# Voltage calcs clarification?

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Registered Users Posts: 17

Here's the system: 36 Sharp 208s (3 in series, 12 in parallel making up three sub arrays), a preassembled power center with 3 MX60s and 2 FX3648s, and 8 Surrette 530s in series. The specs for the modules are: Voc 36.3, Vpm 28.71, Isc 7.99A, Ipm 7.25A. Each string yields 108.9 Voc and 7.99A, each sub array at the combiner box would be 108.9Voc, and 32A. There are two PSPV combiner boxes, one is 60' and the other 40' from the dc disconnect / power center.

When determining the voltage drop between the PSPV and the power center things seem a bit odd: 32x125%x125%=50A at 108.9V, yet all the tables are for 12, 24, or 48V... I've got to be missing something?

Thanks

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: Voltage calcs clarification?

Marks,

Several things:

1) “32x125%x125%=50A at 108.9V” is a non-starter. Voc is “Volts open circuit”, so there’s no current. For calculating the voltage drop between the PSPV and the power center, the numbers to use are 28.71 Vmp x 3 = 86.13 V, and 7.25 A Imp x 4 = 29 A.
2) Use Ohm’s Law (V=IR) and calculate for 86 Vmp, or even just 75 Vmp (summer Vmp).
3) Use Voc (corrected for expected low ambient temperature) for specifying circuit breaker DC voltage ratings.
4) Use Isc for calculating max current ratings for breakers and wiring.

HTH,
Jim / crewzer
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: Voltage calcs clarification?

jim,
he is becoming confused between nec sizing calculators and voltage drop calculators and because you said in 1) to use the imp x 4 and below that in 4) you state to use the isc. i explained that voltage drops are for normal operations while isc is considered for the actual sizing requirements and NOT voltage drop calculations. if it's shorted then who cares at that point if the wire is dropping more voltage? we are in touch with each other and i'm trying to help him out with verifying the voltage drops he'd see in his system.
also note i am recommending in the voltage drop calculator a change to the default temperature for calculation purposes. the default is 50 degrees c and nec recommendations for this use are minimally 60 degrees c. i'm basing this on whiles' article on the matter. http://www.sandia.gov/pv/docs/PDF/CodeCorner83.pdf
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: Voltage calcs clarification?
"he is becoming confused ... because you said in 1) to use the imp x 4 and below that in 4) you state to use the isc."

niel,

What I said in (1) was to use Imp for figuring voltage drop, and in (4) was to use Isc for calculating max current ratings for breakers and wiring. Stated in their entirety, these are different issues and should not be a source of confusion. I hope you're able to help him out.

The voltage drop calculator change sounds good.

Regards,
Jim / crewzer