Voltage drop distance for service

[xptpcrewx]

Let him speak

Sorry, not trying to exclude anyone from the conversation. Just trying to stay on topic.

 

[mbrooke]

Sorry, not trying to exclude anyone from the conversation. Just trying to stay on topic.

Its ok

He makes a good point though, often times conductors can be undersized for the distance or future load. IMO, any L-N fault current below 1000 amps at the service indicates an undersized drop by default.

 

[mbrooke]

not have to worry about any impedance before the service point. If I have historical/empirical data, I use actual conditions, but most of the time there is no guarantee what those will be so it’s safe to assume the minimum allowable service voltage (given that you have no control of it).

Hate to say it, but this method gave the proponents fodder for the creation of electronic AFCIs.

From my perspective this is not an art. Art is largely subjective. I am merely attempting to compare approaches - which is something objective and can be quantified. If you’re interested, let’s discuss the pros and cons of each approach.

As far as voltage drop not being a code requirement… maybe not explicitly; however, if the equipment has a nameplate that could implicitly trigger a 110.3(B) requirement.

I agree that voltage can trigger 110.3(B). However also keep in mind that excessive drop impedance can also cause headaches with 250.4(A)5.

 

[xptpcrewx]

Hate to say it, but this method gave the proponents fodder for the creation of electronic AFCIs.



I agree that voltage can trigger 110.3(B). However also keep in mind that excessive drop impedance can also cause headaches with 250.4(A)5.

I have no idea what you are talking about.

 

[mbrooke]

I have no idea what you are talking about.

I guess I have some splan'n to do.


Standard thermal magnetic breakers were going to meet the requirements of 210.12, until it was brought up that some new homes may have 500 amps at the panel-board due to long or undersized service drops- leading to 75 amps vs 200 amps of assumed fault current at the end of a typical 15 or 20 amp run.

Standard breakers with a 150 amp magnetic pickup will hold on inrush, where as 75 amps will cause a trip. Hence electronics were created to ironically hold on inrush over 75 amps, yet still be able to detect arcing short circuits down to 75 amps.


Between 1000 and 10,000 amps (0.12-0.012ohms) little difference is made on the short circuit current of a branch circuit at the far end, while fault currents under 1000 amps begin to cause a rapid drop in current magnitude.


Low current flow during a fault increases EGC heating and touch voltage, sometimes to the point where it can be in violation of code.

 

[romex jockey]

I guess I have some splan'n to do.


Standard thermal magnetic breakers were going to meet the requirements of 210.12, until it was brought up that some new homes may have 500 amps at the panel-board due to long or undersized service drops- leading to 75 amps vs 200 amps of assumed fault current at the end of a typical 15 or 20 amp run.

Standard breakers with a 150 amp magnetic pickup will hold on inrush, where as 75 amps will cause a trip. Hence electronics were created to ironically hold on inrush over 75 amps, yet still be able to detect arcing short circuits down to 75 amps.


Between 1000 and 10,000 amps (0.12-0.012ohms) little difference is made on the short circuit current of a branch circuit at the far end, while fault currents under 1000 amps begin to cause a rapid drop in current magnitude.


Low current flow during a fault increases EGC heating and touch voltage, sometimes to the point where it can be in violation of code.

Dave Shapiro , past EC writer and IAEI george washington chpt member pointed this out decades ago MBrooke

~RJ~

 

[mbrooke]

Dave Shapiro , past EC writer and IAEI george washington chpt member pointed this out decades ago MBrooke

~RJ~

Well, Google leads me to the the equation Zs=Ze(R1+R2) which comes in at 1,630,000 results....




CMP-2 and CMP-5 seem to have discussed this quite frequently the past three decades. Just expressing my opinion

 

[kwired]

There is a performance difference in sizing for VD for the load calculation and the actual VD when the AC or other larger motor is starting.

I run into this a lot in older installs where the source is like a block away and there is lots of overhead line before getting to the customer service equipment. They want to know why the lights dim when the AC starts, or even had a small repair shop once that had pretty severe dimming whenever the air compressor started. Got POCO to place a transformer closer on that one and it made a huge difference.

 

[romex jockey]

CMP-2 and CMP-5 seem to have discussed this quite frequently the past three decades

so has UL
~RJ~

 

[mbrooke]

so has UL
~RJ~

And Legrand. And Eaton. And EPRI. Just to name a few more.


If the assumption had been made of >1000 amps being the minimum fault current there wouldn't have been way less supporting "evidence" AFCIs.


1000 feet of #14@ 30*C = 2.55 ohms

A typical 125 feet of #14 (L-G) = 0.6375

The service:

120/500=0.24 ohms

120/1500= 0.08 ohms

120/10000=0.012 ohms


So adding 0.6375 to each of the above gives:


136 amps for a 500 amp FC

167 amps for a 1,500 amps FC

184 amps for a 10,000 amps FC

 

[romex jockey]

and...that >R< factor increases trip times.....~RJ~

 

[mbrooke]

and...that >R< factor increases trip times.....~RJ~

Increases trip times by getting the circuit breakers out of their instantaneous magnetic trip threshold. That is the only thing AFCIs are trying to mitigate.

