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# Watt meter..?

#1
12-15-11, 11:59 AM
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Watt meter..?

Another question for any of with some ideas :

I want to install a Reliance Controls MB75 30 Amp watt meter to my home load center for my generator hook up so I can monitor each side. This is the only part I need to finish this install. They of course are and have been on back order forever. I called again today because it was supposed to ship Dec 16th and now they have it estimated Jan 31st.

From the looks of it this is not a complex design. Does anyone have any idea on a website to get parts and make one myself? I have found some that have the current transformers, but more for small electronics. I can make a box, I just need the two appropriate transformers and the meters them. Any help is as always appreciated. Or am I just hosed and going to wait for the endless date pushing?

(I am not using a transfer switch which has the meters built in, but an interlock kit on the home load center)

#2
12-15-11, 12:22 PM
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Is this what your waiting for?

Mike NJ

#3
12-15-11, 12:35 PM
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As to the complexity of designing a watt-meter , pleased be informed that for AC circuits , simply multiplying the line current ( amperes ) X the line-voltage results not in a power measurement of N Watts but a measurment of Volt-Amperes.

Ex: 10 amps X 220 volts = 22OO Volt-Amperes. This would be 2200 Watts if the connected loads were comprised of "pure" resistance such as heating elements, but if the loads included magnetic coils such as motor-windings the "true" load in Watts would be less than 2200.

If your need is to determine the current value of the generator loads so as not to exceed the current-rating of the generator and to "balance" the 120 volts load-currents , consider a hand-held ammeter that "clamps around" the conductor to measure the amperes in that conductor.

#4
12-15-11, 01:16 PM
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Originally Posted by lawrosa
Is this what your waiting for?

Mike NJ
Mike,

Yes that is exactly what I ordered and am looking for. However they have been on back order since early Oct when I ordered mine. Still no idea when they are getting them. First it said would ship Dec 16th, now the website says Jan 31st (estimated). When I called Reliance Controls direct last month they said they have them and are shipping them. Now when I called generators direct where my order was placed they said they are not getting them and have no idea when.
That is why I was wondering if I could make a simple one as well. I have seen the clamp on ones, but that would require me to open my panel each time to see each leg (for balance purposes). This *seemed* to just be the AC current transformers you slipped over each hot lead going to the breaker, the meters and the bond / ground for the box.

#5
12-15-11, 02:06 PM
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Technically, at PATTBAA pointed out, that is not a wattmeter but simply two ammeters, one in each "hot" leg from the generator.

You could search surplus sites for straight AC ammeters with a range somewhat in excess of the peak output of your generator in amperes to allow for slight overloads and simply wire each meter in series with a hot leg.

OR, you could do the same search but instead look for AC ammeters with a 0-5 ampere range and also look for current transformers with XX/5 ampere designation, substituting the peak output of your generator in amperes for the XX or use a slightly higher rated current transformer. The generator wire would go through the "donut" of the current transformer and the leads from the CT go to the meter.

CAUTION! Do not under any circumstances allow power to flow through the CT without having the secondary conductors either connected to the meter or shorted together. Open secondary conductors on a CT can deliver very high voltages.

#6
12-15-11, 02:42 PM
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Originally Posted by Furd
Technically, at PATTBAA pointed out, that is not a wattmeter but simply two ammeters, one in each "hot" leg from the generator.

You could search surplus sites for straight AC ammeters with a range somewhat in excess of the peak output of your generator in amperes to allow for slight overloads and simply wire each meter in series with a hot leg.

OR, you could do the same search but instead look for AC ammeters with a 0-5 ampere range and also look for current transformers with XX/5 ampere designation, substituting the peak output of your generator in amperes for the XX or use a slightly higher rated current transformer. The generator wire would go through the "donut" of the current transformer and the leads from the CT go to the meter.

CAUTION! Do not under any circumstances allow power to flow through the CT without having the secondary conductors either connected to the meter or shorted together. Open secondary conductors on a CT can deliver very high voltages.
I will try looking for that. I figured it was seeing amps, but didn't know how it was displayed on the meters.. since it was called a watt meter, and thank you for the caution note above !

#7
12-15-11, 03:03 PM
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Got confused again. Where do I find these meters that display watts I guess? I understand the unit I ordered uses the CT rings with wire leads to these display meters, but they don't display amps from what I understand but kW or watts on each leg.

#8
12-15-11, 03:22 PM
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I believe what they are telling you is the watt meters are not a good indicator and not at all that accurate.

A amp meter would be a more accurate indicator to gauge your gen use.

depending on what your nameplate says then you can gauge by that. I would say, and if I remember your gen, is to not pull more then 15amps per leg.

I have these watt gauges and I can tell you one is off by 200-300 watts. ( Mine are both over the hot wire and should read the same but they dont)

Mike NJ

#9
12-15-11, 03:27 PM
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Originally Posted by lawrosa
I believe what they are telling you is the watt meters are not a good indicator and not at all that accurate.

