ECN Forum Forums Code Related Discussion NEC & other Code issues Re: Tap?

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.