PV combiner box protection [RE-wrenches]

Phil Undercuffler phil at dankoffsolar.com
Wed Jun 23 11:23:35 PDT 2004


 

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on 6/23/04 10:49 AM, Ray Walters at ray at solarray.com wrote:


> Then, I questioned whether PV was being made to jump through more hoops
> than standard house wiring. As you all know, standard house wiring allows a
> 16 Ga lamp cord to be plugged into a circuit protected only by a 20 amp
> breaker. This poses a very real danger of overload at a much more dangerous
> voltage, yet IS allowed by code in millions of homes including yours. I
> always have taken the NEC relative to solar design with a healthy dose of
> salt.
> Think about it : a 15 amp rated device(solar module) isn't allowed a
> possible 19.5 amps at 24 volt DC, but a 10 amp rated device (16Ga lamp
> cord) is allowed 20 amps at 120 VAC? One is outside on a pole, the other is
> behind your bed.....Hmmmm

Ray,

I understand your reasoning on this, and can agree  with you to a point.
However, that 16 ga lamp cord is attached to a lamp that is Listed to draw a
certain amount of power, and no more.  A 900 watt toaster is only going to
draw 900 watts, and the cable that comes attached to it is designed to carry
that power for as long as the toaster can operate.

If the device does fail, and if it does fail in a short, a LOT more than 20
amps flows through that cable, and the related house wiring too.  That's
intentional, as it's what it takes to trip the circuit breaker.  But that
near infinite current flow only happens for a few short milliseconds until
the breaker interrupts the flow, clearing the fault.

Kurt,

I just received a photo of a j-box where 6 modules were daisy chained, and
the last module in the row smoked its terminal strip.  By my calculations,
it was subjected to 27 amps of current flow at 12v.  I have come to believe
that the combination of low voltage, high current and thermal cycling  make
module interconnections a weak point in our installations.  While series
fusing is intended to protect from a different fault, every day it helps
alleviate this combination by reducing the amperage carried through the
junction box, minimizes the portion of the array affected in case of
failure, and greatly simplifies troubleshooting.  I think it's cheap
insurance at twice the price.

BTW, how many of you use anti-oxidant on your array connections?

If series fusing makes too much "spaghetti" in 12 volt systems, I think
that's Nature's way of saying move to 24 or 48 volt.
 
Phil Undercuffler ext.238
Technical Support
phil at dankoffsolar.com

Dankoff Solar Products
    
The Premier Manufacturer and Distributor of Solar Water Pumps, and
Wholesale Distributor of Solar Electric, Wind, and Solar Heating Equipment

888-396-6611 toll free
505-473-3800 phone
505-473-3830 fax
www.dankoffsolar.com

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