Measuring Wire Length - Multiple Sub Panels

[jeffg]

I went to the store to pick up some wire for a new subpanel project for this weekend and was told that I needed to upsize my wire. The details are:
50A @240V
122' total distance
sourced from an existing 100A subpanel in my shop to a new 50A subpanel in the pump house.

I was planning on running 4AWG AL conductors with a 6AWG AL ground. This appears to be correct on all of the wire calculators and charts I could find.

The salesman told me that I need to upsize to 2AWG AL conductors (or 4AWG CU) because the existing subpanel in the shop is ~100' from the meter and I needed to include that in my distance calculations. Is that right? Do I need to measure all the way to the meter?

 

[JordanOH]

No

 

[wyliesdiesels]

I went to the store to pick up some wire for a new subpanel project for this weekend and was told that I needed to upsize my wire. The details are:
50A @240V
122' total distance
sourced from an existing 100A subpanel in my shop to a new 50A subpanel in the pump house.

I was planning on running 4AWG AL conductors with a 6AWG AL ground. This appears to be correct on all of the wire calculators and charts I could find.

The salesman told me that I need to upsize to 2AWG AL conductors (or 4AWG CU) because the existing subpanel in the shop is ~100' from the meter and I needed to include that in my distance calculations. Is that right? Do I need to measure all the way to the meter?

that salesman is full of it

BTW #6 AL is good for 50a not counting distance. and EGC can be #8 (good up to 60a)

doing the math by hand, #4 AL will give you a 5.612v drop @ full load or 2.34%

If you use #6, it will be 8.78v drop or 3.66%....

#6 with #8 ground should work just fine even though slightly more than 3% recommended drop. the chances of you using 50a or close to it are slim

PS: you could do the math by hand as well. heres the directions

Electrical FAQs

Welcome All I have started this sticky as an FAQ thread for info on the basics that are encountered countless times every week on here. Hopefully this will help some. If u have a topic request, go ahead and post it. And if you have an FAQ you would like covered let me know.... Or if you...

www.garagejournal.com

 

[jeffg]

Thank you guys for confirming. I had a panic moment, thinking I had done this wrong my entire life.

 

[Innovate1]

that salesman is full of it

BTW #6 AL is good for 50a not counting distance. and EGC can be #8 (good up to 60a)

doing the math by hand, #4 AL will give you a 3.77v drop @ full load or 1.57%

If you use #6, it will be 5.9v drop or 2.45%....

#6 with #8 ground will work just fine

PS: you could do the math by hand as well. heres the directions

Electrical FAQs

Welcome All I have started this sticky as an FAQ thread for info on the basics that are encountered countless times every week on here. Hopefully this will help some. If u have a topic request, go ahead and post it. And if you have an FAQ you would like covered let me know.... Or if you...

www.garagejournal.com

I looked at the FAQ and did the southwire calculator which gave 2.32%. Then I tried to follow the FAQ you linked.

#4 AL is 0.46 ohms/1000 ft (from the effective Z column you used in the FAQ example)

2 x 122 x 0.46 x 50/1000 = 5.6V or 2.3%
I could have missed something but how do you get 3.77V drop?

The table on page 11 of the link in the faq seems a bit strange. For #4 AL the DC resistance is 0.508 ohms/1000 ft on page 10. That agrees pretty well with the "Alternating-Current Resistance for Aluminum Wires" column for PVC or aluminum conduit as expected since power line frequency is low so not much skin effect. Why is the value for steel conduit significantly less?

Looking at the column for effective Z the number is less than the resistance for AC or resistance for DC. Any inductance of the run should raise the impedance.

It seems they must be dealing with some factors I am not considering...

As far as accounting for the run from the 100A box to the meter it seems that should be taken into account but using different current and wire size. If this is only done for the run from 100A panel to 50A panel it is only calculating drop for that portion of the feed. If you have sub panels off subpanels and each run is 2% then from main panel to subpanel to another subpanel the drops are additive although each run would be a different calculation. How is this dealt with? Is the drop for each run kept low enough to ignore the additive effects?

Just trying to understand all this better...

 

[mike93lx]

I looked at the FAQ and did the southwire calculator which gave 2.32%. Then I tried to follow the FAQ you linked.

#4 AL is 0.46 ohms/1000 ft (from the effective Z column you used in the FAQ example)

2 x 122 x 0.46 x 50/1000 = 5.6V or 2.3%
I could have missed something but how do you get 3.77V drop?

The table on page 11 of the link in the faq seems a bit strange. For #4 AL the DC resistance is 0.508 ohms/1000 ft on page 10. That agrees pretty well with the "Alternating-Current Resistance for Aluminum Wires" column for PVC or aluminum conduit as expected since power line frequency is low so not much skin effect. Why is the value for steel conduit significantly less?

Looking at the column for effective Z the number is less than the resistance for AC or resistance for DC. Any inductance of the run should raise the impedance.

