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Join Date: Nov 2012
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Testing Amps

05/04/2013 10:10 AM

Hi All, Is There a device that will put a load and determine how many amps the service wire entering a main panel with the breakers for large house , is capable of handling safely , and voltage drop . The laws of physics do not support what I am seeing . ---- There is a main breaker box on a pole under the meter, with 200 amp breaker, cable heads uderground , more than a 1000' away is the house breaker panel with easily 200 amps worth of breakers. [ I will determine the distance later today] 200amp panels usually have lugs to accept 4/0 aluminum cable . A 4/0 cable on a 200a service is only 'good' for about 250' to stay in acceptable voltage drop range. Even if the 4/0 cable is doubled , The 1000'+ Distance is too great for acceptable voltage drop. I do not uderstand how this would pass building codes or electrician installation sign off to pass inspection. The owner , building dept. and power company are of no help. Does anyone have suggestions. Thanks

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#1

Re: testing amps

05/04/2013 10:16 AM

Has low voltage ever been noticed at the house panel?

There may be transformer taps to set the voltage higher at the transformer end, thereby compensating for voltage drop in the cables.

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#3
In reply to #1

Re: testing amps

05/04/2013 10:35 AM

I will look into that, the power co told me there was 100 amp service.

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#8
In reply to #3

Re: testing amps

05/04/2013 11:17 AM

If you have "easily 200 amps worth of breakers" running on what you say, "the power co told me there was 100 amp service", you are in violation of every code I've ever heard of, and no electrician will touch this house, ever, unless it is first brought "up to code".

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#9
In reply to #8

Re: testing amps

05/04/2013 11:28 AM

You are right-- This is not my house, its one I,m looking to purchase, and I would like to determine if what is there is up to code and if its not , what it will take to bring it to proper code. One option is to go overhead with higher voltage wire and transformer closer to house, if I can get easements

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#10
In reply to #9

Re: testing amps

05/04/2013 11:40 AM

I think you need to hire that electrician, soon. No offense, but if what you say is true, you need a pro to give you some help.

We've got some really smart electricians on here (I'm not one), maybe they can advise you.

Proceed with caution.

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#11
In reply to #10

Re: testing amps

05/04/2013 12:19 PM

I will hire someone , but I'm trying to get as much info before hand, Again this is my first day to proceed with inspections, does anyone suggest a pro in N Colorado, FT Collins etc

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#6
In reply to #1

Re: testing amps

05/04/2013 11:05 AM

this is a house I'm looking to purchase, so today is the first day I have to start inspections. what is the best way to measure voltage drop. I was going to use killo-watt meter and a 15 amp tool for starters but that doesn't measure surge, and I need to document this info using electrician . I'm just trying to be informed before I hire someone

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#2

Re: testing amps

05/04/2013 10:27 AM

Use copper wire.

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#5
In reply to #2

Re: testing amps

05/04/2013 10:54 AM

This is an existing underground service to an existing house . 4 years ago before copper went thru the roof, I priced a 200amp service under ground using copper ,1200' , I think it was a M series cable [not sure] the cost of the wire alone was about $ 70.000 . I would just like to figure out what is there and if it was done properly

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#4

Re: testing amps

05/04/2013 10:45 AM

Get an infrared thermometer and check the temperature of the incoming cable and breakers.....if it's heating up you have a problem...I mean other than using aluminum wire...

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#7
In reply to #4

Re: testing amps

05/04/2013 11:13 AM

I have a infrared thermometer, but one would have to draw high amps thru panel,[turn on everything in house ?] and what is acceptable temperature. Aluminum cable is used almost 100% underground in this state [ Colorado] from meter to house

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#17
In reply to #7

Re: testing amps

05/04/2013 5:59 PM

If you're exceeding 130º F at any point, that would be an indication there is a problem...Yes if you want a real test, turn everything on, wait for 10 min or so and check temps....wire connections, lugs, breakers, just scan everything.....this is a great way to find any loose or damaged connections as well.....I would scan all main connections and breakers, make a note of operating temps, and check periodically for any rising temp trends....this will allow you to pinpoint problems before something fails....great for checking motors(170ºF max), refrigeration systems, contactors, really anything electrical....the more you use this method the more familiar you become with operating temps of different components, and will be able to spot a problem right off....