A amp meter would be a more accurate indicator to gauge your gen use.

depending on what your nameplate says then you can gauge by that. I would say, and if I remember your gen, is to not pull more then 15amps per leg.

Mike NJ
Thanks. Not to sound like a dumb ass here, but yes it's a 30amp gen / 15 per leg. I figured what I wanted to be more concerned with was amp draw on each leg, not the wattage. Is that just how Reliance displays it due to ease of understanding because most people buy the gens on wattage etc...? If I get two CTs, and two 0-15 or 20 amp ammeters that should work fine correct?

Again, guys I appreciate all the help, you have cured many head aches of mine

#10
12-15-11, 04:28 PM
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Is that just how Reliance displays it due to ease of understanding because most people buy the gens on wattage etc...?
Yes. A true wattmeter is a fairly expensive device.

#11
12-15-11, 04:32 PM
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Ok. So I need to find the correct CT's, and I guess two AC panel ammeters...? I was looking for what seems to be called edgewise meters like Reliance put in, but I am having a hard time finding them that display AC amps. I found on on Ebay but that was only one.

#12
12-15-11, 04:49 PM
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I believe you would want something like this.

Only an example. And I dont think you need more then 20 amps per leg.

Rectangle Analog AMP Meter AC 20A + Current Transformer | eBay

Mike NJ

#13
12-15-11, 05:01 PM
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Originally Posted by lawrosa
I believe you would want something like this.

Only an example. And I dont think you need more then 20 amps per leg.

Rectangle Analog AMP Meter AC 20A + Current Transformer | eBay

Mike NJ

Nice ! Thanks Mike. Comes with both too. A bit cheaper.

#14
12-15-11, 05:07 PM
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The only issue it will ship from china. Geez how long will it take that package to arrive? Heck it says free shipping....LOL

I am sure there are local places in the US that supplys this type of item.

Mike NJ

#15
12-15-11, 05:22 PM
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Originally Posted by lawrosa
The only issue it will ship from china. Geez how long will it take that package to arrive? Heck it says free shipping....LOL

I am sure there are local places in the US that supplys this type of item.

Mike NJ
I was wondering if I should check the local supply place I got my interlock kit from. Sometimes they look at me like I'm smoking crack when I ask about this stuff because it's standby gen stuff and they mostly deal with normal elec contractors of which I am not.

If not, I did order some led stuff from China via eBay and it came rather fast. I was surprised. I would just like to get this done. I would love the MB75 but with endless delays and no concrete answer on when I might want to do this alternative. I plan on getting the gas piping done this weekend to see if the darn gen will even run as set up...

#16
12-15-11, 05:35 PM
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Bluesea systems makes CT's and meters that you might like. They are mainly designed for AC systems on boats running a generator so they might be just the ticket for your application and they are also relatively inexpensve.

AC Analog Meters - PN - Blue Sea Systems

#17
12-15-11, 06:04 PM
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Thanks everyone. I did manage to find the exact meters and CTs that are in the Reliance product here :

7230 Replacement Analog Watt Meter - 3750W for 20A & 30A Reliance products

I will either do that, or go the eBay route. I will call nooutage first to make sure they actually have them. I may just do that and make my own enclosure since I am looking at Feb or beyond for the Reliance product.

Hopefully my Generac doesn't crap out. There are mixed reviews on them.

#18
12-15-11, 08:29 PM
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Watt meters = useless for generators.

The real killers for generators are motor appliances (Furnace fans, refrigerators, sump pumps). They require 5 to 6 times as many amps as they use after they started. Most watt meters aren't fast enough to detect that.

That's what you have to balance, what appliances are motor based and what are their starting amps, and if you can control when each of them starts.

Example: Furnace fan uses very little power while running but starts with a lot. However you CAN keep the furnace ON 24/7. So start the furnace fan before putting any load on the generator. Once it is started, you can hook up appliances you cant control when they turn on (Refrigerators!!!!!).

#19
12-15-11, 09:05 PM
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Originally Posted by insthink
Watt meters = useless for generators.

The real killers for generators are motor appliances (Furnace fans, refrigerators, sump pumps). They require 5 to 6 times as many amps as they use after they started. Most watt meters aren't fast enough to detect that.

That's what you have to balance, what appliances are motor based and what are their starting amps, and if you can control when each of them starts.

Example: Furnace fan uses very little power while running but starts with a lot. However you CAN keep the furnace ON 24/7. So start the furnace fan before putting any load on the generator. Once it is started, you can hook up appliances you cant control when they turn on (Refrigerators!!!!!).
Yes this will require some manual load control. If needed my main concern was forced hot air furnace, freezer, fridge and well pump. I planned on dumping other loads running well pump to get tank pressure up and then shutting it off and returning other consumers back online. Repeat as needed. PIA for sure, but cheaper than whole house gen and better than what I had which was nada

#20
12-22-11, 09:56 PM
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I ended up finding the Reliance components from the MB75 and making my own.

#21
12-22-11, 10:01 PM
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Looks good.

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