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I'm wanting connect up a bunch of Raspberry Pis (micro computers) to one power outlet in my home. At about .5 amps per computer * 40 computer means I will need to draw 20 amps.

Now, I know that I'm only supposed to draw a max of 12-15amps from my standard 120volt North American outlet, but I'm wondering if there is any way to amplify the amperage and then use it to power all the computers, or alternatively, whether there is another way to somehow achieve this that I'm unaware of.

If there is no possible way to do this, what other method for drawing 20amps of power would you suggest?

EDIT: Computers run at 5V DC, I'm using a 5V, 20amp power adapter which I plug into the wall to get that but I don't want to draw the full 20amps because the wall socket can't handle it.

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  • \$\begingroup\$ 0.5 amp at 120 volts is 60 watts. That seems like a lot for a Raspberry Pi computer. How did you arrive at this value? In any case, there is no way "to amplify the amperage" without another source of power. If you really need that much power, you need to use another outlet powered from a different circuit breaker. \$\endgroup\$
    – Barry
    Commented Aug 12, 2018 at 2:36
  • \$\begingroup\$ @Barry sorry, this is at 5 Volts DC, for each computer, Im using a 5v 20amp power converter - I'll specify \$\endgroup\$
    – Matt
    Commented Aug 12, 2018 at 2:37
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    \$\begingroup\$ You are neglecting Iout is not the same as Iin \$\endgroup\$
    – D.A.S.
    Commented Aug 12, 2018 at 2:46
  • \$\begingroup\$ @TonyEErocketscientist could you expand on that in layman's terms for a noob? \$\endgroup\$
    – Matt
    Commented Aug 12, 2018 at 2:48
  • \$\begingroup\$ Power= volts * amps so converted to input is just a bit more so high input Vac means low input current \$\endgroup\$
    – D.A.S.
    Commented Aug 12, 2018 at 3:48

4 Answers 4

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Not a problem. You have 40 devices drawing 0.5 A each at 5 V.

40 x 0.5 A x 5 V = 100 W

So if your AC to DC converter is 100% efficient then you need to draw 100 W from the mains and in the US that is I= 100/120V = 840 mA which is well within the limits.

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  • \$\begingroup\$ Ok so correct me if I'm wrong here - trying to wrap my head around this. Initially I had read that the cable sizing in the walls meant that exceeding 12-15amps was unsafe, but those amperage numbers are actually determined by what voltage you are drawing the amps at. So 12-15 amps is the max IF you are drawing power at the full 120volt amount, correct? \$\endgroup\$
    – Matt
    Commented Aug 12, 2018 at 2:47
  • \$\begingroup\$ The devices draw 500 mA at 5 V. So a single power supply - which steps down 120 V to 5 V draws 21 mA from the mains (assume 100% efficiency). This is because the power from the mains (P = V_{mains} x I_{mains}) is the same as the power to the device (P = V_{device} x I_{device}) \$\endgroup\$
    – D Duck
    Commented Aug 12, 2018 at 2:51
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    \$\begingroup\$ @Matt Power=volts*amps. This means that 20 amps at 5 volts is the same power as 0.8 amps at 120 volts. You can’t get a 100% efficient converter so it’ll be closer to 1 amp. But bottom line is that when you draw 20 amps out of your power converter, you’ll only be drawing about an amp out of your household outlet. \$\endgroup\$
    – DoxyLover
    Commented Aug 12, 2018 at 2:53
  • \$\begingroup\$ @DoxyLover thank you for the easy explanation! That makes a lot of sense \$\endgroup\$
    – Matt
    Commented Aug 12, 2018 at 3:08
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5V 20A means 0.8A at 120V. you'll be OK

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  • \$\begingroup\$ Ok so correct me if I'm wrong here - trying to wrap my head around this. Initially I had read that the cable sizing in the walls meant that exceeding 12-15amps was unsafe, but those amperage numbers are actually determined by what voltage you are drawing the amps at. So 12-15 amps is the max IF you are drawing power at the full 120volt amount, correct? \$\endgroup\$
    – Matt
    Commented Aug 12, 2018 at 2:46
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    \$\begingroup\$ "those amperage numbers are actually determined by what voltage you are drawing the amps at. So 12-15 amps is the max IF you are drawing power at the full 120volt amount, correct?". This is correct, but I suspect not in a sense you think. You always draw power at the voltage of the line you connected to. It is how much power you draw that determines the current: I = P / V. From this equation you can see that the higher the voltage the less current will flow for the same amount of power. That is why you need 20A at 5V but only 0.8A at 120V. \$\endgroup\$
    – Maple
    Commented Aug 12, 2018 at 11:12
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Think about conservation of power. Power in must equal power out, just as you can't mix 100 g of chemicals and end up with 200 g of result. So the first question to ask is: how much power do the raspberry pies need? (volts * amps * count.) The next question it how much power your outlet can supply. (120 volts * 13 amps, or 10 amps if you have old wiring.) Since you have more power available than you need, all you must ask is how to do the necessary conversions. The common answer is a transformer for high voltage or a switch mode power supply for lower voltage or when lighter weight is needed.

Once you account for inefficiency (some output power is waste heat and waste EMF rather than electricity, a safe assumption being at least 75% efficiency), a transformer's output equals its input. Hence 0.75 * 120 volts * 10 amps == 5 volts * 180 amps. You won't easily find a transformer that meets these specs, but no current will flow except the current required by your load. Your load will absolutely not draw 180 amps. You can either use a power supply that can supply enough current, or use more than one without exceeding the current rating of any individual power supply.

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If you draw more than 15 amps for an extended period of time, the circuit breaker will trip. You can not change the circuit breaker because the wire size will not be sufficient to carry more than 15 amps safely. Kitchen counter and bathroom outlets are wired for 20 amps, but it would be quite unusual to find any other circuit connected to a 20 amp breaker. The most likely possibility would be to find that there is space for another breaker in the box and access to wire from the box to a new outlet in a convenient location.

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    \$\begingroup\$ Above is true, but perhaps not the answer to the question asked. \$\endgroup\$
    – D Duck
    Commented Aug 12, 2018 at 2:55
  • \$\begingroup\$ @DDuck: I think this is an answer to the question that was asked - but not an answer to the question that should have been asked. In any case, the OP needn't worry, as his 5V 20A supply won't draw more than an amp at 120 V. \$\endgroup\$
    – Peter Bennett
    Commented Aug 12, 2018 at 3:17
  • \$\begingroup\$ I think he posted this before I added the edit specifying they were using only 5volts. My fault not his \$\endgroup\$
    – Matt
    Commented Aug 12, 2018 at 6:38

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