It seems they must be dealing with some factors I am not considering...

As far as accounting for the run from the 100A box to the meter it seems that should be taken into account but using different current and wire size. If this is only done for the run from 100A panel to 50A panel it is only calculating drop for that portion of the feed. If you have sub panels off subpanels and each run is 2% then from main panel to subpanel to another subpanel the drops are additive although each run would be a different calculation. How is this dealt with? Is the drop for each run kept low enough to ignore the additive effects?

Just trying to understand all this better...

You need to use the voltage at the start of the run you are calculating for. If you have 240v at the sub, use that. So yes, total wire length is a factor and I can't understand how anyone would argue otherwise. Adding a sub doesn't magically reset the voltage

 

[jeffg]

It sounds like the best approach would be to actually measure the voltage at the existing subpanel and use that for the calculation. If the existing subpanel has a voltage drop, then we are not starting with the theoretical 240V. We are actually starting with something like 235V (or whatever is actually measured). This would account for the wire size and distance from the meter to the existing subpanel.

So as an example, plugging 235V in as the starting voltage, and using 122' of 4AWG wire gives me 227.9V. This is a 3% drop from the exisiting subpanel to the new subpanel, or about 5% drop from the meter? Does that seem right?

Then we need to decide if a 5% drop is acceptable.

Even if we went with 2/0 cable we would still only get close to the same 3% drop we already have at the first subpanel. We can never make up for that loss.

 

[mike93lx]

The one issue there is that voltage at one moment may not be representative and that voltage will change based on the load at your sub

 

[jeffg]

I fear getting stuck in analysis paralysis. The pump in the pump house is only 12A @230V. Add an LED light and a single outlet circuit and I am just going to run 4AWG AL and call it a day.

I am sure there is some academic value in all of this, but I cant account for every possible load situation or I will never get anything done.

Does anyone see a real reason to run 2AWG AL?

 

[exranger06]

The one issue there is that voltage at one moment may not be representative and that voltage will change based on the load at your sub

That's correct. Checking voltage at the sub when there's little to no load on the sub is pretty meaningless. However, if you turn on a bunch of appliances and put some serious load on the sub, and then check voltage with all of those things on, you can get a "worst case scenario" voltage drop, figure out how much additional voltage drop would be acceptable at the pump house, and then figure out which size wire you'd need to not exceed that voltage drop.

 

[Innovate1]

Wyliesdiesels said it was good. I came up with somewhat different numbers but either way the numbers come out ok. And with your small pump load it is way over what you need but leaves room for other things. Yes, sometimes things get run into the ground here.... You could go with quite a bit smaller wire if you wanted but I would still upsize substantially from the minimum size because of start up currents of the motor. Get er done.

 

[wyliesdiesels]

I looked at the FAQ and did the southwire calculator which gave 2.32%. Then I tried to follow the FAQ you linked.

#4 AL is 0.46 ohms/1000 ft (from the effective Z column you used in the FAQ example)

2 x 122 x 0.46 x 50/1000 = 5.6V or 2.3%
I could have missed something but how do you get 3.77V drop?

The table on page 11 of the link in the faq seems a bit strange. For #4 AL the DC resistance is 0.508 ohms/1000 ft on page 10. That agrees pretty well with the "Alternating-Current Resistance for Aluminum Wires" column for PVC or aluminum conduit as expected since power line frequency is low so not much skin effect. Why is the value for steel conduit significantly less?

Looking at the column for effective Z the number is less than the resistance for AC or resistance for DC. Any inductance of the run should raise the impedance.

It seems they must be dealing with some factors I am not considering...

As far as accounting for the run from the 100A box to the meter it seems that should be taken into account but using different current and wire size. If this is only done for the run from 100A panel to 50A panel it is only calculating drop for that portion of the feed. If you have sub panels off subpanels and each run is 2% then from main panel to subpanel to another subpanel the drops are additive although each run would be a different calculation. How is this dealt with? Is the drop for each run kept low enough to ignore the additive effects?

Just trying to understand all this better...

i screwed up on the math because i was in a hurry.

i went back and checked and got the same numbers as you. I fixed my previous comment

 

[Sumboodie]

I fear getting stuck in analysis paralysis. The pump in the pump house is only 12A @230V. Add an LED light and a single outlet circuit and I am just going to run 4AWG AL and call it a day.

I am sure there is some academic value in all of this, but I cant account for every possible load situation or I will never get anything done.

Does anyone see a real reason to run 2AWG AL?

Myself, I'd just run some copper 10 guage UF and call it a day. Good for 30 amps.

Unless you plan on needing that feed to power other things, why run such a big wire?

 

[Innovate1]

Myself, I'd just run some copper 10 guage UF and call it a day. Good for 30 amps.

Unless you plan on needing that feed to power other things, why run such a big wire?

I was thinking the same thing. Unless there are plans to add a lot more loads...