How do you figure amps/load on various extension cords?

EliteRetard

Diamond Member
How does it affect house wiring? I'm talking about all standard 120v US wiring using typical coper (15 amp outlet & breaker).

I see many 100ft 12 gauge extension cords claiming 15 amps, but what if house wiring is 14 gauge?
I see 200ft 10 gauge also claiming 15 amps, but again wouldn't the 14 gauge house wire be a bottleneck?

There's always going to be losses, even if the cord can handle the heat how can it still pull 15 amps from a 15 amp circuit?

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I know they always say never to chain extension cords, and yet practically anybody that's used one has done it before.
How do you calculate a safe load when you do need to connect 2 extension cords? (any size/gauge/length)

In my case I need to go 150ft but only have 2x 100ft 12 gauge, it's certainly not safe to pull 15 amps anymore.
I can't seem to find any information on how to figure what a safe load would be for such a scenario.

Lots of "oh well it depends" answers, but I don't think that's true at all. It's either safe or it isn't.
There should be a safe number of amps to pull no matter what it is that's being plugged in.
Once I know the safe amp draw, then I can determine what items are safe to plug in.

If I had to just guess up a calculator I'd say something like this:

When chaining cords, use the lowest amp/ft cord and divide by total length
-1 amp for each extension cord (14 amp max no matter the extension cord)

Problem is I don't know if electricity works linearly like that or if it covers edge cases well enough etc.
Probably shouldn't attempt chaining or heavy loads if you don't know what a cord is rated for.

In my case, with 2x 100ft 15 amp, my guesstimate would be:

.15 amp per foot, same cords and length, so just divide by 2.
2 "plugs" -2 amps

That would give me a total "safe" draw of 5.5 amps.

Let's say I got another cord instead that was 50ft, 15 amp rated.

.15 amp/ft still lowest, 100ft is 0.6666~ of total 150ft, 15amp x 0.6666~ = 10 amp
Still 2 "plugs", -2 amp

Here the guess would be about 8 amps safe max draw.

But would that be true if the 50ft was 14 gauge?
What if it was a cheapo cord that was 16 gauge?

Would it still be safe to pull 8 amp from a 100ft 12 gauge + 50ft 16 gauge?
Would the order in which the cords are connected matter?

I assume larger first would be best, but then house wiring is usually 14 gauge and comes first anyway.

PowerEngineer

Diamond Member
Generally, the amp rating on a wire or extension cord is related to the amount of heat that the rated current will generate because of the resistance in the conductor. The resistance of the conductor is determined by the inherent resistivity of the material used (e.g. aluminum, copper), directly proportional to the length of the conductor and inversely proportional to the cross-sectional area of the conductor (i.e. its gauge). The amp rating is essentially saying that your insulation is capable of dissipating the heat with that much current flowing through it without melting. It does NOT mean that the current cannot exceed that rated value - you just do not really want it to.

So, if you daisy chain two 15-amp 20-foot extension cords, the resulting 40-foot cord still has a 15-amp rating.

The problem with daisy-chaining extension cords is not with the amperage itself, but in the voltage drop that you will see at the end where you plug in your load. A 16-gauge copper wire has about 4 ohms per 1000 feet, which means a 50-foot 16-gauge extension cord has 0.2 ohms. If your load wants to draw 10 amps, then you can expect a 2 volt drop. If you daisy-chain out to 200 feet then the voltage drop is headed to 8 volts. Many loads (especially motor loads) do not run efficiently when the voltage they see drops too far, and often they draw more current at lower voltages. That 8 volt drop could easily become 10 volts or more.

If we redid the above example with 10-gauge (1 ohm/1000 ft), the 200-foot voltage drop is only 2 volts. Or if instead we reduced to current to 2.5 amps, then it would be 2 volts too.

What you really need to do is consider the amount of current need to run the load you want to put at the end of the extension cords in order to know what gauge you need.

VashHT

snoopy7548

Diamond Member
As an electrical engineer, I often find that the simplest solution is the best one. Here you go:

PowerEngineer

Paperdoc

Platinum Member
OP, your calcs are complicated. The simplest answer is to rely on the labels.

