An outside vendor is trying to convince my company that it is OK to plug a 30 amp rated UPS into a 20 amp branch circuit. "They will Make us a pigtail to convert their plug 30 amp 250 volt to our customers 20 amp 250 volt receptacle" My position is this violates the NEC and I feel would effect the UL rating also. The UPS would supply power to a Medical device that draws 12 Amps.
oveloading a branch circuit
- Thread starter jbhall
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: oveloading a branch circuit
Despite the fact that the only thing they presently intend to plug in is a 12 amp load, there is no way to stop anyone from plugging in more load. The 30 amp UPS is a 30 amp load. I would call this a violation.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: oveloading a branch circuit
If the UPS is listed then it has a 30 amp cord cap for a reason. You would violate the UL listing of the UPS. Also the input to the UPS will be more than the output due to conversion losses, and the input has to sized to charge the battery and carry the load.
And who will control the load on the UPS? 12 A today, 20 A tommorrow, 25 A next year.
If the UPS is listed then it has a 30 amp cord cap for a reason. You would violate the UL listing of the UPS. Also the input to the UPS will be more than the output due to conversion losses, and the input has to sized to charge the battery and carry the load.
And who will control the load on the UPS? 12 A today, 20 A tommorrow, 25 A next year.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: oveloading a branch circuit
A pretty common situation.
I don't see it as a major problem as long as the UPS is there to serve a single device and the end user understands that the branch circuit CB may trip if they add load at some point in the future to the UPS.
In reality, this would seem to be a situation where you can't control what people plug into an outlet so it does not appear to really be a NEC issue, since the NEC "stops" at the outlet. If it was hardwired, I'd say it was an issue, but being plug and cord connected, I say its a problem for the end user to deal with if they ever load the UPS up enough to trip the branch circuit OCPD.
This is not a lot different then someone plugging two hair dryers into the same outlet. Its not going to hurt anything when they turn them both on at the same time as the OCPD will trip to prevent excessive current flow.
A pretty common situation.
I don't see it as a major problem as long as the UPS is there to serve a single device and the end user understands that the branch circuit CB may trip if they add load at some point in the future to the UPS.
In reality, this would seem to be a situation where you can't control what people plug into an outlet so it does not appear to really be a NEC issue, since the NEC "stops" at the outlet. If it was hardwired, I'd say it was an issue, but being plug and cord connected, I say its a problem for the end user to deal with if they ever load the UPS up enough to trip the branch circuit OCPD.
This is not a lot different then someone plugging two hair dryers into the same outlet. Its not going to hurt anything when they turn them both on at the same time as the OCPD will trip to prevent excessive current flow.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: oveloading a branch circuit
I agree that we are not responsible for what the owner or tenant plugs into an outlet. However, I do see one key difference between the proposed installation and plugging in two hair dryers. The difference is that each of the hair dryers has a plug that is designed to connect to a standard 20 amp, 120 volt outlet, and the outlet is designed to accept such plugs. On the other hand, jbhall is dealing with an outlet designed to accept a 20 amp plug, and the vendor is proposing to insert a 30 amp plug, after having created a pigtail ? or what I would call a ?cheater plug? ? to make the connection possible. I think this crosses the line, for the reason I cited, and even more for the reason that Tom cited.
I agree that we are not responsible for what the owner or tenant plugs into an outlet. However, I do see one key difference between the proposed installation and plugging in two hair dryers. The difference is that each of the hair dryers has a plug that is designed to connect to a standard 20 amp, 120 volt outlet, and the outlet is designed to accept such plugs. On the other hand, jbhall is dealing with an outlet designed to accept a 20 amp plug, and the vendor is proposing to insert a 30 amp plug, after having created a pigtail ? or what I would call a ?cheater plug? ? to make the connection possible. I think this crosses the line, for the reason I cited, and even more for the reason that Tom cited.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: oveloading a branch circuit
I hope moderator Dereck picks up this thread he has a lot of experience with UPS, if not you should PM him.
The I have a 120V 30A input UPS. Its designed at 30 amps to power the output loads and charge the battery. Its rated above 20 amps. If there was a power outage and the UPS had to rechage it would overload the branch circuit.
I see it as a problem.
I hope moderator Dereck picks up this thread he
I hope moderator Dereck picks up this thread he has a lot of experience with UPS, if not you should PM him.
