultch]

Some of the information regarding the 2014 NEC rules outlined in 705.12(D)(2) in this thread is incorrect. For reference, see slides 4-17 in the following link.

http://www.slideshare.net/solpowerpeople/interconnection-rule

705.12(D)(2)(1) concerns feeders, where a PV system interconnection point is made at a feeder conductor. Note that this is not a tap, and the tap rules don't apply.

705.12(D)(2)(2) talks about how to treat a non-PV related tap in a feeder that also has a PV system interconnection (not considered at tap) made elsewhere on the feeder.

The following reference to 705.12(D)(2)(2) is from the Apr/May 2014 edition of SolarPro:

"Pre-exiting taps or taps added to a feeder with a PV supply source must be sized per 705.12(D)(2)(2) and 240.21(B), based on the sum of 125% of the inverter(s) output-circuit current plus the feeder OCPD rating. See Figure 3 for an example tap conductor calculation. Note that inverter output-circuit conductors, which must not have an ampacity less than the rating of the circuit OCPD, cannot meet the definition of a tap conductor."

I don't believe the 120% rule applies under the NEC 2014 in the OP's case. With the addition of a 200A breaker protecting the portion of the feeder conductors between the interconnection point and the sub-panel, I believe the PV system output current can be up to 160A protected by a 200 amp fused disconnect. The meter main has no provision for supplying branch circuits, and so is not limited by the 120% rule. This situation is the only one I am aware of where a system in excess of the 120% rule limitation is allowed on a load-side connected system.

In slide 5 of your link, is there any permissible length that the yellow highlighted section can be, and still have #3/0 be sufficient?

Note that slide 8 shows such a situation, but doesn't highlight the portion between the PV interconnect breaker and the 200A feeder protection breaker. There is a length that is about a quarter inch on my screen, and the slide doesn't tell you that it has to be 600kcmil or a common lug assembly. One might assume by omission that it can continue at #3/0, if the length is in reasonable limits. Possibly with the tap rules as a guide, as it follows the same logic as a tap conductor that 3/0 would be sufficient. Under normal operation, there is no reason for more than 200A of current to flow on that section of conductor. A serious fault would be detected by the upstream 200A breaker in the 1000A panel and the PV interconnect breaker, and the downstream breaker provides overload protection.