Wire size from combiner box to E-panel

[hammick]

I plan to use the Midnight Solar E-panel which will be 15' or less from my combiner box. Panel wattage is 1710 (six 285 panels). I think I can go to nine panels before CC is maxed out (60a). System is 48v with Conext SW4048 inverter and XWMPPT60A charge controller.

Assuming I might add three more panels what size wire should I use? My panel installer told me I could use inexpensive 6/3 from home depot but I'm pretty sure he wasn't factoring in the future addition of three panels.

Thanks

[Naptown]

Most AHJ's will require DC to be in metallic raceway and marked as such every 10 feet.
Check with them.

[hammick]

Most AHJ's will require DC to be in metallic raceway and marked as such every 10 feet.
Check with them.

I won't have any inspections being off the grid and remote but my panel installer said he would be required to use conduit but an electrician wouldn't. He said as a homeowner I was not required to use conduit.

What is the reason for metallic raceway vs AC subpanel wire that isn't required to be in conduit?

[Naptown]

safety
There is no real way to turn off the DC under load from the panels.
Your electrician is just trying to reduce costs and not IMHO in a safe way.
Inspections or not do you intend to insure the property?
If so I suggest following the NEC to the letter or in the event of a fire you will be SOL

[Mike90250]

To actually answer this question, we need to know the voltage and amps coming from the Combiner Box. Then it's a simple look up for the gauge wire that can handle the amps.

[hammick]

To actually answer this question, we need to know the voltage and amps coming from the Combiner Box. Then it's a simple look up for the gauge wire that can handle the amps.

OK let me see if I can figure this out from the panel specs. Six Solar World 285 panels (two strings of three).

Standard test condition specs:

Maximum power: 285wp
Open circuit voltage: 39.7v
Max power point voltage 31.3v
Short circuit current 9.84a
Max power point current 9.20a
Module efficiency 17%

Series/parallel power (two strings of three):
Volts: 93.9
Amps: 18.4

So using this calculator: http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

It looks like I can add 18 panels total to my CC and with 3% acceptable loss use 6ga wire.

For 1% acceptable loss I need 6ga for six panels; 4ga for nine panels and 1ga for 18 panels.

Have I calculated this correctly?

Thanks.

[sensij]

The 60 A limit on the charge controller is an output limit, not input. I haven't looked too closely at the specs, but usually, the input specs are wide enough to allow the maximum output to be achieved.

I'm guessing you won't hit the 60 A output until you double your array size, as explain in one of your other threads. In addition to voltage drop, you should consider the NEC requirements as well. Combined Isc (NEC PV Output Circuit) for four strings of three (NEC PV Input circuits) would be 9.84 * 4 = 39.36. Wire ampacity should be at least 1.56 * 39.36 = 61.04. 6 AWG (assuming 90 deg insulation) satisfies that. If you want to go by voltage drop, the actual Imp on the PV Output circuit will not be more than 4 * 9.20 = 36.8 A. Voltage drop of 36.8 A though 15 ft (one way) of 6 AWG is about 0.5%, so that works too.

[hammick]

The 60 A limit on the charge controller is an output limit, not input. I haven't looked too closely at the specs, but usually, the input specs are wide enough to allow the maximum output to be achieved.

I'm guessing you won't hit the 60 A output until you double your array size, as explain in one of your other threads. In addition to voltage drop, you should consider the NEC requirements as well. Combined Isc (NEC PV Output Circuit) for four strings of three (NEC PV Input circuits) would be 9.84 * 4 = 39.36. Wire ampacity should be at least 1.56 * 39.36 = 61.04. 6 AWG (assuming 90 deg insulation) satisfies that. If you want to go by voltage drop, the actual Imp on the PV Output circuit will not be more than 4 * 9.20 = 36.8 A. Voltage drop of 36.8 A though 15 ft (one way) of 6 AWG is about 0.5%, so that works too.

Thanks Sensij. 6ga it is. Just received an email from my installer and he also says 6ga. However his amp calculation seems off. See below. Maybe I am not understanding how to calculate the number of panels my CC can take.

