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We are putting a sign and several lights in our parking lot and need a 60 amp panel to be located 400 feet from the breaker box. Allowing for the voltage drop, what size wire do we need to run?

Thank you. North America is the location. The sign is 120 volt, 20 AMP in a dedicated circuit There will be three 120 volt, 30 watt lights, one 115 feet from the panel, one 100 feet from the panel, and one twenty feet from the panel. We have considered setting another pole but would rather not

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    Unfortunately the information you are giving us really doesn't mean anything. In order to run a proper voltage drop calculation. We need to know the voltage being used, the number of fixtures and their power information, and a site plan or description giving us the distance between the fixtures. Commented Mar 10, 2021 at 13:11
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    240v? Single Phase? What is the actual amperage of the load? North America? Commented Mar 10, 2021 at 13:11
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    Ok 3 30w lights but what about the sign? The 3 lights draw under 1 amp why do you think you need a 60a panel for a 120v circuit? Most lighting today uses universal ballast or drivers for led or fluorescence so far a single 15a branch circuit with a disconnect at the sign will be more than enough. Unless the sign is also going to be a homing beacon for the space station.
    – Ed Beal
    Commented Mar 10, 2021 at 15:51
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    Do your lights have to be 120V? Because at these distances, running at a higher voltage will definitely save you a lot of money in wire. Parking lot lights are very often higher voltage for exactly that reason, so it shouldn't be too hard to find some.
    – Nate S.
    Commented Mar 10, 2021 at 16:46
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    Who told you a 60A panel was required? Commented Mar 10, 2021 at 17:00

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Do we care about money here?

An oddity of wire economics is that 2-2-2-4 "MH Feeder" (90A; 100A if service) is about the same cost as 10-3 UF-B cable (30A). The difference is aluminum, which is absolutely fine for large feeders of #2 size. (not so good for #10, but not our problem).

Big big problem: your loads are 120V

Given your application, the power usage is 120V and wildly lop-sided onto one leg. This means you really need to do your voltage drop calculations on 120V not 240V. Which really throws a wrench into it, unfortunately.

The 120V load is probably at least 10A (since it's demanding a 20A dedicated circuit) but should not be more than 16A continuous as that's too much for a 20A circuit.

And when we consider voltage drop at 120V, that's a very grim picture. The minimum legal size is #10 copper, but that's right out, giving 11% voltage drop at 16A. We have to go clear up to #6 just to get 4.71% drop, but that makes more sense since it's much more expensive than the MH feeder I mentioned earlier.

If we go with the 2-2-2-4 MH feeder, we're at 3.13% which is completely acceptable anywhere but maybe Canada, and even there it's within the margin of error, for Pete's sake.

The aluminum feeder will work fine since we are going panel-to-panel, so we're running between lugs that are either made for aluminum, or are aluminum.

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Assuming since this is commercial that it is probably 208v 3Ø, and that whoever told you you needed a 60A panel knew that signs and lights were rated as continuous loads, and they told you 60A panel based on 125% of actual load, and your voltage drop tolerance is 3%, then you are looking at about #2 awg copper or 2/0 aluminum.

But there's a lot of assumptions there.

Edit: Your update changes things. Serious explanation is needed for why anybody would specify a 60A panel!

120v, 16A, 400ft, 5% loss fed from service panel, gets you #6 copper. You shouldn't need more than #12 for the 115 Ft. feed from the sign to the lights if installed on opposing leg of 240v feed.

If the sign will be able to be turned on separately from the lights then you would need to calculate your voltage drop on the singe 120v sign circuit. If the sign company specs 20A circuit which would allow 16A I would be hard pressed to calculate at less than 16A on the 20A circuit.

If you are feeding the sign from a service panel then you could easily use voltage drop allowance of 5%, but if fed by a subpanel you may have to consider some additional loss feeding that panel and increase one size or more. The NEC recommends 3% loss on feeders, and 5% total loss including branch circuit wires. Canada has some more strict rules.

