Breaker Size

aci2468

OK, question.  I have 2  250 watt  24vdc  panels in parallel,  # 6 wire,  30 ft  PCM60X, what would be my breaker size?    ( midnight solar)  20 amp?

Vic

Hi aci..,

OK,  so you are now thinking of wiring those two PVs in PARALLEL.   This will double the current output of the PVs,  into the Charge Controller (CC).

SO,  you should multiply the Isc rating of a single PV by 2.  and then multiply this by 1.25,  and round to the next higher standard Midnite breaker size.

This breaker on the input of the CC is handy,  as a disconnecting switch.

The DC circuit breaker that you MUST have,  is  the one on the output of the CC,  that connects to the battery (this might be the breaker size that you are asking about in this Thread).   This breaker will need to be sized to protect the cable that connects between the CC and the battery.   The size of this breaker will depend upon any codes that you need to follow,  the temperature rating of the cable,  the number of cables that are run together,  plus a few other variables.   This output breaker is needed for fire safety reasons.

BTW,  just to nit-pick a bit,  PVs that have 60-cells (PVs with a Vmp of about 29.5 - 31 volts),  are NOT real 24 Volt PVs.   Real 24 v PVs have a nominal Vmp of around 36 volts +/-- a half of one volt,  or so.

What voltage is your battery bank?

Is the above of any help?   Vic

aci2468

My battery banks is  4  12vdc 210 amps each (deep cycle),  2 in series  then doubled to 24 vdc

aci2468

I have 4  250 watt helios panels to go to a 60 amp mmpt , battery bank is 24vdc.   isc  8.67      vmpp  30.8     mmp8.12  voc   37.4

Vic

Hi aci,

Thanks for the added information.

Your 24 volt battery  generally will not be fully-charged by PVs with a 30-ish Vmp PV array,  when those two PVs are wired in PARALLEL.  And,  if the PVs are wired is series,  then you would want a real MPPT CC.

There are number of inexpensive CCs from parts of Asia that claim to be MPPT CCs,  but are NOT,  so be careful when shopping.

If you do not own the PVs,  yet,  you may be able to find some real 24 V PVs  --  those with 72 cells.  Then wiring two of these PVs in PARALLEL  with a PWM CC  should work well.

FWIW,   Thanks,   Vic

aci2468

Thanks for the info. and your time.

vtmaps

Vic said:

if the PVs are wired is series,  then you would want a real MPPT CC.

There are number of inexpensive CCs from parts of Asia that claim to be MPPT CCs,  but are NOT,  so be careful when shopping.

I have never seen one, but the PCM60X claims to be an MPPT controller, and it's not all that cheap (but less expensive than Midnite or Outback).  http://www.mppsolar.com/v3/pcm60x/

As Vic points out, putting those panels in parallel will not give satisfactory charging, so put them in series.  Regarding the breaker size, are you asking about the breaker between the panels and the controller, or between the controller and the battery, or both?

--vtMaps

aci2468

i am going to put individual breakers on the PV panels and a single  larger breaker after the pv breakers and the CC  and a breaker also between the batteries and the CC

aci2468

Sorry  rewrite that,   I am going to put individual breakers on the PV panels and a single  larger breaker after the pv breakers and before the CC  and a breaker also between the batteries and the CC

Vic

OK,  thanks for the info aci..,

Technically,  with a 24 volt battery,  and two PVs with about a 30.6 Vmp,  you will want to wire the two PVs in SERIES.  so,  there is no need for any breakers at the PV modules.  A breaker at the input of the CC  is a very good idea,  but not absolutely required.

With two PVs in series,  the current in that circuit,  and into the input of the CC is that of one PV  --  with two PVs in series,  the current remains the same as that of one PV,  but the voltage doubles.  Therefore,  we are back to using a breaker rated at 1.25 times the Isc of one of the identical PV modules,  rounded up to the next standard breaker size.  For many 250-ish watt PVs,  this is 15 amps maximum.  Just use a MidNite MNEPV,  or MNEDC breaker and you should be fine.

The breaker at the output of the CC is required,  and it is also nice that this breaker acts as a disconnect (switch).  This breaker needs to be sized to match the current rating of the cable that you are using to connect the CC to the battery.

FWIW,   Vic