"#6 is fine because you will be coming into a 60 amp breaker before your controller. 9 panels @ 48 volts would be 53 amps, 12 panels would be 71 amps. You may loose a little if you go up to twelve but I would say that is the maximum."

Here is what my CC manual says about array size (the way I read it I can have 12 panels and get max power):

"although the solar charge controller can harvest a maximum of 3500 watts, the PV array size can be as high as 6720 watts (based on a 48a x 140Vdc = 6720 watts)"

My calculation is 12 x 285 = 3420 watts.

[Mike90250]

It's my thought that 6ga into a 60A breaker is going to be marginal, safety wise. Depending on the conduit fill, and insulation spec, 55A could be the limit for 6ga.
Check the tables for final decision.

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

[sensij]

It's my thought that 6ga into a 60A breaker is going to be marginal, safety wise. Depending on the conduit fill, and insulation spec, 55A could be the limit for 6ga.
Check the tables for final decision.
https://lugsdirect.com/WireCurrentAm...ble-301-16.htm

When you apply conditions of use to wire ampacity, the PV Output circuit rating is 1.25 * Isc [2011 NEC 690.8(B)(2)], the extra 1.25 does not need to be applied. However, you bring up a good point. The OCPD also needs to be sized at 1.56 * Isc, and the OCPD needs to protect the conductor. Since 1.56 * Isc = 61 A, a four string system would require something more than a 60 A breaker (at least 65 A). With a 65 A breaker, 6 AWG might be ok under some conditions of use, but 4 AWG is safer, especially if there are any unmarked terminations that need to be rated at the 60 deg value.

The installer's calculations do not look correct for the PV Output circuit, it looks like he is trying to calculate the ratings for the circuit out of the charge controller into the battery. Since the CC is limited to 60 A output, the the CC output should be protected by a 75 A breaker (60*1.25), with wire ampacity to match.

[Naptown]

When you apply conditions of use to wire ampacity, the PV Output circuit rating is 1.25 * Isc [2011 NEC 690.8(B)(2)], the extra 1.25 does not need to be applied.

The correct way is to take the ISC of the combined strings and multiply by 1.56
If you are talking about the output from the inverter or from battery to incverter then 125%

[sensij]

The correct way is to take the ISC of the combined strings and multiply by 1.56
If you are talking about the output from the inverter or from battery to incverter then 125%

By 2011 NEC, when looking at ampacity,

690.8(B)(2)(a) requires the 1.56 factor to be applied, and conditions of use can be ignored.
690.8(B)(2)(b) requires only a 1.25 factor, and conditions of use must be considered.

Whichever requires the larger conductor must be used.

FWIW, 690.8(B)(2)(c) says that no matter what the circuit rating is, the conductor must be protected by the OCPD when conditions of use are considered.

When looking at OCPD, the 1.56 factor must be used [690.8(B)(1)(a)].

[hammick]

The midnight solar E-panel wiring diagram shows an 80a breaker after the PV+ input of the charge controller and a 100a breaker on the Battery + output of the charge controller. Of course they were using a MS150 CC.

Can I use 4ga wire and 80a and 100a breaker?

Hoping to order this stuff this afternoon.

Thank you very much for the help.

[sensij]

The midnight solar E-panel wiring diagram shows an 80a breaker after the PV+ input of the charge controller and a 100a breaker on the Battery + output of the charge controller. Of course they were using a MS150 CC.

Can I use 4ga wire and 80a and 100a breaker?

Hoping to order this stuff this afternoon.

Thank you very much for the help.

No, 4 AWG is definitely too small to be protected by 100 A OCPD, and might be too small for an 80 A.

If you'd like more coherent help from this community, please sketch up a drawing of the entire system. Piecemeal information results in inconsistent advice. Please consider hiring an electrical engineer to do the design, given the risks involved for a system of this size.

[hammick]

No, 4 AWG is definitely too small to be protected by 100 A OCPD, and might be too small for an 80 A.

If you'd like more coherent help from this community, please sketch up a drawing of the entire system. Piecemeal information results in inconsistent advice. Please consider hiring an electrical engineer to do the design, given the risks involved for a system of this size.

Sorry the 100a breaker was for the MS Solar CC which is 96a output. My CC will be 60a output.