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#18
In reply to #17

Re: testing amps

05/04/2013 10:35 PM

Thanks for the info, This will be helpfull

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#12

Re: Testing Amps

05/04/2013 12:57 PM

Hi Jazzman.

I think you are overly concerned here.

4/0 Aluminum wire is service rated for 200 amps.

Voltage drop over a 1000' run (or any length) will become a larger factor when the load approaches maximum.

For instance, when drawing 100 amps over 1000 feet using aluminum wire at 95 degrees F: If you have 240 VAC at your source, you will have 220/110 VAC at the service entrance.

Even though you have a 200 amp service, it would be highly unusual to be drawing maximum power at one time (AC, Oven and all stove tops, Water heater, Dryer and Washer, microwave, all lights, TVs and stereos ALL on at the same time).

If you ever try to turn everything on at the same time, you will have approximately 200/100 VAC to your appliances, which should still be ok.

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#13
In reply to #12

Re: Testing Amps

05/04/2013 1:22 PM

Thanks for the calculations and info, I always used a rule of thumb to limit voltage drop to 5%. just to be safe in case heavy loads were being used, especially surges on pumps , welders table saw, etc. Its my understanding that the armatures wil excessively arc with the lower voltages causing pitting leading to premature failure. And what happens to sensitive electronics ,computers, tv, etc when the pump kicks in, surges and causes temporary voltage drop ? Is this harmful to electronics ?

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#14
In reply to #13

Re: Testing Amps

05/04/2013 2:16 PM

Every motor in your house has a "locked rotor" infinite current draw, but only for an instant.

Ever notice a light bulb momentarily flicker when your AC kicks on?

To be more specific, yes, an underpowered motor will degrade faster, but unless this "house" is going to be primarily a workshop, I believe your service entrance cable is fine.

Another tidbit...if you will be using 50-75% of your rated power on a regular basis:

1. You will go broke...so buy stock in your local power company.

2. Every 6 months or so, tighten the connection lugs on your service entrance cable. Aluminum will expand and contract more than copper, and will work loose. A loose connection at these lugs will do far more damage to your appliances than any motor kicking on.

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#16
In reply to #14

Re: Testing Amps

05/04/2013 2:46 PM

Thanks for the info, I usually put some removable threadlock on the lugs when using aluminum cable , is that a safe practice?

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#15
In reply to #13

Re: Testing Amps

05/04/2013 2:17 PM

Also, its my understanding that the current elect code requires minimum 100amps @ 240V , not 200v. I will be talking with bld dept on monday to get further info. All your comments are welcome and appreciated

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#19

Re: Testing Amps

05/04/2013 10:58 PM

"...200 amps' worth of breakers..." is not any sort of indicator for the required service to the panel. Adding up the ratings of all of the breakers will not get the desired result; it would be HIGHLY unlikely that every circuit would be loaded to capacity at any given time. If the construction was approved by the constructor/installer and the inspector as being "up to code", the odds are good that you are OK.

Most electrical devices in the United States will work just fine from about 105 to 130 Volts (Double these numbers for "240-Volt" appliances.). If your maximum load causes the voltage to drop below 110 Volts, you might need a larger-gauge service.

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#20

Re: Testing Amps

05/04/2013 11:06 PM

Iazzman, you are quite willing, but a bit incoherent to my ears. Let me read back, what I hear. The aluminum wire is rated for 200 Amps, good. The service entrance is rated for less, good. Beyond that who cares.

