100amp subpanel going to shed 135ft away from main panel. Is voltage drop a concern?

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JohnDS

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So this is actually for my own shed. I'm wondering if I should even bother worrying about voltage drop. And this depends if I need to upsize the 1¼ pipe to shed. Please advise. Thank you. John.
 
Sorry, I was in a rush.

Ok, so I'm running a 100a subpanel. Pipe is not in. Trench is open. I dont know the load. I was going with 3awg and then 1¼ pipe. According to my calculations:

CMVD = 2KIL
VD = 2KIL / CM
VD = 2(12.9)(100)(135') / 52,620
VD = 6.62

Should I be worried?
 
JohnDS said:
Sorry, I was in a rush.

Ok, so I'm running a 100a subpanel. Pipe is not in. Trench is open. I dont know the load. I was going with 3awg and then 1¼ pipe. According to my calculations:

CMVD = 2KIL
VD = 2KIL / CM
VD = 2(12.9)(100)(135') / 52,620
VD = 6.62

Should I be worried?
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If you don't know the load, how can anyone know if you need to be worried?
If it's just lights & receptacles with an occasional saw or welder, I wouldn't give it a second thought.
 
As Little Bill notes, if you don't know the load you cannot calculate voltage drop.

If you are just guessing at loads, the circuit rating is not an unreasonable guess. The operating loads should be less than the circuit rating, so it is a good place to start.

IMHO unless you have a huge motor on that 100A panel, your voltage drop is fine.

Jon
 
Thanks guys. I dont think it would be an issue either. Understood about not knowing the load, but wouldn't assuming a 100a load cover the worse case scenario. With that said, is this an issue?
 
JohnDS said:
Understood. Wouldn't assuming a 100a load cover the worse case scenario. With that said, is this an issue?
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No but that would be a foolproof way to determine that when at maximum load the conductors are within the design range for voltage drop. If you knew the actual calculated load you could use that instead in your calculation and end up with possibly smaller conductors. In this case with your #3 AWG conductors the point is moot because you cannot use smaller conductors anyway.
 
I always calculate based on the amperage of the breaker. I don’t know if this is necessary but I figure the 100 amp breaker will allow up to 100 amps so I use whatever conductor can get me 135 feet with less than 3% voltage drop. In your case that would be #1 aluminum with a #6 ground or #3 copper With a #8 ground. You could use 1-1/4 inch pvc for either of these. Don’t forget the ground rods at the shed.
 
Doc13067 said:
I always calculate based on the amperage of the breaker. I don’t know if this is necessary but I figure the 100 amp breaker will allow up to 100 amps so I use whatever conductor can get me 135 feet with less than 3% voltage drop. In your case that would be #1 aluminum with a #6 ground or #3 copper With a #8 ground. You could use 1-1/4 inch pvc for either of these. Don’t forget the ground rods at the shed.
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If the loads are unknown then going by breaker size gives you worse case, but may be overkill. If it's aluminum wire, who cares.

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Thanks guys for the help. By the way, has anyone seen the prices for 1 1/4 pvc lol? $16-$18 for a stick lmao. I was going to run two 1-1/4, one for data or cameras or whatever, other for #3s. After seeing the price, I got 1-1/4 and 1" instead. Cost me $400! And #3 thhn is over $1000. What a bummer. I really wanted to put a nice 100amp out there, but now I think I'll return everything and just get 8/3 UF for $2.28/ft.🙄
 
JohnDS said:
Thanks guys for the help. By the way, has anyone seen the prices for 1 1/4 pvc lol? $16-$18 for a stick lmao. I was going to run two 1-1/4, one for data or cameras or whatever, other for #3s. After seeing the price, I got 1-1/4 and 1" instead. Cost me $400! And #3 thhn is over $1000. What a bummer. I really wanted to put a nice 100amp out there, but now I think I'll return everything and just get 8/3 UF for $2.28/ft.🙄
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Use some AL USE and DWV PVC if you want to save some money. If you use DWV get pressure couplings.
 
DrSparks said:
If the loads are unknown then going by breaker size gives you worse case, but may be overkill. If it's aluminum wire, who cares.

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I like worst case so you know you can add to it if need be. Actually I think you have to do it that way. If you calculated the voltage drop based on a lower rating, say 30 amps, wouldn’t you then have to put a 30 amp breaker in rather than a 100? Otherwise you’d be setting up a circuit that would allow for excess VD and someone could inadvertently hook up additional components and fry the circuitry on those additional component. The more I think about it, I would definitely make sure the conductors can handle anything the breaker would allow.
 
JohnDS said:
Thanks guys for the help. By the way, has anyone seen the prices for 1 1/4 pvc lol? $16-$18 for a stick lmao. I was going to run two 1-1/4, one for data or cameras or whatever, other for #3s. After seeing the price, I got 1-1/4 and 1" instead. Cost me $400! And #3 thhn is over $1000. What a bummer. I really wanted to put a nice 100amp out there, but now I think I'll return everything and just get 8/3 UF for $2.28/ft.🙄
Click to expand...
8/3 UF will get you 30 amps at that distance.
 
Doc13067 said:
I like worst case so you know you can add to it if need be. Actually I think you have to do it that way. If you calculated the voltage drop based on a lower rating, say 30 amps, wouldn’t you then have to put a 30 amp breaker in rather than a 100? Otherwise you’d be setting up a circuit that would allow for excess VD and someone could inadvertently hook up additional components and fry the circuitry on those additional component. The more I think about it, I would definitely make sure the conductors can handle anything the breaker would allow.
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Well for a "Fisher Price my first 100' feeder" sure calculate the VD based on the supply breaker, but get into some bigger stuff and you will spend $1000's or tens of thousands more and it will be a complete waste to use the full feeder rating at 3% VD.
 
Doc13067 said:
I like worst case so you know you can add to it if need be. Actually I think you have to do it that way. If you calculated the voltage drop based on a lower rating, say 30 amps, wouldn’t you then have to put a 30 amp breaker in rather than a 100? Otherwise you’d be setting up a circuit that would allow for excess VD and someone could inadvertently hook up additional components and fry the circuitry on those additional component. The more I think about it, I would definitely make sure the conductors can handle anything the breaker would allow.
Click to expand...
Actually no. The NEC does not require us to compensate for voltage drop. It is only recommended. Remember Code does not instruct us on how to make a functional and/or efficient electrical installation.

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