Current ratings for short length of 10AWG PV wire?

[fafrd]

Problem:

I’ve got a Soladeck through-roof junction box with cable glands that accept wire OD of 7.4mm (0.2913”) maximum.

I’m trying to feed a new 3P1S parallel string into that junction box which means I’ll need a short length of 10AWG PV wire running out of the junction box into a Staubli MC4 connector rated for 60A up to 50C.

10AWG PV wire is rated for 55A up to 90C if freestanding in air (not in conduit, single + and - short 10AWG wires will be run from junction to 6AWG within junction box out through cable glands into air where they will connect to Staubli Y connectors rated for 50A @ 85C).

If I use the Iscmax x 125% x 125% x 3 panels in parallel calculation, wire needs to be sized to handle at least 41.17A and I believe I should be OK (as long as I am using the correct Saubli MC4 connector), but I’d appreciate a reality check on anything I may be overlooking tag would translate to this circuit being unsafe.

Here is the datasheet for the Staubli MC4 connectors: https://www.staubli.com/content/dam/spot/PV_SOL-LVDC-en.pdf

Section 4.1 indicates that the PV-KBT4/6X MC4 connector has a cable gland supporting cable OD range of 5.7mm to 7.4mm which is the size I need to fit my 10AWG PV wire with an OD of 0.28” = 7.112mm.

The Current Derating Diagram on page 9 shows that the MC4 connectors connected to 10AWG / 6mm2 PV wire (purple) are rated for ~50A @ ~50C and ~30A @ ~90C while the table at the top of page 10 specifies a rating with 6mm2 cable of 60A @ 50C and 31A @ 90C.

Ambient temps on my roof never exceed 120F / 48.9C, so a temperature derating to 50C should suffice.

Does anyone see a reason that putting a 3P1S string with a worst-case ISC current of 26.3A (41.166A @ 125% x 125%) through one of those Staubli MC4 connectors connected to 1-2 feet of 10AWG PV wire freestanding in air (and shaded by a PV panel) could be a safety hazard I might come to regret?

 

[sunshine_eggo]

Does anyone see a reason that putting a 3P1S string with a worst-case ISC current of 26.3A (41.166A @ 125% x 125%) through one of those Staubli MC4 connectors connected to 1-2 feet of 10AWG PV wire freestanding in air (and shaded by a PV panel) could be a safety hazard I might come to regret?

No.

 

[OzSolar]

26.3A (41.166A @ 125% x 125%) through one of those Staubli MC4 connectors connected to 1-2 feet of 10AWG PV wire freestanding in air (and shaded by a PV panel) could be a safety hazard I might come to regret?

26.3 is STC rating? NOCT will be ~20% lower.

 

[fafrd]

26.3 is STC rating? NOCT will be ~20% lower.

I thought all of the code calculations were based on STC, not NOCT?

But to answer your question, yes, 26.3A is 3 x Iscwc @ 50C (the panels are older 60-cell 245W panels…).

 

[fafrd]

No.

Thanks.

My thinking is that it is the output of the final Y connector and the final 10AWG MC4 connector that are the Mia critical components in the entire string circuit and that all MC4 connectors including the first MC4 Y connector will be fine rated for only 30A - would you agree?

I may end up using 8AWG MC4 extension cords for some or all of those extensions, but that would be merely to reduce I2R resistive losses and voltage drops, not to increase safety…

 

[OzSolar]

I thought all of the code calculations were based on STC, not NOCT?

But to answer your question, yes, 26.3A is 3 x Iscwc @ 50C (the panels are older 60-cell 245W panels…).

They are. Isc x 1.56.

Do you want to the code answer or what would I be comfortable doing on my house answer?

 

[Hedges]

Code doesn't allow over 30A in 10 awg.
Ampacity of 90 degree wire in bundle is 40A. Higher for single wire in free air. (before derating for elevated temperature.)

Bundle factor is allowed to be ignored for up to 2' of nipple. Not addressed is what the wire should look like beyond the nipple. Seems to me "short" is not good, want long wire with better cooling at either end.

Seems to me what is OK for a vehicle might be an issue only for code compliance.

 

[sunshine_eggo]

Thanks.

My thinking is that it is the output of the final Y connector and the final 10AWG MC4 connector that are the Mia critical components in the entire string circuit and that all MC4 connectors including the first MC4 Y connector will be fine rated for only 30A - would you agree?

Yes.

Do you want to the code answer or what would I be comfortable doing on my house answer?

These are always my answers...