HPWC installed on partly used 100amp subpanel. Measuring amps?

[gordo]

HPWC installed on lightly used 100amp subpanel. How to measure total current draw?

So my model S arrives end of this week (yay) and I got an electrician out to install my HPWC the other day on an existing 100A subpanel in my garage... Based on my rough calculations, i figured that the subpanel was highly underutilized, and I'd easily be able to draw 40A to charge the car at night... However I have dual chargers and was hoping to make partial use out of them by setting up the charger DIP switches to 64A. I actually had the electrician run 100A wire and 100A breaker as well (in case of future upgrade), so technically the hardware is there to support up to 80A charging.

As I understand from a previous thread, the maximum intermittent load I can draw for household items (intermittent) while the car chargers draw 64A would be 100A - (64A continuous, which is considered 80A intermittent) = 20A intermittent load.

So, I got myself a clamp-on ammeter attachment (Fluke i400) which reads up to 400A AC and clamped it around the leads coming into the subpanel (the thick red, black and white wires toward the top of the image). The subpanel looks like this:

From reading, I believe this is called split phase wiring, but not entirely sure. In any case, when I take current readings from each wire, each of them reads a different number of amps (thankfully quite small by the looks of it when measured late at night). The question I have is, should I simply add them all up to determine the total current being used by the panel or just the red and the black?

Note: The thick black and red wires coming out of the 100A breaker set near the bottom of the image go to the HPWC.

 

[sefs]

No, do not add up the two current measurements. You are correct, this is split phase. Split phase works by taking two ends of the transformer secondary, with a grounded center tapped neutral conductor. Voltage from each end of the transformer secondary, or each line/leg, will give you 120VAC nominally. When you see a difference of load between lines A and B, this is caused by 120VAC loads, or single pole breakers. Say you measure 50A on line A and 55A on line B. When you measure the neutral conductor, you will measure 5A (if you don't have some bonding issue, but that's another discussion). If you were only charging you car, you would measure 64A on line A and 64 A on line B, and no current on the neutral. Each line conductor can only conduct, in your case, 100A. It doesn't matter if you have 0A on one line, if you have 101A on the other, the circuit breaker will trip (circuit breakers don't trip like this in actuality, but this is for illustration purposes) opening both lines via the handle tie that ties them together making it a two-pole breaker. If this imbalance is large, you can rearrange the single pole circuit breakers to balance the loads.

 

[Cottonwood]

I agree with sefs, but will say it a little differently.

Black is L1, Red is L2, and White is Neutral.

L1 - L2 = Neutral

Because sign does not matter Neutral can be + or -. 240 V loads put current on L1 and L2. 120 Volt loads put current on L1 or L2.

The 100 Amp limit is the max of L1 or L2. Therefore, balancing the 120V loads on L1 and L2 will give you more panel capacity.

 

[gordo]

Ah, that makes sense. Thanks! So, just to confirm my understanding, that means I would not want to charge the car at full rate (64A) if either the red or black line reads above 20A (at least to stay within spec)...

 

[FlasherZ]

You're taking the fairly literal approach to determining a load calculation, but that's ok, and yes, you're correct - if either of the black or red current readings are above 20 for extended periods of time, then you will have capacity issues.

 

[gordo]

Cool. When i get home later im going to try to max out the panel (except for the hpwc) and see where we stand. Last night with the lights off i measured less than an amp on both black and red, so assuming i charge only late at night, i will have no issues... Only concerns would be a washer, dryer (gas) that also share the panel.

 

[tga]

Just to be sure - you are only putting the meter on one wire at a time, correct? Running multiple wires through the clamp will give you incorrect readings.

 

[gordo]

Yes thats my understanding.

 

[sefs]

A bit off topic here, but does anyone with a HPWC know if the pilot signal will change during charging if the dip switches are changed? If it does, it wouldn't be too hard to realize a design that consists of two split core CT's around the feeders to measure feeder current and set it up to reduce charging current when feeder ampacity approaches its limit, and then when the current drops, charging current could be increased again. Could do it all with relays or an Arduino.

 

[FlasherZ]

A bit off topic here, but does anyone with a HPWC know if the pilot signal will change during charging if the dip switches are changed? If it does, it wouldn't be too hard to realize a design that consists of two split core CT's around the feeders to measure feeder current and set it up to reduce charging current when feeder ampacity approaches its limit, and then when the current drops, charging current could be increased again. Could do it all with relays or an Arduino.

No, you have to restart the HPWC by power-off/power-on after changing the DIP switches.

- - - Updated - - -

Cool. When i get home later im going to try to max out the panel (except for the hpwc) and see where we stand. Last night with the lights off i measured less than an amp on both black and red, so assuming i charge only late at night, i will have no issues... Only concerns would be a washer, dryer (gas) that also share the panel.

Just a reminder that you don't have to plan for maximum possible power load, only maximum likely power load.

 

[gordo]

Just a reminder that you don't have to plan for maximum possible power load, only maximum likely power load.

So I just maxed out the house (without going too crazy like plugging in hair dryers or anything...) and here's what i came up with:
Black wire: ~32A
Red wire: ~30A

When I turn most of the lights off and leave everything else running (floor heaters, dryer, fridge):
Black wire: ~12A
Red wire: ~8A

When I wind it all down except for a few lights (most realistic load at midnight):
Black wire: ~1A
Red wire: ~1A

So it seems that I should be just fine to charge overnight, but may need to be somewhat mindful of energy usage during the (likely very rare) day charging situation.

 

[NigelM]

So it seems that I should be just fine to charge overnight, but may need to be somewhat mindful of energy usage during the (likely very rare) day charging situation.

Doesn't sound like you have air conditioning? That may come on automatically if it's thermostat controlled and there's a spike in usage when systems start up.

Tip: clearly label the HPWC so that no-one accidentally plugs in in the middle of the day when you're not around. You'd only trip the breakers if there was a problem but it's good to have a reminder in place.

 

[ChrisPDX]

I noticed something surprising about the numbers provided. Assuming you only switched off the lights, you had about a 5kW drop on that alone. That's a lot of power for lights! Do you have 50 100W incandescent bulbs in the house? Maybe something else switched off before you measured it.

 

[gordo]

Doesn't sound like you have air conditioning? That may come on automatically if it's thermostat controlled and there's a spike in usage when systems start up.

The A/C doesn't run off the garage subpanel. It's on the main panel, so wouldn't affect the HPWC.

- - - Updated - - -

I noticed something surprising about the numbers provided. Assuming you only switched off the lights, you had about a 5kW drop on that alone. That's a lot of power for lights! Do you have 50 100W incandescent bulbs in the house? Maybe something else switched off before you measured it.

Good catch. The house in total actually has 192 50watt MR16 halogens on a Lutron Homeworks system. This was put in by the previous owner. I have converted some to LEDs, but most are still halogens. I've programmed the system to run those lights at 45% by default, which provides more than enough light for day-to-day evening use, but it's still quite inefficient and I'm considering expanding the LEDs as cost permits.

Fortunately, as far as the HPWC goes, most lights will be off when it starts charging.