Voltage drop calc, from subpanel or service entrance?

[jvitez]

The physics of voltage drop calculations are perplexing me a bit.

I want to install a subpanel in my attached garage, but also hope to build a detached garage at some point. The size of subpanel I install somewhat depends on whether I run power to a detached garage from the attached garage subpanel, or directly from the service panel, which will also depend on whether I heat with NG or electricity, depending on whether I use in floor radiant or forced air unit heaters, which depends on.......anyway, you get the picture.

Here are the parameters:

1. Service panel to attached garage subpanel: 100 ft. Subpanel to detached garage: 140 ft, so 240 ft. total.

2. Service panel to detached garage directly: 190 ft.

I know costs will vary depending on size of wire, but for this aspect please ignore cost effectiveness. I'm just interested in the physics.

So, in number 1, do I calculate voltage drop using 240 ft or 140 ft, from the service panel or subpanel? My calculations take me over 3% depending on which wire I run. Thanks!

 

[w1im]

The Electrical Engineer (not Electrician) answer is that you calculate voltage drop from the transformer to the end of the line. You can get the voltage at the transformer by measuring at your service panel with no load (all breakers off) and then figure out the voltage drop based on the max current draw. You will need to do do calculations on each different size of wire and each current draw (may be different between panels).

For instance, if the transformer to service panel is 0AWG and is drawing 200A, and the service panel to sub panel is 4AWG and is drawing 50A, then you calculate the voltage drop for each section and add the drops to figure out the total drop at the sub panel.

 

[pattenp]

What are the amp capacities you want at each sub-panel?

Edit: After thinking about it with all the unknowns on the detached garage, you may be better off running both subs from the main service.

 

[jvitez]

w1im: cool! That's the technical type answer I was looking for. Thanks!

pattenp: I'm planning for 18kW to the attached garage, and either 17kW or 8kW to the detached garage, depending on heating energy source. I have 400 amp service to the house, though it's divided into two 200 amp main breaker service panels double tapped from the meter. One panel has fewer heavier loads, the other has multiple light loads, which effects where I feed from.

In hindsight, I wish I would have specified a 400 amp main breaker and a larger panelboard, but the electrician said double tapping was the cheapest way, and how he usually does it.

Whichever way I go, I'll be crying at the cost, and the weight of wire I'll be lugging!

 

[r_olson_06]

If you run #4 copper to the attached it should serve the mininal load, if ran seperate feeders to each from service with about 2% dropped to the attached. And a #3 copper to the detached with 2.75% voltage drop. If ran together a 1/0 copper with a 150amp sub panel with a 1.5% to the attached garage and a #4cu with a 100 amp sub panel in the detached garage.

 

[Aceman]

I'm planning for 18kW to the attached garage

Based on your calcs, I'd recommend #2 AL SER minimum. If you're considering adding more load in the future, 1/0 AL SER may be worth the upgrade.

and either 17kW or 8kW to the detached garage

I'd run from the main panel:
17kw=1/0 AL USE RHH RHW x 3, #4 AL USE RHH RHW ground.
8kw=#2 AL USE RHH RHW x 3, #6 AL USE RHH RHW ground.

We'll usually drop the neutral down one size or two if there's some 240v load in the panels.

it's divided into two 200 amp main breaker service panels double tapped from the meter.

In hindsight, I wish I would have specified a 400 amp main breaker and a larger panelboard, but the electrician said double tapping was the cheapest way, and how he usually does it.

That's how most are done. 400 amp panels aren't used very often, so that makes them expensive.

Whichever way I go, I'll be crying at the cost, and the weight of wire I'll be lugging!

Use aluminum. Copper isn't worth it for this stuff.

You'll see once you price aluminum, it's pretty easy to go with larger wire sizes for not much more money, if you think want to upgrade. It usually runs somewhere around 1/3 the price of copper.

With long runs the savings add up pretty quick!

 

[pattenp]

You can use aluminum instead of copper if cost is an issue. Copper is just very expensive. There's nothing wrong with using aluminum. For the 100' run from main to attached garage feeding a 150A sub would take 3/0 AL and would have around 1.7% drop at 240V. The 190' run from main to detached garage feeding a 100a sub would take 2/0 AL and would have a voltage drop of around 2.58% at 240V. With the same 150A & 100A subs wired one to the next you will need to use 4/0 AL the entire run to keep the drop below 3% to the last sub.

 

[Aceman]

You can use aluminum instead of copper if cost is an issue. Copper is just very expensive. There's nothing wrong with using aluminum. For the 100' run from main to attached garage feeding a 150A sub would take 3/0 AL and would have around 1.7% drop at 240V. The 190' run from main to detached garage feeding a 100a sub would take 2/0 AL and would have a voltage drop of around 2.58% at 240V. With the same 150A & 100A subs wired one to the next you will need to use 4/0 AL the entire run to keep the drop below 3% to the last sub.

I'm curious how come your calcs are calling for such large wire?

 

[pattenp]

I used the Southwire voltage drop calculator and I was a little surprised at the sizes. The calculator doesn't have a lot of options as to wire type and how the wire is run. It probably over states the size.

Edit: Oh.. and I used 150A and 100A as the supplied current to the end of cable. I didn't use the 18kw or 17kw-8kw, your numbers are good for that.

Here's the calculator: http://www.southwire.com/support/voltage-drop-calculator.htm

 

[Aceman]

That explains it. Imagine that, a wire manufacturer recommending oversized wire.

I do all calcs manually, as I've heard the online calculators are always a little "off" on their sizes. On long runs, even tiny fractions of a calc, can mean big money.

 

[pattenp]

I'm too lazy to manually calculate them. I should have stated that I was giving an example using the 150A and 100A.

 

[jvitez]

Thank you gents, very much, for all the helpful and detailed information. Much appreciated!

I checked into the specs of Cu vs Al: 1/0-1/0-1/0 SER Al would be almost half the weight of 2-2-24 Cu. And if it's 1/3 the cost, aluminum it is!

Since I'm unsure of when a detached garage can happen, it also makes more sense to supply the attached garage with what it needs alone, than to install huge cable on a "what if" basis that might not ever be needed.

This forum is awesome. I've learned so much since joining. Thanks again everyone.

 

[pattenp]

Just to make sure you know that you need to run 3 conductors w/ground SER so you need 1/0-1/0-1/0-2 AL SER. That will get you a full 100A to your sub-panel

Thank you gents, very much, for all the helpful and detailed information. Much appreciated!

I checked into the specs of Cu vs Al: 1/0-1/0-1/0 SER Al would be almost half the weight of 2-2-24 Cu. And if it's 1/3 the cost, aluminum it is!

Since I'm unsure of when a detached garage can happen, it also makes more sense to supply the attached garage with what it needs alone, than to install huge cable on a "what if" basis that might not ever be needed.

This forum is awesome. I've learned so much since joining. Thanks again everyone.

 

[jvitez]

Sheesh. The nomenclature of SER cable is tripping me up. Thanks! In my head, I know I need 120/240 for a subpanel, so a 3 conductor plus ground feeder cable is needed. I'll be more careful when looking at product information.