I went back to the original post, and found that 75mV for 15A reduces to 5mV per amp, a very convenient scale factor. Really, you could use both shunts in series, the 15 amp one to set your current and then the 20 amp one for the actual check.
Really, the main inconvenience of shunts is needing to break into the circuit. AND, of course, the fact of the large common mode voltage possible. But using a well isolated meter reduces that problem.

 

I would mount the shunt in a plastic box and bring the sense leads out to pin sockets on the box (such as below) with 10kΩ in series with each terminal.
That will minimize the chance of electrocution while taking the measurement with only a minor effect on the measurement accuracy when using a standard multimeter.

 

My experience with using shunts has been that the big hazard is the load connection. The measuring connection is usually a 2 conductor insulated cable, typically #24 wire, that I plug into the DMM before switching on the power. That way I don't need to be anywhere near the shunt.

 

Personally I would never consider using a resistive shunt for AC current detection when there are now so many non-invasive ways to do it electrically or electronically.
Max.

 

Several digital clamp-on meters for ~$15.00 on Ebay 273139012635 etc.
Max.

 

Determining the source impedance of your electrical system will be an interesting exercise. Good luck with the project, please post the results that you find.

I used an oil heater and a micathermic heater as loads. The shunt and CT were connected in series. Starting and load voltages were measured on the adjacent outlet. My results were as follows:

Starting no load voltage: 121.3V
Load voltage: 113.2V

Therefore 8.1V dropped on wiring from transformer outside house all the way to the outlet in bedroom

Shunt voltage (20A/50mV): 42.2 mV = 16.88A
8.1V @ 16.88A = 0.47986 ohms

Therefore the prospective short circuit current at 121.3V is 252.78206A

====

CT voltage (20:5 using 0.2ohm burden): 846 mV = 16.92A
8.1V @ 16.92A = 0.47872 ohms

Therefore prospective short circuit current at 121.3V is 253.38402A

Based on the above it looks like the prospective short circuit current at this outlet is around 250A @ 120V. Does this seem reasonable for a residential outlet in Canada/USA ?

 

The calculated value is reasonable, although during a short circuit condition a lot of other things are not very predictable. For starters, the transformer may go into saturation before that much current flows, limiting the available current a bit. In addition, the calculated resistance based on your measurement may include a weak point that may fail before that much current can fail. And even if no "weak link" lets go, there may be other protective things in the circuit that will cut off the current when they function.
I don't recall what the stated purpose of this determination is, but the safe guess is that the short circuit current might be up to 250 amps or so for a very short period of time.
In addition, MAX is certainly entitled to use whatever device he chooses to measure AC current, while using a series shunt can certainly provide an accurate reading, but with quite a bit less convenience and more safety precautions required. Fortunately, at least where I live, we are each free to choose the level of technology we use, at least in our own areas. Just because some technology is newer and possibly more accurate does not make previous methods worthless or poor choices.

 

In addition, MAX is certainly entitled to use whatever device he chooses to measure AC current!

Max.

 

The Ideal Surtest does exactly what you want. http://www.idealindustries.ca/produ...cuit_analyzers/suretest_circuit_analyzers.php

Not sure exactly how it does it.

 

The Ideal Surtest does exactly what you want. http://www.idealindustries.ca/produ...cuit_analyzers/suretest_circuit_analyzers.php

Not sure exactly how it does it.

A good series shunt does the job, at least at power frequencies, and it is totally clear exactly how it does it. Also, I got my collection of accurate shunts for a very low price, and I already have the accurate isolated meters.

 

I don't recall what the stated purpose of this determination is, but the safe guess is that the short circuit current might be up to 250 amps or so for a very short period of time.

I don't believe I indicated my main purpose which was to determine the appropriate fuse to use on residential mains wiring (for projects that I build and keep in my room). In this case, to my understanding, even a glass fuse can be used (doesn't have to be ceramic and/or classified as HRC) as long as the interrupting rating is above 250A @ 120V AC. For example, I believe the following line of fuses would be safe to use:

https://belfuse.com/resources/datasheets/circuitprotection/ds-cp-5mf-5mfp-series.pdf
The linked datasheet indicates glass fuses with an interrupting rating ranging from 300A to 10kA @ 125VAC (depending on the ampere range)

 

@van53 https://www.littelfuse.com/products/fuses/cartridge-fuses/3ab_3ag-6_3x32mm-fuses.aspx
Max.

 

@van53 https://www.littelfuse.com/products/fuses/cartridge-fuses/3ab_3ag-6_3x32mm-fuses.aspx
Max.

Thanks Max. I tried out the I-Design part selector on the right (after going to the URL you provided) which I found useful.