You worry about oxydised alu carrying current. No problem, follow the power company standard termination. They use a Al/Cu cold welded crimp termination on the high power lines. The Al side gets a bathtub crimp, cold welding al to al. The Cu side gets bolted where it is needed. You tape insulate it. Cadillac treatment.

Past the hyperventilation, To make sense, I need to know the voltage drop at 10 or 100 Amps. That is the next step to make sense, and give you sensible advice.

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#21

Re: Testing Amps

05/04/2013 11:39 PM

I see there are a number of common electrical service misconceptions in play here.

One is that all though the main service supply point, Meter box, has a 200 amp breaker the limiting point of the house would be whatever the primary breaker is in the actual service panel in the house which if it has around 200 amps of combined breakers could still only be a 100 amp panel with a 100 amp primary breaker in itself.

Now if so at 1000' using 4/0 aluminum a 100 amp service would still fall within the code books allowable voltage drop given the typical 80% rule for the primary 100 amp breakers rating. That is to say that a 100 amp breaker is only intended for continuous loads of 80 amps not the full 100.

Second relating to the 1000' feet of cable heating up whether it's 1 foot long or 10,000 feet long it's still going to only heat up by whatever the per foot power loss is. That being that if it is loosing 1 watt per foot at X amount of amps it won't get any hotter over 10,000 feet than that one foot would get. The total power loss and voltage drop at 10,000 feet would be 10,000 times more than what occurs over the one foot length but the cable as a whole would not be any hotter.

Personally I have more questions as to why or how this residence managed to get its primary meter placed so incredibly far from the house itself? Around here a electrical service would have had a HV primary line bringing power to the stepdown transformer and meter box well within the X amount of feet limit of whatever size of service was used. Which in this case being that 4/0 cable being the common upper limit and 250' being the distance limit for a 200 amp service on a 4/0 feed the transformer and meter would have been place less than 250 feet from the house.

Now relating to the $70,000 quote for a 1000' of three conductor 4/0 copper line WTF? 4/0 three conductor cable costs around $25 a foot. Even less for 4/0 aluminum. So where is the other $45K+ going?

You can outright buy a smaller 4WD tractor type commercial trencher for under $20K brand new or rent a walkbehind one for around $200 for a day.

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#23
In reply to #21

Re: Testing Amps

05/04/2013 11:54 PM

You are with me in the detail analysis. But with Iazzman, you sailed straight over his head, and not his fault.

I am sure, your and mine, that Iazz's paying ability is simply short on desirable and acceptable by us.

So is his capability in our fields, in reverse.

I wish his talent to improve here, as my talent to improve there.

Yeah, right, good luck.

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#27
In reply to #21

Re: Testing Amps

05/05/2013 11:26 AM

"Personally I have more questions as to why or how this residence managed to get its primary meter placed so incredibly far from the house itself?"

I'm curious as well.

It might be worthwhile of the OP to see if he can get the power company to move the meter closer to the house. That way the losses in the wire are their problem not his. He'll still see the voltage drop as the current increases, but at least he won't be paying for the lost power.

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#28
In reply to #27

Re: Testing Amps

05/05/2013 12:57 PM

RE: Meter location. Dollars to donuts, the meter was installed as a construction site meter and just left in place instead of relocating it.

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#22

Re: Testing Amps

05/04/2013 11:39 PM

Hire a load bank and keep increasing the load on your main breaker from zero to 200Amps and record the voltage drop as well as temperature of main service cable.

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#24
In reply to #22

Re: Testing Amps

05/05/2013 12:08 AM

Actually all he needs is a electric heater an amp clamp and voltmeter.

If the open circuit voltage is 120 volts and a 10 amp load pulls it down .5 volts it pretty reasonable to assume a 100 amp load will pull it down 5 volts and a 200 amp load will pull it down 10 volts.

Plus if he doesn't already have one its a good excuse to go and buy a basic Digital Voltmeter/amp clamp unit. They only cost less than $20 with shipping at Harbor Freight and can be ordered online.

