NEC Guidelines For PV Cable

[tictag]

I noted in this thread that "Input conductors and circuit breakers breakers must be rated at 1.56 times the short circuit current of the PV array (per NEC)".

When I'm sizing PV cable I usually ensure that the cable is rated for the maximum possible current (let's say Isc = 10A) using tables such as this one, which implies that a minimum of 16AWG (max current loading = 15A) would be suitable.

If this statement is correct then 1.56 x 10A = 15.6A, which means that, even though 16AWG is rated for the max possible load current (Isc), are the NEC guidelines suggesting that the minimum should be 14AWG (max current loading = 24A)?

 

[FilterGuy]

I know your post is rather old, but I have been researching this lately and this is what I believe NEC is saying:

Max current of a solar panel can be greater than Isc due to things like temperature and amount of sunlight. Therefore the NEC defines the max current to be: Imax = 1.25*Isc.

Since the conductors will be in operation for multiple hours, it is considered continuous use so the NEC says the conductor capacity must be 125% of Max current Icondutor=1.25*Imax= 1.25*(1.25*Isc) = 1.56*Isc

But it is actually more complicated than that. The NEC actually says the PC conductor must be sized for the greater of:
1) Imax x 1.25
or
2) Imax with all of the temperature and conduit adjustments.

Figuring out all the adjustments is a rather long and torturous process.

Generally speaking if 1) your ambient temp remains below 105F (40C), 2) the wires are not on the roof, and 3) there are no more than 3 wires in conduits, no further adjustments are needed. To select wire size you can use Isc x 1.56 and the Ampacity table for the wire type you have.

However, if any of the following conditions exist, the wire size may need to be larger (sometimes significantly larger):

• Ambient temps above 105F/40C.
• More than 3 current carrying conductors in a single conduit or raceway
• Wire on the roof (particularly if it is in conduit).

 

[Jim Burrow]

That's for the feedback. I will copy and paste your comments into my notebook.
Although I could use 14 AWG wire as my panels draw 9 amps max (I measured the ISC), I will be using at least #12 AWG for just the reason you mentioned above. But I will apply your calculations before I buy the wire just to make sure I don't need to use 10 AWG. (I have 6 40v, 300w panels at 9a ISC). They will be arranged as 2 in series for one branch and will have 3 parallel branches. That will give me 80v at 27a panels. - of course, in reality, the output or useful power will be less.

 

[Hedges]

I noted in this thread that "Input conductors and circuit breakers breakers must be rated at 1.56 times the short circuit current of the PV array (per NEC)".

When I'm sizing PV cable I usually ensure that the cable is rated for the maximum possible current (let's say Isc = 10A) using tables such as this one, which implies that a minimum of 16AWG (max current loading = 15A) would be suitable.

If this statement is correct then 1.56 x 10A = 15.6A, which means that, even though 16AWG is rated for the max possible load current (Isc), are the NEC guidelines suggesting that the minimum should be 14AWG (max current loading = 24A)?

Do you have an NEC table that goes down to 16 awg? Mine stops at 14 awg. And although 14 awg 90 degree C may be good for 35A (single conductor in free air), we are required by NEC to use 15A OCP.

Inside appliances, smaller gauges are used.

 

[wade0000]

Since my run was less than 25 feet, I just oversized and used 10 AWG for my combined wiring. Isc=9.2 So 9.2A x 125% x 125% x 2 banks in parallel = 28.75 A

I believe 10 AWG can handle like 40 amps under normal conditions. I believe the bigger the wire the lower the loss/resistance. just my experience. Your run might be much longer and therefore the cost increase much higher.

Regardless, good luck!

 

[FilterGuy]

Do you have an NEC table that goes down to 16 awg?

This is the NEC 2017 table

 

[Hedges]

Hmpf!

If I use 14 awg I have to fuse it at 15A.
But if I save money with 16 awg I can fuse it at 18A?

I found a table with asterisk for 16 and 18 awg

Conductor Ampacity Based on the 2017 National Electrical Code - North America Standards Reference

www.productinfo.schneider-electric.com

Haven't found official footnote, but this one says 10A and 7A

https://www.electricalassociation.com/Images/images/MEA/PDFs/Powerpoints/OvercurrentPart1.pdf

"15.6A, which means that, even though 16AWG is rated for the max possible load current (Isc), are the NEC guidelines suggesting that the minimum should be 14AWG"

Close to the 15A OCP requirement, but I would use 12 awg. I have lots of it for AC wiring.

 

[FilterGuy]

Haven't found official footnote, but this one says 10A and 7A

The footnote says to **Refer to 240.4(D) for conductor overcurrent protection limitations.

I looked up 240.4D but decided not to chase the links any further.... but here is 2017 240.4

240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
(A) Power Loss Hazard. Conductor overload protection shall not be required where the interruption of the circuit would create a hazard, such as in a material-handling magnet circuit or fire pump circuit. Short-circuit protection shall be provided.
FPN: See NFPA 20-2007, Standard for the Installation of Stationary Pumps for Fire Protection.
(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cordand-plug-connected portable loads.​

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).​

(3) The next higher standard rating selected does not exceed 800 amperes.​

(C) Devices Rated over 800 Amperes. Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent
(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
(1) 18 AWG Copper. 7 amperes, provided all the following conditions are met:

(1) Continuous loads do not exceed 5.6 amperes.​

(2) Overcurrent protection is provided by one of the following:​

a. Branch-circuit-rated circuit breakers listed and​

marked for use with 18 AWG copper wire​

b. Branch-circuit-rated fuses listed and marked for use​

with 18 AWG copper wire​

c. Class CC, Class J, or Class T fuses​

(2) 16 AWG Copper. 10 amperes, provided all the following
conditions are met:

(1) Continuous loads do not exceed 8 amperes.​

(2) Overcurrent protection is provided by one of the following:​

a. Branch-circuit-rated circuit breakers listed and​

marked for use with 16 AWG copper wire​

b. Branch-circuit-rated fuses listed and marked for use​

with 16 AWG copper wire​

c. Class CC, Class J, or Class T fuses​

(3) 14 AWG Copper. 15 amperes
(4) 12 AWG Aluminum and Copper-Clad Aluminum. 15 amperes
(5) 12 AWG Copper. 20 amperes
(6) 10 AWG Aluminum and Copper-Clad Aluminum. 25 amperes
(7) 10 AWG Copper. 30 amperes
(E) Tap Conductors. Tap conductors shall be permitted to be protected against overcurrent in accordance with the following:

(1) 210.19(A)(3) and (A)(4), Household Ranges and Cooking Appliances and Other Loads​

(2) 240.5(B)(2), Fixture Wire​

(3) 240.21, Location in Circuit​

(4) 368.17(B), Reduction in Ampacity Size of Busway​

(5) 368.17(C), Feeder or Branch Circuits (busway taps)​

(6) 430.53(D), Single Motor Taps​

(F) Transformer Secondary Conductors. Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a
single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the device defined in 240.6.
(G) Overcurrent Protection for Specific Conductor Applications.
Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G)

 

[wade0000]

Yep, I oversized by using AWG 10 USE-2 which is rated for 40A as I thought