Max # of PV Wires in Tray

[Matt Donovan]

690 permits PV wire in tray, but 392 offers no guidance on how many #10 wires can fit in a tray (unless you follow "sum of diameters may not exceed tray width", which is not practical.) So is there a best practice for how many #10 PV wires should be permitted in a 2", 4", 6" tray?

 

[Carultch]

690 permits PV wire in tray, but 392 offers no guidance on how many #10 wires can fit in a tray (unless you follow "sum of diameters may not exceed tray width", which is not practical.) So is there a best practice for how many #10 PV wires should be permitted in a 2", 4", 6" tray?

For signal and control wiring, the NEC allows you to fill cable trays to 50% full (in terms of cross sectional area). This is the most ambitious cable tray sizing allowance, of any cable type which could be used in a tray. Even though circles theoretically can be packed up to almost 90% by area, in practice that is not going to happen with wiring. A tray is going to "look full" when filled to 50% by cross sectional area, and you are probably going to want to size to max 40% to be conservative. Structurally, as long as you can fit the wires in the tray, you can use a tray of that size, when using the allowance for listed PV wire in trays.

Ampacity-wise, it is uncharted territory, and that is a problem, if you have a serious situation where you need to justify your installation. The NEC offers no calculation on how to determine ampacity when taking credit for the PV wire rule. Does one get to take credit for airflow, given the fact that it is an open wiring method instead of a closed raceway? Not specified. Does the bundling derate apply, just as it would if it were a closed raceway? Again, not specified. In wireways and troughs, you can ignore bundling derates up to the 30'th wire, but once you add the 31st wire, it suddenly puts your derate factor to the value for 31-40 wires. I would expect the same thinking would apply to cable trays of PV wire as well, and if anything, the fact that the wiring method doesn't have closed walls would improve the ampacity compared to the same situation in a wireway.