The limiting factor is not the current-carrying capacity or the heat generated, really. As PowerEngineer said, it's the voltage drop from a long cord, and what that does to the load at the end. And actually, the same rules apply to house wiring. We DO use 14 Ga wire for a 15 A circuit, be it in-the-wall stuff in the house or an extension cord. But one rarely sees a house wiring circuit that runs over 50 Ft of wire from breaker panel to the end, and almost never over 100 ft. If you run a buried cable from your house panel out to the garage for power there, you would normally use the same practice of a heavier cable for that because it is longer than an in-the-house run. As far as heat generated goes, think of it this way. A 200 ft cable has twice the resistance and hence twice the heat generated and lost, but the heat is spread over twice as long a cable, so the heat PER FOOT is the same, and there is NO "hot cable". However, as Power Engineer says, the VOLTAGE drop across a 200 ft. cable is twice what it would be for 100 ft of the same gauge, and at heavy current near the rated max that amount can be significant for certain loads - especially motors! When a motor turns a bit slow because its voltage supply is reduced, the motor itself actually acts like its own "resistance" is slightly less so it draws slightly more current, and THAT can increase heat generated inside the motor. This effect gets worse if that motor is slowed down further by a heavy workload. Trying to START a heavy motor under heavy load (like a compressor) with light-weight wiring is VERY hard on a motor!

SOME loads are less worry this way. For example, I have a cord about 150 ft. long I string out from the house to my back-lane parking spaces in the winter for four engine block heaters (nominally 400 W each) on a timer, plus three floodlights (100 W each). That's 1900 W, or 16.5 A at 115 V supply, over the limit of the 14/2 Ga 150 ft. cord. More recently I switched to LED floodlights that only use 17 W each, so its down to 14.3 A. But it works. Note that ALL of those load items are basically simple resistive load (NO motors) that can function very well at slightly reduced voltage. BUT when I need to use that line briefly to plug in the electric starter on my snowblower, I make sure to disconnect some items temporarily - it's a small motor, but I don't know its current draw under load. Same thing if I'm using a shop vac to clean a car, or a power washer - MUST disconnect all other "regular" loads on that extension cord. In the summertime to reach the furthest corners of our lot with my elecric lawn mower I end up stringing two 100 ft 14/2 Ga cords together for only that far space. But the max load of that mower is maybe 11 A, and it's working that way for 30 min or less, NOT under continuous heavy load.

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Diamond Member
I think all the above posters covered it beautifully. There are lots of online calculators that will cover both voltage drop over a length of cord, as well as heat generation across a length of cord for a given current.

If you are doing a short term load, eg using a 12 amp circular saw for a 50% or less duty cycle (1 min on, 1 min off) then you probably will never even need to worry about it. If you are trying to run an air conditioner at 80% duty cycle, then I'd much more worry about overheating your cord. Lucky for you, as temperature increases in a conductor, so too does the resistance and current loss, so you'll probably trip a breaker before you cause a fire inside the home itself.

Exterous

Super Moderator
Came for the "keep plugging things in until they stop working or your house burns down and make note of the amps" type answers and left disappointed by the actually helpful ones. What happened to you ATOT?

uclaLabrat

highland145

Lifer
just keep plugging things in until they stop working or your house burns down and make note of the amps.

snoopy7548

Exterous

Super Moderator
just keep plugging things in until they stop working or your house burns down and make note of the amps.
I appreciate the effort

uclaLabrat

VirtualLarry

No Lifer
just keep plugging things in until they stop working or your house burns down and make note of the amps.
works for me... j/k.

These things concern me a little bit. I've got a 240V metered PDU plugged into an ostensibly 20A 240V -capable A/C outlet circuit, and from there, most of my mining rigs. The meter reads 9-10A on the 240V. (For four rigs.)

Paperdoc

Platinum Member
Virtual Larry, that should not be any problem. Amp ratings and volt ratings are separate issues - there is not an overall Watts rating limit. So the 10 A max current is entirely acceptable in any cord rated for 15 A. Separately, then you need to know the Volt rating limit for the cords, and ensure that is met.

