I have a question. You have an 800 amp 480 volt 3 phase service. The first piece of equipment in the service is a fusible disconnect that feeds a CT cabinet, then goes to a large 2 piece MDP cabinet. Now if someone taps the busbar inside of the MDP and goes to a separate fusible 400 amp disconnect which then feeds a beaker panel which is connected to a large PV system. That feeder is a tap. Right? It is in pipe and troughs and all that is in the service room. However can you run that unfused tap wire inside of the same trough as all the other wires that all have proper OC protection on them? You are trying to run, fused and non fused wires in the same trough.
Now can you cite sec. 230.7 of the 2011 NEC? The 400 amp wire is a feeder, but is it a tap or a service conductor? It feeds a PV system and other than the original 800 amp fuses, there is no other protection on that 400 amp wire. Plus it runs through a trough with all the other conductors. I know it would be safer if it was in a different conduit run away from the other conductors.

What is your thoughts?

I think the short answer is you can't run unprotected service conductors in the same raceway as load side conductors.

If the tap is ahead of the service disconnecting means then it is service entrance conductors...right.They do not belong in the same conduit as non service conductors.

But if the feeder is a tap from the existing feeder...I don't know of a restriction of feeder taps with other circuits?

Also if the service has a 800amp main and 400amps of solar on the backfeed the bus in the MDP must be rated for minimum 1000amps.

2011 NEC 705.12(D)(2) 120% rule:

1000amp bus x 120% = 1200amp
800ampmain = 400amp solar = 1200amp

Greg,

That is one of my main concerns, however I would need a code section to cite. Since the "tap" is after the main fusible disconnect, it is not technically a service conductor. It would be a feeder wire. Yet, there would only be an 800 amp OCP protecting that feeder and it is running through a trough with all the proper OCP wires.

Short circuit,

I don't believe the 400 amp feeder wires land on a bus bar but are connected on the bottom of the MDP where there are some extra lugs. That is what the EC told me.

OK I misunderstood the situation. If all of the conductors are on the load side of the first O/C device, I am not sure there is a violation.

Greg,

Neither is the EC. Yet, is there a danger? Should the job fail? or pass? You have a 400 amp feeder wire with 800 amp OCP running through a trough with regular branch circuit wires.

Harold:
You have to get into the tap rules (240.21-(B) etc) may help.

As the PV grid tie conductors are protected at 400 amps at the PV AC disconnect, and the conductors are 500Kcmil CU, this sounds like a normal PV grid tie.

Are the conditions in the field the same as the submitted plans?

Also, what are you refering to as 'regular branch circuit wiring'?

If I understand how you are explaining the arrangement then....

This arrangement would still need to comply with 705.12(D)(2)

The bus and the tap conductors would need to be rated for minimum of 1000amps.

800amp main + 400amp solar = 1200amp of power feeding the "bus" and "conductor".

If the "bus" and "conductor" was rated for 1000amps you multiply those values by 120% which equals 1200amps...

It sounds like a load side connection to me. Read 705.12(D)(2) 2011 NEC

You have to have the connection at a dedicated circuit breaker or fusible disconnect as outlined in 705.12(D)(1)

The bus in that MDP must be rated 1000amps minimum.

This is a hazard IMO.

He could switch out the 800amp main for a 600amp main if the building load calculation was under 600amps. Then he would be only 40amps out of compliance to the 120% rule.

600 + 400 = 1000amps of breakers...

800amp bus rating x 120% = 960amps...

1000amps - 960amps = 40amp difference....

For 2014 we will use the inverter output rating x 125% for the backfeed value to use instead of the PV breaker rating...

So if that job had a 240KW inverter it would be 240KW divided by 831= 361amps

361 + 600 = 961

Opps...forgot to show the x 125% in last post...(PV inverters are rated continous operation so x 125%)

So if that job had a 240KW inverter it would be 240KW divided by 831volts (480volts x 1.73) = 288.80amps x 125% = 361amps ( note this 361amp figure would land on a 400amp breaker)

361 + 600 = 961

Harold:
You need the buss rating and the service conductor rating to proceed with this. I didn't understand the intent of your OP, and took it as concerns over protected & non-protected conductors within the same enclosures or raceways.

