Odd amperage drop across breakers in combiner box
Raj174
Solar Expert Posts: 795 ✭✭✭✭
I know, sounds ridiculous right? I have a 3600 watt array, 12 300 watt panels, 6 strings of 2 panels each. I have had consistent output of 2100 to 2200 watts with clear skies, occasionally brief periods of 2400 to 2500 watts. Also, I have seen outputs upwards of 3000 watts with cloud edge effects. I have always thought this was low for this size array.
Today I was checking the amperage on the 6 strings with a clamp meter, and all 6 were between 6 and 7 amps, average 6.5.
So that adds up to about 39 amps. However, on the # 4 AWG cable terminated on the plate that combines the 6 breakers, I have 33 amps. This makes no sense to me. Next, I switched all the breakers off, except #1 and checked for amps on a single string. I got 6.4 on the string and 5.4 on the #4 CC input cable. Did this with all the other strings with similar results. 39 amps on the PV strings and 33 amps on the #4 cable. I am perplexed by the 1 amp drop across the Midnite 15 amp breakers. Seems to me that, if something were dragging the amperage down, it would happen on both sides of the breakers. The voltage at the time was consistent on all cables at 61 volts. The combiner box is a Midnite MNPV12.
Anyone have any ideas?
Rick
Today I was checking the amperage on the 6 strings with a clamp meter, and all 6 were between 6 and 7 amps, average 6.5.
So that adds up to about 39 amps. However, on the # 4 AWG cable terminated on the plate that combines the 6 breakers, I have 33 amps. This makes no sense to me. Next, I switched all the breakers off, except #1 and checked for amps on a single string. I got 6.4 on the string and 5.4 on the #4 CC input cable. Did this with all the other strings with similar results. 39 amps on the PV strings and 33 amps on the #4 cable. I am perplexed by the 1 amp drop across the Midnite 15 amp breakers. Seems to me that, if something were dragging the amperage down, it would happen on both sides of the breakers. The voltage at the time was consistent on all cables at 61 volts. The combiner box is a Midnite MNPV12.
Anyone have any ideas?
Rick
4480W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH nominal LiFePO4, Kohler Pro 5.2E genset.
Comments
And then there are Real World Watts.
Are the panels hot ?
I have heard of Voltage Drop, but not "Amperage Drop".
It sounds like you cannot accurately measure the DC Amps with your meter.
Is there any metal near the #4 cable?
It appears that you are reading only ~90% of actual on the #4 cable.
Amps IN must equal Amps OUT.
You have a measuring problem.
They are 300 watt STC of course, and obviously less in the real world. I have a good DC clamp meter and I trust it.
I do agree with you, amps in must equal amps out. This evening I will temporarily put all the positive wires on an insulated buss bar for testing.
Rick
I don't know where it could be going except for a short circuit on that one string. If you turn off all the breakers in the combiner box, are the open circuit voltages all the same on all six strings, or nearly so?
He says, he measured each string, one at time and each string is ... 6.4 amps "in" but and only 5.4 amps "out".
Shouldn't he have seen 0 Amps in #4 cable for the shorted string?
That did not happen.
This sounds like a measuring error.
Second, unless I'm reading it wrong, the #4 cable is on the combined side, going to the charge controller. Fact is, I can't see it making sense any other way. So I don't think there is a "#4 cable for the shorted string", but rather one #4 cable for all the strings.
For both current and voltage, it all has to add up to the same total. For voltage, if you see a lower voltage on one end, there has been some voltage loss along the way. Likewise, all the current has to go somewhere, so if you have less at one end than at the other, there is another path that it has taken.
In the end it could be a measurement error, but I can't see how that error would repeat consistently across all the strings.
Quite a mystery.
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Rick
Either they are all the same, or some or one is different, and different would be bad,
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Just a couple thoughts;
First, I DO question the accuracy/repeatibility of many/most Clamp DC Ammeters.
Second, assume that the Classic was in Bulk - MPPT, but still, the Solar conditions could have changed, causing changes in the amount of PV power available.
If the CC was in Absorb, and you were feeling that a fairly rapid series of measurements would essentially equate to constant charge current conditions, this could be a factor.
However, I have NO experience with LiFePO4 batteries, so some of the above may just be NOISE,
FWIW, Vic
IF there was a "Shorted String" ( as someone else had suggested ) then
the amps measured in the #4 cable would have been "0 Amps"
when the breaker for only that "shorted string" was ON.
But that did not happen, therefore there is no "shorted string".
Amp in must equal amps out,
so how can a bad connection change the amps flowing through the circuit?
Thanks for your comments,
Rick