Now, even that depends on details of that power line. You MAY have a special outlet that supplies only 240 V AC up to 20 A, with a special outlet fixture, to feed a PDU that deals only with 240 VAC and thus has a special plug on the end of its cord to match the special wall outlet. But another more common way to do something like that in North American households is what is called a "Split Duplex" outlet. The common wall outlet consists of two actual plug sockets in a "duplex" arrangement, one above the other. The construction of that fixture in the box connects both outlets to a single power supply line in parallel, so both have 120 VAC available fed from ONE Hot side of the breaker panel. For that the combined max amps is the rating of the breaker, normally 15 or 20 A. But the alternative is to break off a small connector bridge between the two outlets on their Hot sides, making them separate electrically. Then the box and fixture can be fed from the breaker panel by a duplex breaker that draws power from both of the panel Hot buses and feeds both of those separately to the duplex outlet on a 3-wire (plus bare Ground) cable with TWO hot wires - Black and Red - and a common Neutral (White). At the now-Split duplex outlet, the two Hot lines are connected to the separated outlet sockets, and they share the common Neutral. Thus EACH of the two duplex sockets becomes a single 120 VAC source limited to the amps rating of its own half of the duplex breaker back in the panel - typically 15 or 20 A EACH line. So in that one outlet box you have TWO individual circuits to use, but only one outlet per circuit. The intent is NOT to supply 240 VAC to something. It is to supply two 120 VAC lines to separate devices. So, although there is a 240 VAC voltage between the two Hot lines in the cable from the breaker panel to the outlet box, each outlet supplies only 120 VAC to whatever is plugged into it. The most common use of these is in places where you might have several power consumers close together and plugged in, possibly for simultaneous use. An example is over a Kitchen counter with many appliances like a mixer, toaster, microwave oven, electric frying pan, heavy-duty blender, etc. Or in a workshop with several power tools. In your case, VirtualLarry, your PDU may have ONE cord fitted with a normal 3-prong 120 VAC plug on the end going to ONE of those outlets, so it is working only on 120 VAC power from ONE of the outlet circuits, and is limited to 15 or 20 A total to the PDU and its loads. It is also possible - I don't know design details of what that unit is - that is it designed to use that same concept and connect to BOTH Hot lines plus Neutral from a Split Duplex outlet (or even a special-design outlet) and then internally to distribute 120 VAC power to two separated groups of normal 120 VAC 3-prong outlets. If designed that way, then each separate PDU outlet group is limited to the rating of its breaker of 15 or 20 A. Either way, IF the items you plug into this device all have normal 3-prong 120 VAC 15 A plugs on their ends, then they all are using only 120 VAC, and the VOLT rating of their cords needs only to be suitable for that Voltage.

The only time you would need to be concerned about the Volt rating of the cords plugged into that PDU is if they have very different plugs designed for use on 240 VAC circuits. THEN the cords, also, need to be rated that way.

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VirtualLarry

No Lifer
In your case, VirtualLarry, your PDU may have ONE cord fitted with a normal 3-prong 120 VAC plug on the end going to ONE of those outlets, so it is working only on 120 VAC power from ONE of the outlet circuits, and is limited to 15 or 20 A total to the PDU and its loads.
No, it's explicitly a 240V outlet, I looked it up. It's for a dedicated A/C circuit. They come in 120V or 240V varieties, with the 240V being more efficient.

dank69

Lifer
just keep plugging things in until they stop working or your house burns down and make note of the amps.
Trip the breaker = 1 too many.

highland145

EliteRetard

Diamond Member
Thanks for all the responses so far.

I don't have much else to add right now.

skyking

Lifer
There is only one solution to a question with so many variables. Do practical load testing.
Given X number of volts at no load, test the cord combinations for voltage drop under load. You can also test the voltage at the outlet under load. That will give you quantifiable data to work with.
Next: everybody had one of those infrared thermometers. Do heat rise testing, particularly at the blade connections of the extension cords. Those are some really crappy connections, BTW. Condition of the cords and connections comes into play.
As voltage drops, amperage rises, temperature of the cords and connections rise.
Motor loads are inductive, and also feature starting spikes. Resistive loads such as space heaters are constant, but once again the voltage drop-amperage rise- heat rise cycle quickly gets out of hand.

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Paperdoc

Platinum Member
VirtualLarry, rather than diverting this thread, I sent you a PM.

Fritzo

Lifer
Plug it in- if it doesn't spark or melt it works.

JEDI

Lifer
100ft 12 gauge extension cords claiming 15 amps
200ft 10 gauge also claiming 15 amps, but again wouldn't the 14 gauge house wire be a bottleneck?

the 14gauge house wire/15amp breaker at the house wouldnt be a bottleneck.
gauge of the wire is for heat dissipation.

the longer the extension cord, the more heat produced due to resistance.
to compensate, the manufacturer uses thicker copper.
the thicker the copper, the less resistance.

so daisy chaining two 100ft 12 gauge cords will produce more heat than rated.

if you see the extension cord smoking, STOP WHAT YOU ARE DOING and turn off the device.

but 12gauge should be fine even at 200ft.
12gauge is rated for 20amps.

the problem is daisy chaining cheap extension cords that use 18 gauge wire.

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highland145

Paperdoc

Platinum Member
Jedi said, "If you see the extension cord smoking, STOP WHAT YOU ARE DOING ". Good advice. However, the cause of that is not "daisy-chaining" together two 100 ft extension cords. It IS true that doing that creates twice as much resistive loss (converted to heat) as only one cord of 100 ft would do, BUT that same double heat is distributed evenly over twice as long a cord, so the termperature rise it produces in each cord is exactly the same as for one cord of 100 ft. The real impact of such a long cord is on the reduced voltage available at the end for the load device, and how that affects ITS performance and possibly ITS internal heat generation.

I fully agree with Jedi's last statement, "The problem is daisy chaining cheap extension cords that use 18 gauge wire." And there the source of the problem is using a smaller-gauge wire that is NOT rated for the amps load.

PowerEngineer