 

The proper fuse to use to protect the mains wiring is usually dictated by both the entrance cable from the pole and the first electrical panel that the incoming wires from the meter connect to. In the non-service upgraded homes in my neighborhood the original fuse box had 60 amp main fuses. That was fine until electric stoves became popular that needed either 40 or 50 amp fusing became popular. Then came central air conditioning and many houses now have their service upgraded to 100 amps, and the new-built monster sized houses have 150 or 200 amp service. But the wires on the poles are mostly the same size as always, skinny copper, with the high voltage wires at the very top being mostly #6 wire. But I don't know what the voltage on that top wire is, except that it will draw an arc to bare dirt in my yard when they fall down.

 

Power poles and pole transformers have virtually been eliminated in most major cities and are generally only found in rural areas, at least in this country.
I have a local distribution one sitting at the end of my yard
And due to the inability to obtain house insurance, knob and tube wiring and Wired Service Main fusing has virtually been eliminated.

The exception being cities in poorer areas of the world.!
Delhi for e.g.
Max.

 

Max is "sort of correct" in that there are areas of high priced residences where none of the power distribution system is visible. I have actually seen such an area and so I know that they exist. But in most of the USA, including everywhere except Max's neighborhood, power lines run on poles and transformers are small enough to be pole mounted. Here in automation Alley, Michigan, there are power poles nearly everywhere. Of course, not looking at all like the picture of Delhi that Max posts.
But for those folks who are in that top 1%, and even out to those in the top 10%, poles and transformers are the way it is done.

 

there are areas of high priced residences where none of the power distribution system is visible.
But for those folks who are in that top 1%, and even out to those in the top 10%, poles and transformers are the way it is done.

Practically all Canadian cities have under ground service at the residential/business level.
Not necessarily high priced residences, I would have thought most of the U.S. would be likewise?
Even rural communities here of any size here do not have visible service supplies!
But we do generate all our own power with a total of 15 hydroelectric generating stations. Even sell the US some!
Max.

 

But in most of the USA, including everywhere except Max's neighborhood, power lines run on poles and transformers are small enough to be pole mounted.

Maybe Max doesn't live in the US. Max also mentions:

And due to the inability to obtain house insurance, knob and tube wiring and Wired Service Main fusing has virtually been eliminated.

Here in the US knob and tube is still common in older homes and is insurable. Many of the older homes on the street I live on still have knob and tube wiring and still have the original 60 Amp service from the power company. As my wife and I refurbished our house room by room I ripped out the old knob and tube and replaced all the wiring. This was followed by new entry service which required a new meter and service to the pole.

Ron

 

Practically all Canadian cities have under ground service at the residential/business level.
Not necessarily high priced residences, I would have thought most of the U.S. would be likewise?
Even rural communities here of any size here do not have visible service supplies!
But we do generate all our own power with a total of 15 hydroelectric generating stations. Even sell the US some!
Max.

Actually down here while there are plenty of underground utilities the majority are above ground poles. During the 50s and early 60s there were entire communities going in on Long Island with everything underground but here we are 60 years later with no shortage of utility poles.

The entire US power grid system scares me to where I have my own backup power. Huge sub stations surrounded only by a fence with exposed large transformers. Years ago this was no big deal but today is a different story. Where I live, much like up in Canada it gets real cold during our winters. You want to freak people out? Remove the power in January in sub zero F. temperatures. Pipes start freezing real fast and people hate dark and cold. The large sub stations are sitting ducks for any type of terrorist attack. Hell, how many major black outs have we seen where multiple states lose power?

Canada is also blessed with plenty of Hydro-Electric. While we have some we lack the rivers Canada has. Anyway, we are now way, way off topic.

Ron

 

At home, a 60 YO house, poles run in the back yard with high voltage at the top level and phone at the lowest. A trasformer feeds about 4 houses. As transformers get replaced, HV disconnects are added. There is one big fuse at the transformer primary. Then no fuses until it reaches inside the house.

This house is still fuses and only GFCI's for the porch (which is where yard power comes from if needed) and the front yard outlet.

Initially, everything was 2-wire polarized, metal boxes, with a ground run to ground the box. There is NO tapped hole in the metal boxes for ground, so grounding is tricky, nonetheless. Ideal Industries discontinued there term-a-nuts which i really liked.

My father didn't wirenut ground in one outlet and that connection went bad. Really tough to find.

One room, I upgraded to tamper proof, 3-prong outlets when painted. Tamper proof doesn't allow you to put stuff in one side to expose a conductor. Basically "baby and child proof".

The bathroom doesn't have a GFCI either. the hair dryer has an integral GFCI and the toothbrush chargers are mostly sealed. A shaver charger unusually isn't in place except when charging. I don't consider it unsafe.

Now, there is a 1950's stove in the house with no oven pilot light and no pilot safety light. That could be a problem. Estimated cost for custom parts to retrofit is about $500.00 USD for a conventional pilot system.

Dad really knew how to run wires. He was really good at it.

The CO and cumbustable gas alarms need replacing.