Batteries, Charge Controllers and the 2011 NEC™
Off Grid Code Compliance
with Kenny Grigar
Those that were logged into the session live benefited from a little pre-lecture code talk from our presenter Kenny Grigar, his co-presenter Melissa Grigar, and our host- Sarah Raymer (AKA Ms. MOOC). We were discussing a problem involving minimum conductor size.
The problem goes as such:
What is the minimum conductor size for THWN-2 conductors carrying a 200A PV Output Circuit load coming from a PV Combiner Box to a the DC disconnect in a large Central Inverter?
This problem was taken from an old #SolarMOOC problem set that can be found here: Installing Electrical Components
*Note: In the previous problem set, it was not clarified which conductor type. I have changed that as well as a few little things in some more of the questions from the original problem set.
My answer is below:
200A x 1.25 x 1.25 = 312.5A
See Article 310, Table 310.15(B)(16)
75º column must be used because of terminal limitations
350 kcmil is limited to 310A
400 kcmil allows up to 335A
If you have questions or comments about this please submit them to our Forum where we can all help eachother out.
For example- check out this discussion: Ampacity Correction Factors for High Temperatures
A few mentions that stemmed from discussion in the lecture:
We talked about requirements for OCPDs for source strings: It is true that PV Source Circuits do not absolutely HAVE to be protected with an OCPD unless there are more than 2 strings. This is because the inherent qualities of the modules mean that they are ‘current limited’.
See NEC 690.9 for details: When you have 3 strings in parallel, if there is a fault in one string, the current of the other 2 strings can back up onto that string and overload the conductors. If you have 2 strings only, each string is always capable of dealing with that fault current from the other string.
Inverters have moved to NEC 705
When connecting an inverter with a load-side connection, you have to be sure not to overload the busbar. According to NEC 705.12(D)(2), we are allowed to go to 120% (Richard Stovall will be reviewing this in Thursday night’s lecture!).
There is an exception, and this was mentioned by Kenny:
“Exception: Where the photovoltaic system has an energy storage device to allow for stand-alone operation of loads, the value used in the calculation of bus or conductor load shall be 125% of the rated utility interactive current from the inverter instead of the rating of the overcurrent device between the inverter and the bus or conductor.”- NEC 705.12(D)(2)
Some CHAT Questions that were addressed:
Chat Question #1:
When is the battery bank at it’s most dangerous state? Can you speak a little about the various degrees of risk with flooded batteries? Can they explode under minimal loads or just while equalizing?
A: Batteries must be adequately vented – see NEC Article 480, NEC 480.9 (A) and NEC 480.10
When treated properly, batteries should not explode. This is why we vent them- to avoid a dangerous collection of hydrogen gas that can be very dangerous.
The battery shown in the presentation had exploded because of it being short circuited. If you were working with a non-insulated tool, and you dropped it onto the battery connectors so that a conductive connection was made between the positive and the negative, a serious KABOOM could happen.
Here’s a little information sheet on off-gassing that we found from Jupiter Batteries: Off-Gassing
Chat Question #2:
Trojan L16 6V batt spec sheet says, Absorption charge = 2.35 and Float Charge = 2.20, but the batt is 6V, what should you set on the FM80 charge controller? Times these numbers by 3 ?
A: The answer is- yes. Each cell = 2V, so 6 Volt = 3 cells, so times 3 for charge controller setting.
Most of what you would need to know for batteries and charge controllers from the 2011 NEC Code™ can be found in articles:
480: Storage Batteries
690.10: Stand Alone Systems
Part VIII: Storage Batteries
690.72 Charge Control
690.74 Battery Interconnections
700.12(A): Source of Power, general requirements, storage battery