Sub Panel & Voltage Drop

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pridelion

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Hello, I need to install a sub panel. It will be fed from a GE main panel which limits the largest circuit breaker size to a 70 amp 2P. The 70 amp breaker maximum conductor size is #3. Here's the problem: the sub panel will be roughly 165' away from the main panel, and every voltage drop calculation I do it shows that the voltage drop will be too great. (usually ends up being 114.6 to 115.3 V) Is this correct or am I using the wrong V Drop calc. or am I missing something? How will I get this sub panel fed without a crazy voltage drop?
Thank you
 
sub panel and voltage drop

sub panel and voltage drop

Hello, I need to install a sub panel. It will be fed from a GE main panel which limits the largest circuit breaker size to a 70 amp 2P. The 70 amp breaker maximum conductor size is #3. Here's the problem: the sub panel will be roughly 165' away from the main panel, and every voltage drop calculation I do it shows that the voltage drop will be too great. (usually ends up being 114.6 to 115.3 V) Is this correct or am I using the wrong V Drop calc. or am I missing something? How will I get this sub panel fed without a crazy voltage drop?
Thank you
 
Run the size conductor you need, splice it to the max. size conductor the cb will take in the panel.
Done.
 
pridelion said:
Hello, I need to install a sub panel. It will be fed from a GE main panel which limits the largest circuit breaker size to a 70 amp 2P. The 70 amp breaker maximum conductor size is #3. Here's the problem: the sub panel will be roughly 165' away from the main panel, and every voltage drop calculation I do it shows that the voltage drop will be too great. (usually ends up being 114.6 to 115.3 V) Is this correct or am I using the wrong V Drop calc. or am I missing something? How will I get this sub panel fed without a crazy voltage drop?
Thank you
Click to expand...

Splice a short piece of #3 onto the end of each larger conductor to attach to the breaker. That will make it fit and will not change the VD.
As long as there is enough space you can make the splice inside the panel.
 
The voltage drop will change in the amount of current that is being drawn off of the sub panel. If you had a 60 amp load on the sub panel, the voltage drop would be greater than if you had a 5 amp load. If you are worried about the voltage drop because of the length than increase the wire size to the next standard size. There isn't a "maximum" for wire size, there is a minimum. Take a look at 250.122(b) if you are going to increase wire size for voltage drop.
 
How do you do a voltage drop calculation without knowing the actual load? The 3 percent voltage drop on the feeder is an informational note only and not enforceable. Unless you are sure the sub-panel is going to be at or near full load I wouldn't worry about it too much. If you do have a calculated load for the sub panel, often times the actual load is less than 50 percent of the calculated load, depends on the amount of diversity of the loads. Like the previous post states if you increase the ungrounded conductors for any reason you must increase the EGC proportionality.
 
pridelion said:
Hello, I need to install a sub panel. It will be fed from a GE main panel which limits the largest circuit breaker size to a 70 amp 2P. The 70 amp breaker maximum conductor size is #3. Here's the problem: the sub panel will be roughly 165' away from the main panel, and every voltage drop calculation I do it shows that the voltage drop will be too great. (usually ends up being 114.6 to 115.3 V) Is this correct or am I using the wrong V Drop calc. or am I missing something? How will I get this sub panel fed without a crazy voltage drop?
Thank you
Click to expand...
Probably don't need to go any higher than #4 CU or #2 AL. There is no NEC rule for voltage drop. Is the actual load any where near 70A?
 
Pridelion,

Welcome to the forum, we prefer it if you don't double post, it only makes things confusing.

I have merged your two threads into one.
 
pridelion said:
Hello, I need to install a sub panel. It will be fed from a GE main panel which limits the largest circuit breaker size to a 70 amp 2P. The 70 amp breaker maximum conductor size is #3. Here's the problem: the sub panel will be roughly 165' away from the main panel, and every voltage drop calculation I do it shows that the voltage drop will be too great. (usually ends up being 114.6 to 115.3 V) Is this correct or am I using the wrong V Drop calc. or am I missing something? How will I get this sub panel fed without a crazy voltage drop?
Thank you
Click to expand...

Hummm - I'm not seeing doom and distruction.

With the information we have:
System is 120/240?
What is the load on the 70A feeder?
What voltage drop calculation are you using? I've never trusted internet calculators much - can't see what they are doing.​

Just using ohm's law, Table 8 - #3 CU is .245 ohms/1000'.

Resistance for a round trip is 2 x 165 x (.245/1000) = 0.08 ohms

A 70A load on 165 feet of Cu has a Vd of .245 ohms/1000) x ((165 x 2)/1000) x70 = 5.7V

For a 70A load on 120V, 5.7Vd => 4.7%
For a 70A load on 240V, 5.7V => 2.4%

So don't run a 120V, 70 A load with nothing on the other pole. Split the load up - balance it. If you do that, then the each side will only be a 2.4%Vd.

Better yet, as other have said, also calculate the loading. It could well be less than a 100% 70A load.

ice
 
iceworm said:
Hummm - I'm not seeing doom and distruction.

With the information we have:
System is 120/240?
What is the load on the 70A feeder?
What voltage drop calculation are you using? I've never trusted internet calculators much - can't see what they are doing.​

Just using ohm's law, Table 8 - #3 CU is .245 ohms/1000'.

Resistance for a round trip is 2 x 165 x (.245/1000) = 0.08 ohms

A 70A load on 165 feet of Cu has a Vd of .245 ohms/1000) x ((165 x 2)/1000) x70 = 5.7V

For a 70A load on 120V, 5.7Vd => 4.7%
For a 70A load on 240V, 5.7V => 2.4%

So don't run a 120V, 70 A load with nothing on the other pole. Split the load up - balance it. If you do that, then the each side will only be a 2.4%Vd.

Better yet, as other have said, also calculate the loading. It could well be less than a 100% 70A load.

ice
Click to expand...

Hi and thank you. I'm not sure what the final connected load will be. It's a small coffee shop thats relocating in the building. I just wanted to make sure that given the restrictions on the main panel, (70 amp breaker max), that the sub panel and eventual loads willnot be experiencing a bad voltage drop, as I want to get the sub panel work done ahead of time and then do the branch circuits at a later date/as needed.
 
pridelion said:
.... I'm not sure what the final connected load will be. ... I want to get the sub panel work done ahead of time and then do the branch circuits at a later date/as needed.
Click to expand...

That makes it tough. My crystal ball is in a perpetual fog bank - hopefully your's does better. Don't forget, that the best you have available for a continuous load is .8 x 70A = 56A.

ice
 
pridelion said:
Hi and thank you. I'm not sure what the final connected load will be. It's a small coffee shop thats relocating in the building. I just wanted to make sure that given the restrictions on the main panel, (70 amp breaker max), that the sub panel and eventual loads willnot be experiencing a bad voltage drop, as I want to get the sub panel work done ahead of time and then do the branch circuits at a later date/as needed.
Click to expand...
Put an amp meter on the existing coffee shop's panel, that should give you a good idea.

Look at their electric bills.
 
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