At the hazard of making a fool out of myself...

IIRC, by their very design, PV (wave matching) inverters are sync'd to the utility's waveform.

If this fails to be true, they will trip out.

If that is true, then when a bus is backfed grid inter-tied PV power it can't stack up on top of the amps provided by the utility. For the inverter will not be functioning at the exact same voltage -- but a tad higher. Hence, the PV inverter will displace current back up towards the Poco.

Should the load-side go into fault mode, (bolted short) IIRC, the inverter trips open.

So stacking of the amperages sounds over-engineered -- for PV inverter-interties. (only)

Beyond that, there's a general mis-understanding WRT PV current during a bolted short. Unlike an infinite bus (Poco power) PV arrays are semi-conductor devices -- which S A T U R A T E curret flow under a bolted short.

Restated: they are not at all capable of generating the transitory amperages possible with inductive elements. (Transformers, alternators, generators) Unlike PV cells, inductive elements store EM energy -- which has to be absorbed during a bolted short. Hence, fuses and C/Bs.

PV cells have no collapsing field effects at all. The only inductive and capacitive elements would be in the inverter -- and, IIRC, the factory has them internally protected/ fused.

Since I don't work the PV side of the street, I'd appreciate comments from installers that do.

John,

Figure a MDP with a trough above it. All of the branch circuits run through this trough. All the branch and feeder circuits have proper OC protection. Then you take this 400 amp wire and run it through the same trough. The 400 amp wire feeds a fused disconnect which then goes to a sub panel. This has a 400 amp breaker being back fed from a large PV system on the roof.

Where the 400 amp. feeder originates is at some lugs on the bottom of the MDP cabinet. I didn't see if the lugs are attached to because the cover was on and the EC didn't have tools to take it off. I also didn't have any extra paperwork with me it was in the office at the time. I don't know if an engineer signed off about the amperage of the bus bar. I will check today.

Harold:
Yes, in theory the 'tap' is unprotected from the tap to the 400 amp CB. This is an issue with 'line tap' solar grid ties, Is it compliant? By the tap rules, it is. I can't come up with an article off the cuff to cite (unprotected) and protected within a raceway, but I will look into it as soon as I have some time.

I have to agree with Teslas theory above regarding the buss, but we have the 'book' to conform to.

Telsa,

Are you saying that the power from the grid inverter will pass thur the panel without feeding any loads off the panel? So there is no need to worry about the 120% rule?

The buss in the panel would have to experience some heating due to the inverter load passing thru it...right?

shortcircuit

The bus, in this instance, has two line-side connections: the Poco and the sync'd inverter.

The load is invariant -- doesn't change -- whether the power is flowing via the inverter -- into the main bus -- and out and down to the dependent circuits ... or whether all of the power comes from the Poco. (Sundown)

The amperage seen by the bus simply can't exceed that flowing under sundown conditions. During noontime, even full amps -- from the PV inverter -- merely shove power backwards -- onto the bus. If the local load is way down, then the net result must be that power flows backwards -- up the line -- and into the Poco's grid.

In the most extreme case, that would be, say half of the bus capacity.

For PV inverters that are sync'd to the Poco frequency -- any breakdown in that synchronization must cause them to trip out. Other wise, arc welding would begin.

Sync'd inverters just can't over load the bus -- unless NEMA standards have really let us down.

(Exception: a seriously overdesigned PV array could over power the bussing. However, that level of overdesign would be wildly uneconomic -- crazy, in fact.)

Lets say the loads on the buss at the middle point between the utility supply breaker (800amp) and the inverter input breaker(400amp) exceeded the value rating of the buss(800amp) at 10am to 2pm on a sunny day...the buss would be overloaded and heating would cause damage...right.This is how it was explained to me.

Seems like you are describing a fault scenario?

