Does maximum amperage for a breaker change when using a 220V outlet?

Question

For some context, I am planning to move some workshop tools into a storage unit.

The unit manager says that the maximum amperage on the outlet is 30A, and that they're wired for 120V. I was told they could re-wire the outlet for 220V to suit my needs.

The machine I'm looking at is 3 horsepower, which at 220V tells me that it would be suitable to run on a 15A outlet.

The technical specifications for the machine suggest running it on a 30A circuit. This would make sense to me, but only if it were a machine running on 120V.

Power-wise I would believe that a 120V/30A outlet converted to be 220V (and presumably ~15A) would be sufficient for this machine, but the specs have me scratching my head and believing I am missing some piece of information.

My question then, do power breakers only really care about amperage, and if I were to have the unit manager get an electrician to re-wire the outlet to be 220V, it would still have a maximum amperage of 30A despite doubling the maximum wattage? I.e. could I run my 3hp 220V machine on this outlet once rewired?

Answer 1

One key, which seems to be missing here, is that there are no 220V receptacles typically in the US. Current voltage is nominally 120V for a hot-neutral circuit and 240V for a hot-hot circuit (which may also include neutral to allow for both 120V and 240V - e.g., for a typical clothes dryer). So 220V just doesn't fit. Any modern equipment designed for 220V is really designed for a range that includes 220V and 240V and a little higher and lower.

There is another variant, primarily in commercial buildings but sometimes in residential (e.g., apartment buildings): 208V. In this case, the nominal voltage is 120V hot-neutral but 208V (not quite double) hot-hot. Most, but not all, motors and other devices designed for 220V/240V can also handle 208V (or a little below).

The interesting twist here is 30A. That is unusual for a 120V circuit in the US, but not unheard of. But since wire size depends on current, once you have wire in place for 30A (typically 10 AWG) it can be easily switched between 120V and 240V.

Replace the single breaker with a double-breaker and:

• If using a /2 cable, connect black and white to the breaker, mark the white on each end to indicate it is a "hot" wire and replace the receptacle with a 6-30.
• If using a /3 cable (i.e., planned ahead for 120V/240V use, or perhaps used that way previously), connect black and red to the breaker, white to the neutral, and replace the receptacle with a 14-30.
• If in conduit, add a second hot wire, connect both hot wires to the breaker, white (if used) to the neutral and replace the receptacle with a 6-30 or 14-30.

If in conduit, they really could give you more than 30A if you need it. Which means either they are using cables (and then there would be a significant cost to run a new cable) or they have other reasons to limit total power usage.

Now we get to the question of your device. While 3 HP should be fine with 240V @ 15A, there may be other factors such as startup current involved. Assuming the device is properly listed (e.g., UL or ETL), the manufacturer instructions should be reliable. But sometimes instructions are not clear, or inadvertently mix up 120V vs. 240V requirements. (There was a cooktop question recently where the instructions did not match the requirements as marked on the device.)

So check with the manufacturer, or provide a link to the manual and we may be able to provide more information.

Answer 2

The machine I'm looking at is 3 horsepower, which at 220V tells me that it would be suitable to run on a 15A outlet.

That only works in watts. However, we provision circuits in VA. VA is volts x amps. The simplest way to say it is that AC power arrives in a "sine wave". VA is the whole sinewave that must be delivered. "Watts" is the part of the sinewave the machine actually uses. For wire thermal considerations, VA is relevant and not watts.

The technical specifications for the machine suggest running it on a 30A circuit. This would make sense to me, but only if it were a machine running on 120V.

Because NEC is a practical document, and motors are weird.

You need to refer to NEC Article 430, table 430.248 I believe, Full-Load Currents in Amperes, >>Single-Phase<< Alternating Current Motors.

3 horsepower ---> 230V ---> 17A

Not sure why motors are still specced at the old voltage, but like I say, motors are weird.

Most things require a 125% derate which puts us at 21.25 amps - too much for a 20A breaker and needing 25A at least.

Here you must also follow NEC 110.3 "follow the labeling and instructions". Those are not "suggestions" (unless they say they are).

The NEC motor rules allow astounding oversizing of breakers to avoid nuisance tripping. It's honestly too much, so the UL approved instructions and labeling will limit that further, and yeah, that overrides NEC.

My question then, do power breakers only really care about amperage, and if I were to have the unit manager get an electrician to re-wire the outlet to be 220V, it would still have a maximum amperage of 30A despite doubling the maximum wattage? I.e. could I run my 3hp 220V machine on this outlet once rewired?

Yes. Really.

This is why AC power won the War of the Currents. You can 5x wire's capacity simply by going 5x voltage (up to wire insulation limits). DC could not do that, so Edison's DC installations could not scale.

Answer 3

If the owner is willing to re-wire to suit your needs, you probably don't need to worry too much about the details. Just tell them what you need, and give them your machine's written specs.

If your machine requires a 4-wire cable (requires neutral) you'll need to run a new cable to the unit, so it doesn't matter what's there now. It probably doesn't need that, so the cable that is there now is hopefully suitable for 30A (since there is a 30A breaker on it, we can hope) and in that case the cable can be used at 240V for 30A. A cable capable of 30A does not need to be thought of as a 15A cable if 220V is applied. The cable is still good for 30A.

So you will hopefully be in good shape. The hardest part, running a new cable to the unit, may not be necessary. You'll need to put in a new 2-pole breaker and a new outlet. If they said they are willing to make changes, they can't be anticipating less work than that.