 

[Fred B]

and...that >R< factor increases trip times.....~RJ~

Can I add a stupid question please? That fact above a given, can say for a motor load, you want to increase your trip time, would increasing wire length be a viable means to do such in absence of a variable trip breaker? (I know it is used as method to reduce AFC.) Or might the voltage drop be too much by time you get enough wire added to get a significant enough of R to have a meaningful desired increase in trip time? (Usualy increases in trip time in other applications undesirable for safety reasons.) Was just wondering given the big issue getting materials currently if this could be a short term fix until you get an appropriate breaker.

 

[kwired]

And Legrand. And Eaton. And EPRI. Just to name a few more.


If the assumption had been made of >1000 amps being the minimum fault current there wouldn't have been way less supporting "evidence" AFCIs.


1000 feet of #14@ 30*C = 2.55 ohms

A typical 125 feet of #14 (L-G) = 0.6375

The service:

120/500=0.24 ohms

120/1500= 0.08 ohms

120/10000=0.012 ohms


So adding 0.6375 to each of the above gives:


136 amps for a 500 amp FC

167 amps for a 1,500 amps FC

184 amps for a 10,000 amps FC

What is typical magnetic trip setting of a 15 amp thermal magnetic breaker? (Is lower for standard 1 pole QO than most others)

I highly doubt it is 1000 amps, would guess in the 100 - 150 range though.

Lower capacity separately derived system or other smaller alternative sources may not even be able to deliver 100-150 amps fault current.

 

[kwired]

What is typical magnetic trip setting of a 15 amp thermal magnetic breaker? (Is lower for standard 1 pole QO than most others)

I highly doubt it is 1000 amps, would guess in the 100 - 150 range though.

Lower capacity separately derived system or other smaller alternative sources may not even be able to deliver 100-150 amps fault current.

But lets remember the OCPD is primarily protecting the conductor. less current means more time before damages, so thermal curve of the breaker is still protecting the conductor.

 

[mbrooke]

What is typical magnetic trip setting of a 15 amp thermal magnetic breaker? (Is lower for standard 1 pole QO than most others)

I highly doubt it is 1000 amps, would guess in the 100 - 150 range though.

Lower capacity separately derived system or other smaller alternative sources may not even be able to deliver 100-150 amps fault current.

According to the listed time current curves starts around 100-150 amps, as you stated, which is correct.

https://library.industrialsolutions.abb.com/publibrary/checkout/GES-9886B?TNR=Time%20Current%20Curves%7CGES-9886B%7CPDF&filename=GES-9886B_v2%209-09.pdf

https://library.industrialsolutions.abb.com/publibrary/checkout/GES-9887B?TNR=Time%20Current%20Curves%7CGES-9887B%7CPDF&filename=GES-9887B_v2%209-09.pdf

Before the 90s 15 and 20 amp single pole breakers started around 20x-25x, or 400-600 amps.


Incident energy aside, if a separately derived source can not deliver 100-150 amps the voltage on its output terminals will sag resulting in a lower touch voltage at the fault point- thus increased trip time may not physiologically harm occupants.

 

[ptonsparky]

Can I add a stupid question please? That fact above a given, can say for a motor load, you want to increase your trip time, would increasing wire length be a viable means to do such in absence of a variable trip breaker? (I know it is used as method to reduce AFC.) Or might the voltage drop be too much by time you get enough wire added to get a significant enough of R to have a meaningful desired increase in trip time? (Usualy increases in trip time in other applications undesirable for safety reasons.) Was just wondering given the big issue getting materials currently if this could be a short term fix until you get an appropriate breaker.

Yes, an increase in distance should give a decrease in the initial startup amperage. It will also increase the length of time in that start mode and drop back to a normal motor current

 

[retirede]

Yes, an increase in distance should give a decrease in the initial startup amperage. It will also increase the length of time in that start mode and drop back to a normal motor current

I have a chop that would only start without tripping the breaker if I plugged in to the receptacle farthest from the panel. Until I replaced the breaker with a hi-mag.

 

[romex jockey]

Can I add a stupid question please? That fact above a given, can say for a motor load, you want to increase your trip time, would increasing wire length be a viable means to do such in absence of a variable trip breaker? (I know it is used as method to reduce AFC.) Or might the voltage drop be too much by time you get enough wire added to get a significant enough of R to have a meaningful desired increase in trip time? (Usualy increases in trip time in other applications undesirable for safety reasons.) Was just wondering given the big issue getting materials currently if this could be a short term fix until you get an appropriate breaker.

It far from a stupid Q Fred, the fact is , it's been the fodder for many 'run length Vs trip times' UL studies

~RJ~

 

[kwired]

I have a chop that would only start without tripping the breaker if I plugged in to the receptacle farthest from the panel. Until I replaced the breaker with a hi-mag.

About 50 feet of 16-3 cord also reduces starting current enough to not trip in most cases where it otherwise trips frequently.