Harbor Freight Digital Multimeters

I have one of the $10 six function 300 VAC 400 Amp mini clamps and I have yet to kill it!

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#25

Re: Testing Amps

05/05/2013 9:59 AM

national electric code branch circuit and feeder calculations.

The laws of physics assume that just because a wire is capable of handling (say) 200 amps doesn't mean that it actually IS handling 200 amps at any given time. The code limitations have more to do with how hot the wires get, and how easily they can cool down rather than voltage drop in most cases. And since voltage drop is directly dependent upon amperage, you may have to do some calculations.

This table shows ampacities of common service wires.

There is a lot to consider. Are the wires copper or aluminum. Are they in a conduit? How full is that conduit? What is the load? Is it continuous (like street lighting), or should you use a formula (article 220 NEC) to determine "average" load?

If you find that under normal use that your voltage is fluctuating wildly, you may have to increase the ampacity of your service lines.

This problem is often encountered when a house is subdivided into flats...the old service is not good enough. If there has not been any major changes in occupancy the last couple of years, you may not have a problem. However, a GOOD home inspector will do exactly as you are doing, and check out everything. I am surprised that your local building department and power company are not being of any help...you should be able to request a certificate of inspection from them.

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#26

Re: Testing Amps

05/05/2013 11:23 AM

I know this is an American system but the UK's BS7671 on site guide gives a quick reference to volt drop and diversity calculations.

The OSG is a small reference book to save carrying the 500+ page regulation book.

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#29

Re: Testing Amps

05/05/2013 7:54 PM

I wish I had more time for this one, wow, If I recall code right you can go up to 125% of the mains rating when adding up all your amps, but you say you have 200 amps worth on 100 amp service???? I advise to to shut it off and hire a real electrician to come untangle this mess before you melt something!

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#30

Re: Testing Amps

05/05/2013 8:21 PM

Jazzman,

  1. If you want to go through the calculations, borrow a copy of the National Electrical Code from your local library, or sit down in the reference department with it and some basic information about the house. With it, you can calculate the service size your prospective house will need. This is based on--square footage; large electrical loads such as range, dryer, water heater, A/C, and others; and other required circuits.
  2. In a residence the basic load is 3w/sq.ft. Unlike a commercial building, the receptacles and lights all are part of this load (except for kitchen, laundry, and a couple other locations). There is no limit to the number of circuits you can have, as long as the overall main for the house is larger than the computed load.
  3. Circuit breaker sizes are limited by the gauge of the wire in the circuit and by the type of device you are wiring to--20A for 12AWG wire, 15A for 14AWG, etc. Regular 15A receptacles are OK on either a 15 or 20A circuit, but larger than 14AWG must be in a screw clamp or under a screw head.
  4. Voltage drop is calculated by Ohm's law, and can be measured with any decent meter. Other posts here suggest measuring the voltage, then turning on a significant load (oven) then measuring it again. Easy and quick way to do this is to turn on the oven, go to the breaker box and measure the voltage then turn off the breaker for the range and see what the voltage changes to. Before turning it off, read the clamp-on meter to see how many amps it was drawing.
  5. Others' suggestions about IR scanning are very good. The spot-check thermometer is valid but not nearly as informative as an actual camera. No matter what method you use, remember that IR temperatures are HIGHLY subjective, because of large variations in the emissivity of the surface being measured. A very reliable method to minimize this type of error is to compare surfaces of the same material and finish condition--one under load and one passively sitting there.
  6. If your indoor panel has no main, then you need to check its inside label to see what size it is rated at, whether 100, 125, 200 or ?? amps. Even if it has a main, this check is still worthwhile.

There are many other things to look at when doing a good electrical inspection of the house, such as light bulb sizes, GFI locations and condition, recessed lights, . . . . . If you don't have extensive experience in this, hire a qualified electrician (inspection departments won't normally recommend one, but they can give you a list of a few, and commercial services exist to compare the qualifications of contractors).

--JMM

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