0+0+0+0+...=23W? Any way to improve small-load accuracy?
Posted: Fri Jan 03, 2020 1:43 pm
I installed my new GEM system just before the new year, and have been happy with it overall. So nice to be able to monitor individual circuits now! I kept the Split-200, Split-60, Micro-80s and Micro-50s in place from my 6-year-old ECM-1240, and added Micro-40's to all remaining circuits, spread across two panels. I'd originally ordered it with Micro-50's, hoping for the best accuracy, but these were unfortunately out of stock, so I agreed to substitute Micro-40's.
This is the first time I've used the Micro-40. Is this particular CT known to be less accurate with very small loads in the range of 1-2 watts or less? If so, is there any way to improve it, either by adjusting settings (preferred) or changing the way it's hooked up? Since most of the M-40's are on 15A and 20A branch circuits that don't need its full 40A range, I thought about putting in my own burden resistors of double what the GEM has between pins 2-3 of each input, connecting this and the CT in parallel to pins 1-2, then raising the CT Range value by one. Would this help? It'd be a bit of a wiring mess, since the GEM's push-in terminals don't look like they're meant for more than one wire.
Would it be worth considering an upgrade to Micro-50s, once those are available again?
What I'm seeing is a main-breaker measurement for my subpanel that's always anywhere from 13W to 25W higher than the total of all 17 individual breakers on that panel. Most breakers are reading 0W at any given time (Watt-seconds counters not incrementing at all), but turning these off one-by-one does reduce the residual load measured at the main by about 1W per breaker.
Most of these zero-reading breakers do have small phantom loads on them, such as GFCI receptacles, remote-controlled receptacles, appliances without mechanical switches, unloaded chargers, surge-protector strips with indicator lights, etc. I guess it's less than 1W on most circuits, but some must be slightly more, because there aren't enough breakers to fully account for the residual load sometimes hitting 23-25W.
Is the GEM capable of tracking fractional watt-seconds at all (using any variety of CT), such that, say, a 1/2 watt load would increment the W-s counter every other second? One of Teken's old posts mentioned a best-case precision of 0.27W, but I don't know how he figured that.
My GEM is mounted directly above the subpanel in question, with CT wires trimmed to less than 2' each. The subpanel main breaker has a pair of Micro-50's on it (leftover from ECM-1240, and formerly the only CTs in this panel), with Micro-40's on all individual breakers. This is my 120V-only, protected-loads panel that's kept alive from a hybrid solar+battery-based inverter during blackouts, and it has no load circuits greater than 20A. Both red & black "hot" legs coming into it come from the same inverter bus-bar.
In the main panel, a few feet away (12 mixed CT sets there are spliced into three Cat5 cables, about 10' each, run through attic & wall cavity to reach the GEM) I have a Split-60 on the single-pole 60A breaker feeding the inverter panel's AC-In, and indirectly the subpanel when it's operating from normal grid power. When no power is being supplied or drawn by the inverter panel, the Split-60 (feed to subpanel) and paralleled pair of Micro-50's (subpanel main) usually agree with one another to within a couple of watts. So, I'm inclined to trust those mains readings, and suspect individual circuits are reading a bit low rather than the main breakers a bit high. The GEM's PT and power supplies are plugged into a circuit from the subpanel (so that they'll keep running in "off grid" mode), which usually has no other active loads on it.
My inverter is a true-sinewave type (Outback GVFX-3524, 3.5kW) that's certified for feeding power back to the grid as well as operating standalone, so I don't think it's a factor here. Also, there's little or no difference in the residual (Main minus sum-of-loads) subpanel reading between when the inverter is active during sunny hours, vs. dormant at night and acting as only a pass-through (its transfer relay connecting AC-In straight through to AC-Out).
All inputs of the GEM are of equal accuracy and precision, right? It's not like the ECM-1240 where Ch1/Ch2 do a little better than Aux1-Aux5? I wired main panel loads starting to inputs 1, 2, 3... and subpanel loads counting down from from 32 (sub main), 31, 30, 29... so that leftover positions in the middle could be used to expand either panel while keeping contiguous groups.