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  • In commercial you can't assume 208V. Commercial voltages in the US are a dog's breakfast of 240 "wild leg", 120/240 split phase, 120/208 wye, 277/480 with stepdowns, and even 480 delta (which I have in abundance). Commented Mar 10, 2021 at 16:43
  • @Harper-ReinstateMonica Thought I covered that with "But there's a lot of assumptions there". Couldn't leave parameters empty in calculator, had to make some assumptions. Commented Mar 10, 2021 at 16:55
  • Yeah, that's true. The bigger problem (very typical on long-distance feeds of small ampacity such as 30A) that OP's 120V loads are not gonna be balanced. An extremely lopsided load will only load 1 hot and the neutral, and will suffer voltage drop as if it were 120V. The 60A panel is because, well, actually 60A panels aren't even a thing anymore because no PoCos are left still giving out 60A service. Subs these days start at 70A because of #6Cu wire ampacity at 75C. Commented Mar 10, 2021 at 22:11
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You really don't need a 60A subpanel. You generally don't (but technically could) put in a subpanel for 20A. 60A is a standard size and the usual minimum size.

In fact, you may not need a subpanel at all. However, you likely do need a shutoff switch (which the main breaker in a subpanel provides). You can also, generally, only run one circuit of a given type to a separate location. Since having a separate circuit breaker for the sign vs. the lights is beneficial (so that if the sign is undergoing maintenance the other lights will still work), a subpanel logically makes sense. There are grounding and other requirements which apply once you install a subpanel.

However, your stated loads are not anywhere near 60A. In fact, your actual load is probably way below 20A! The sign requires a 120V 20A circuit. That should mean actual load load is 16A or less. My guess is, unless this is a HUGE sign, that the actual load is far less than that. The other lights can all share a single 15A or 20A circuit because the total usage is < 1A!

That being said, having some extra room for future expansion - e.g., more lights, security cameras, etc. is not a bad idea. Keep in mind also that 3% is, in most cases, a guide and not a strict rule. Speaking of security cameras: If you need data for cameras, signage (e.g., if the sign is really a giant computer screen), etc. then running a second conduit for fiber is a very good idea. Communications and power can share a trench but they can't (easily) share a conduit. At 400 feet from home base, I wouldn't rely on WiFi, and cellular costs will add up over the long term - a wired (fiber at this distance) connection can make a lot of sense.

How you actually run the wires is also a big factor. There are, generally, 3 options:

  • Direct bury cable - This may seem easiest, but has the catch that any future upgrades will require trenching again.
  • Conduit - This is higher initial cost, because you are paying for conduit and wires, but allows future upgrades relatively easily & inexpensively, provided the conduit is large enough.
  • Overhead cable - This avoids trenching, but the cost of installing poles (unless there are some already in place that you can legally & safely use) is significant.

Using this calculator:

  • 16A, 3% drop = 6 AWG
  • 16A, 4% drop = 8 AWG
  • 10A, 3% drop = 8 AWG
  • 8A, 3% drop = 10 AWG

However, as Harper has pointed out, this is based on 240V - i.e., a balanced load. With an unbalanced load - e.g., 16A on one leg, 1A on the other, the voltage drop really should be based on 120V. Which gives you:

  • 16A, 3% drop = 3 AWG
  • 16A, 4% drop = 4 AWG
  • 10A, 3% drop = 6 AWG
  • 8A, 3% drop = 6 AWG

which is much bigger wire. Which really means: Move to aluminum. Which also means: Consider the specific feeder cable that Harper suggested.

On the other hand, a lot of modern lighting, including LEDs, can work just as well at 240V as at 120V. Your sign might work at 240V too! In fact, if the sign actually requires 16A @ 120V (which we're guessing based on the 20A circuit), it will only need 8A @ 240V. *heck with the sign company about 120V vs. 240V, and also find out the actual power requirement, not just the recommended circuit size.

So if you really don't expect to have much more than what you already described, plus perhaps a few more LED lights added later on (actually, quite a few would still be under 10A), you can run 8 AWG wire and be perfectly fine.

On the other hand, if you actually had a 50A draw (which is possible on a 60A panel - e.g., if you connect a big electric car charger) then you would need 4 AWG just to stay within 5% drop!

One option is to use a decent size conduit - e.g., 1" - and 8 AWG wires. Then if you ever need a significant upgrade, you can replace the wires. You may even be able to get away with 10 AWG, if you check first with the sign company to find out how much power the sign actually uses.

Note that the breaker in the main panel for the feed to the subpanel needs to be sized based on the wires. The subpanel can be 60A (or 100A or whatever) but if you use, for example, 10 AWG wire, then you need to have a 30A breaker to protect that wire.

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