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I have an old conveyor dryer, which my office purchased used some time ago, just sitting in storage. The dryer is for silk screen t-shirts and has a large heating element and a variable speed conveyor that runs underneath, rather like a small pizza oven. I'm interested in getting it running, but I'm perplexed about the plug and circuit requirements.

The panel on the side of the dryer indicates that the full load amp is 30 amps. So why would the plug be a NEMA 14-50R, which handles 50 amps?

All the 220 outlets on the building have 30 amp breakers and run 10 gauge wire. Will I need to have someone install new breakers and run 8 gauge just for this dryer even though it says it won't pull more than 30 amps?

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On the machine does it have a WATT'age rating? This will clear things up. Also-is the 50A fuse original or is it a plug in cord like kettle lead to computer that is additional? Also some photos maybe would clarify things – ppumkin Jun 15 '12 at 8:01
This might help. NEC 2008 430.22 Single Motor. (A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1). 30A * 125% = 37.5A. 80% of 30A breaker is 24A, 80% of a 40A breaker is 32A, 80% of 50A breaker is 40A. – Tester101 Jun 15 '12 at 12:22
up vote 8 down vote accepted

30 amps is where a 30 amp breaker should cut off, and to avoid that, you should only use 80% of the breaker's capacity. So for a 30 amp breaker, you shouldn't be using more than 24 amps. Using a 50 amp breaker and a dedicated outlet ensures that you don't exceed the capacity of the circuit with that appliance.

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The 50-amp circuit also ensures that the circuit can handle any transient startup jolt; you see these in electric motors, not so often with heating elements. – KeithS Jun 14 '12 at 14:24
@KeithS I beg to differ. a 30A rated device should never exceed 30A at any point in its cycle.. otherwise how would you match sensitive breakers to it. It is usually the max wattage + 20% for safe rating. So its possible the device only uses a 5Kilowats and 25Amp fuse is absolute minimum with "jolt" taken into account- it should never burn out or trip breakers. – ppumkin Jun 14 '12 at 22:12

The fact that a 30-amp device is shipped with a 50-amp plug shouldn't bother you at all.

The most likely reasons for the "overkill" plug are the following:

  • the manufacturer decided that it won't hurt to have a better ampacity plug - some extra reliability just in case and it doesn't really cost much
  • the manufacturer used 50-amp plugs for all his high-power devices just to simplify the logistics (also see this similar question on Electronics SE).
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If it's a piece of industrial/commercial equipment, then the odds are high that someone replaced the plug or entire cord at some point in time. This type of equipment gets modified all the time, and not necessarily the correct way.

I just went back and re-read your question...a 50 amp plug is probably appropriate for a 30 amp equipment...I wouldn't run the circuit at full capacity, even if it is a dedicated circuit.

I would rely on the legend plate for the accurate power requirement spec and not the cord end. If you are not confident enough to switch the plug your self to one that is compatible with your supply outlets (and meets the spec for the equipment) then an electrician should be able to do it for a very reasonable price (and he could inspect the used equipment for other electrical safety issues at the same time).

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making the 50amp redundant because it would never fail.. so if the mahcine exceeds 30amp-- say 45amps(a nice wattage increase) due to a micro short the plug would not fail turning the cable into a nice heatin element, causing a fire – ppumkin Jun 14 '12 at 15:56

Besides other possibilities some already mentioned in this trend, given that it was bought already used, another possibility is that where it was working originally they had installed a 14-50R and preferred to change the male plug; or, maybe they were not using a dedicated circuit and the fuse would blow or the breaker open and they decided to change fuse and breaker from 30 to 50 and then the plug too.

There are a good number of possible speculations about the matter, but the practical point would be, is what I would do, to connect the dryer with a 14-30P cord, but without working load for a work cycle and see if the breaker trips, if no problems are seen then turn it on again now with full working load and wait and see. If there are still no worries, there is no need to change anything more than the plug; if with or without working load the fuse and or breaker open, then the machine needs a careful examination of its connections, motor and wiring, looking for shortcuts or burned up electric elements, or mechanical problems that could be overloading the motor. As ppumkin said, a device marked to draw 30A maximum should not at all run over that, so to re-wire in 8 gauge and install 50A breakers would just be inviting catastrophe if there's some problem underlying, or if there isn't any would be an idle and wasteful expense.

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Your worries are correct here indeed. Why is there a 50amp in there? And I will tell you that it should not be there! In no home appliance rated for single phase @ 220v have i seen a 50amp fuse! You might as well stick a paper clip in there or piece of wire to bypass the plugs fuse.

50Amps = 11000WATTS! What the?

30Amp I have seen in like hot water boilers and those always run on dedicated 5mm solid core cable with 30AMP breaker, isolater and internal fuses.

So I would suggest to drop the 50amp to 30 amp.. or even 25amp.. but dropping it to low could blow it on start- as start always take a massive spike in electricity.

Either way if your main 30Amp is not tripping then the 50Amp is redundant because it will never fail.

What fuse is INSIDE the machine is the question to be asking! :-)

In the end plug it in and run it WITHOUT any other appliance on that breaker line!

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"In no home appliance"... I don't have the impression that this is a standard home appliance. – BMitch Jun 14 '12 at 15:35
Home appliances plug into normal home sockets. If it has a non normal socket such as a round one with 3/4/5 pins then we are talking about industrial standard appliances that run of dedidcated phases. A whole house runs of 1 Phase @ 220Vol.. An industrial water pump runs of 3-5 phases! – ppumkin Jun 14 '12 at 15:50
All the 220 outlets on the building have 30 gives me the impression he is not talking about industrial standard appliance but large home appliance – ppumkin Jun 14 '12 at 15:51
As much as I'd like to defer to your expertise since I don't have industrial experience, two things bug me: 1. they make 50amp 220v plugs so they must have a purpose somewhere, and 2. this device already has that plug so someone already thinks it's appropriate. – BMitch Jun 15 '12 at 1:59
@ppumkin -- in the US, electric ranges run on 50A circuits, and can push those circuits to their normal operating limits when running full-blast. – ThreePhaseEel Mar 4 '15 at 23:56

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