Interestingly, the Main-minus-sum-of-circuits figure on my main panel is usually in the -5W to 5W range. It has mainly 240V loads, though, most measured by M-80s, with the few 120V circuits there (dishwasher, washing machine, outside lighting, charger for 12V battery system) on the opposite hot leg from the one going to my inverter and subpanel, for the sake of balance.
This is the first time I've used the Micro-40. Is this particular CT known to be less accurate with very small loads in the range of 1-2 watts or less? If so, is there any way to improve it, either by adjusting settings (preferred) or changing the way it's hooked up? Since most of the M-40's are on 15A and 20A branch circuits that don't need its full 40A range, I thought about putting in my own burden resistors of double what the GEM has between pins 2-3 of each input, connecting this and the CT in parallel to pins 1-2, then raising the CT Range value by one. Would this help? It'd be a bit of a wiring mess, since the GEM's push-in terminals don't look like they're meant for more than one wire.
Would it be worth considering an upgrade to Micro-50s, once those are available again?
What I'm seeing is a main-breaker measurement for my subpanel that's always anywhere from 13W to 25W higher than the total of all 17 individual breakers on that panel. Most breakers are reading 0W at any given time (Watt-seconds counters not incrementing at all), but turning these off one-by-one does reduce the residual load measured at the main by about 1W per breaker.
Most of these zero-reading breakers do have small phantom loads on them, such as GFCI receptacles, remote-controlled receptacles, appliances without mechanical switches, unloaded chargers, surge-protector strips with indicator lights, etc. I guess it's less than 1W on most circuits, but some must be slightly more, because there aren't enough breakers to fully account for the residual load sometimes hitting 23-25W.
Is the GEM capable of tracking fractional watt-seconds at all (using any variety of CT), such that, say, a 1/2 watt load would increment the W-s counter every other second? One of Teken's old posts mentioned a best-case precision of 0.27W, but I don't know how he figured that.
My GEM is mounted directly above the subpanel in question, with CT wires trimmed to less than 2' each. The subpanel main breaker has a pair of Micro-50's on it (leftover from ECM-1240, and formerly the only CTs in this panel), with Micro-40's on all individual breakers. This is my 120V-only, protected-loads panel that's kept alive from a hybrid solar+battery-based inverter during blackouts, and it has no load circuits greater than 20A. Both red & black "hot" legs coming into it come from the same inverter bus-bar.
In the main panel, a few feet away (12 mixed CT sets there are spliced into three Cat5 cables, about 10' each, run through attic & wall cavity to reach the GEM) I have a Split-60 on the single-pole 60A breaker feeding the inverter panel's AC-In, and indirectly the subpanel when it's operating from normal grid power. When no power is being supplied or drawn by the inverter panel, the Split-60 (feed to subpanel) and paralleled pair of Micro-50's (subpanel main) usually agree with one another to within a couple of watts. So, I'm inclined to trust those mains readings, and suspect individual circuits are reading a bit low rather than the main breakers a bit high. The GEM's PT and power supplies are plugged into a circuit from the subpanel (so that they'll keep running in "off grid" mode), which usually has no other active loads on it.
My inverter is a true-sinewave type (Outback GVFX-3524, 3.5kW) that's certified for feeding power back to the grid as well as operating standalone, so I don't think it's a factor here. Also, there's little or no difference in the residual (Main minus sum-of-loads) subpanel reading between when the inverter is active during sunny hours, vs. dormant at night and acting as only a pass-through (its transfer relay connecting AC-In straight through to AC-Out).
All inputs of the GEM are of equal accuracy and precision, right? It's not like the ECM-1240 where Ch1/Ch2 do a little better than Aux1-Aux5? I wired main panel loads starting to inputs 1, 2, 3... and subpanel loads counting down from from 32 (sub main), 31, 30, 29... so that leftover positions in the middle could be used to expand either panel while keeping contiguous groups.
Interestingly, the Main-minus-sum-of-circuits figure on my main panel is usually in the -5W to 5W range. It has mainly 240V loads, though, most measured by M-80s, with the few 120V circuits there (dishwasher, washing machine, outside lighting, charger for 12V battery system) on the opposite hot leg from the one going to my inverter and subpanel, for the sake of balance.