Just to let everyone know, I am calling back the PV company and getting them back out there to look inside of the 800 amp disconnect and ask, if anyone calculated the bus bar for addition power and if so, I would like a letter from an EE stating that fact that the bar can handle it. Also the install does not match the drawings that I got back at the office. So I also need them to re-draw up the plans again. The print showed a tap before the 800 amp main, but in reality, it is after it. Also the plan was drawn up by a company in Calif., but it does have a NJ seal on the plan.

Harold,
The EE can't state that the buss can handle more than 800amps. They must follow whats prescribed in 705.12(D)(2)

705.12(D)(2) says the added value of the breakers feeding a buss can't exceed 120% of its rated value.

This is probally why it was drawn on the original plan as a line side connection. But a line side connection would be in front of the CT cabinet for metering which is cold sequence connected with the 800amp main.

They can't connect before the cash register.

This happened in our area and the MDP was upgraded to an MDP with 1200amp bussing.

Anyway...keep us informed on how this works out.

Telsa, my last post was for you?

shortcircuit

Harold:

A 'line side tap' cannot be before the CTs. Line side taps are installed after the meter.

The first thing to be determined is the rating of the buss.

If the job was not installed to the submitted & approved plans, you definetly need 'as-builts' which require review and approval. (The review may be a "fee" depending on your Twp fee schedules & ordinances)

The EC should have been at the site for the inspection!

I understand what Tesla is saying, and agree with him.

Hotline...

A "line side tap" or "supply side connection" which is the description used by the NEC, is allowed by 230.82(6) for a PV system

This is allowed to be before the meter, but if done like this it would need to be set up as a seperate service, which is allowed under 230.2(A)(5) requiring the configuration to have its own metering and service disconnecting means, etc.

In my area, with a similar installation as Harolds (400amps of backfed solar load side connected to an 800amp MDP with cold sequence metering), an 800amp MDP was replaced with 1200amp equipment to comply with 705.12(D)(2)(known as the 120% rule) because they didn't want seperate metering for the PV.

shortcircuit

Shortcircuit:
Yes, an additional meter can be installed for a PV, as you described, and will be compliant.

Usually, the intent of the proposed installation as Harold explained is addressed in Plan review. Sometimes, as we are all human, it may get missed, but caught at field inspection.

The solution you stated above, is a usual remedy.

Harold has to get the actual rating of the buss from the existing MDP for starters.

John,

Called up the PV company and told him that his plans showed a line side tap, but they were claiming a load side tap. I told him that either way, he has to re-draw the plans to show what is really there. I will also go back out to make sure that the plan matches the work done there.

Harold:
Did you check the mfg nameplate data on the piece of gear? It may have a higher buss rating then the main CB.

John,

When I go back, I will check that out. I also have the EE drawing up new set of plans to show what has really been done. I also want a letter stating that the bus can handle the extra load.

The Jan/Feb edition of the IEIA has a good article on the 120% rule by John Wiles on page 52

Short circuit,

I don't remember if I read that article or not, but I do have the magazine and I will check it out.

I would also question tapping the buss in the MDP. Does the manufacturer allow tapping the buss? If not, it may void the listing on the equipment.

A FE by UL may be in order if the equipment was modified.

Shortcircuit:
Yes, modifications to buss cause issues with the listing. Alas, it has been 'common practice' for some time.

Dependent on having a PE/EE provide methods and materials, and a signed sealed drawing, it is acceptable in most cases.

A few jobs required the mfg to provide the materials and make the mods, and provide UL field certification. That is when $$$ is no object, as it it very costly

Harold, how did this job work out?

I went back yesterday. I had a new set of drawings that showed the correct way it was all actually installed. There were some 800 amp quick blow fuses before the tap for the PV system. So the feeder to the PV was protected. They also didn't tap the bus, but bugged the incoming wires. II have signed and certified drawings from an EE for this job.

Thanks for the follow up Harold.

The EE still has to follow the 120% rule.

How did this work out Harold?

Short circuit,


The EE did indeed follow the 120% rule and he marked it out on the new set of plans that he drew up.