The I have a 120V 30A input UPS. Its designed at 30 amps to power the output loads and charge the battery. Its rated above 20 amps. If there was a power outage and the UPS had to rechage it would overload the branch circuit.
I see it as a problem.
I hope moderator Dereck picks up this thread he
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: oveloading a branch circuit
Go to any RV supply store and they can hand you adapters UL listed to change 30 amp plugs to fit 15 amp receptacles.What would be the differance here ?
Go to any RV supply store and they can hand you adapters UL listed to change 30 amp plugs to fit 15 amp receptacles.What would be the differance here ?
Re: oveloading a branch circuit
I was setting around the house tonight and my ears started burning
NEC violation, not IMO. NEC does not have anything to do with what happens after the plug. UL listing problem, yes. Manufacture warranty void, absolutely.
Lets look at operation for a moment. It really depends on what type of UPS it is such as stand-by, interactive, or dual conversion. They all behave differently. For the sake of argument I will assume it is a higher end dual conversion type. The problem you would encounter is when the AC fails and the UPS inverters are supplied by the batteries to supply the load. The batteries would discharge. Once power is restored the rectifiers would go to full current limit to supply the load, and recharge the batteries. The batteries will take all the current that the rectifiers will supply minus the load current. This means the AC input would go to FLA until the batteries are charged.
I think you can see the problem. The UPS would be drawing 30-amps on a 20-amp circuit. It will not work even if you turn the load off.
I was setting around the house tonight and my ears started burning
NEC violation, not IMO. NEC does not have anything to do with what happens after the plug. UL listing problem, yes. Manufacture warranty void, absolutely.
Lets look at operation for a moment. It really depends on what type of UPS it is such as stand-by, interactive, or dual conversion. They all behave differently. For the sake of argument I will assume it is a higher end dual conversion type. The problem you would encounter is when the AC fails and the UPS inverters are supplied by the batteries to supply the load. The batteries would discharge. Once power is restored the rectifiers would go to full current limit to supply the load, and recharge the batteries. The batteries will take all the current that the rectifiers will supply minus the load current. This means the AC input would go to FLA until the batteries are charged.
I think you can see the problem. The UPS would be drawing 30-amps on a 20-amp circuit. It will not work even if you turn the load off.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: oveloading a branch circuit
Dereck You hit that one on the head.
When I installed 4 APC UPS on the computers here I had the same problem about two years ago when I first hooked them up every thing went ok for about 6 month's but then we had a power outage that drained the batteries (as I wasn't home) And when the power came back on line it tripped the breaker. I was perplex as to why. And after doing some checking I found that each 1500 va UPS unit was drawing about 12 amps when also charging, and all three was connected to the same 20 amp circuit. As you can see this will be a problem. But when the batteries was fully charged the load was only about 2.6 amp's per unit. So I ran a #12/4 NM to split up the load with another #12/2 NM to have three circuits at that location. (I try to keep multiple circuit runs twisted together to keep the magnetic coupling effect from near by lightning strikes down when they feed equipment that has common conections between them) I never had a problem since.
One other question? Is the output of this UPS 120 volts? if it is then the load on the input will be half the output but the charging cycle will still push the input load over the edge.
Dereck You hit that one on the head.
When I installed 4 APC UPS on the computers here I had the same problem about two years ago when I first hooked them up every thing went ok for about 6 month's but then we had a power outage that drained the batteries (as I wasn't home) And when the power came back on line it tripped the breaker. I was perplex as to why. And after doing some checking I found that each 1500 va UPS unit was drawing about 12 amps when also charging, and all three was connected to the same 20 amp circuit. As you can see this will be a problem. But when the batteries was fully charged the load was only about 2.6 amp's per unit. So I ran a #12/4 NM to split up the load with another #12/2 NM to have three circuits at that location. (I try to keep multiple circuit runs twisted together to keep the magnetic coupling effect from near by lightning strikes down when they feed equipment that has common conections between them) I never had a problem since.
One other question? Is the output of this UPS 120 volts? if it is then the load on the input will be half the output but the charging cycle will still push the input load over the edge.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: oveloading a branch circuit
I don't see how a cheater cord causes the warranty to be in jeopardy, but I think Derek's point that the thing may well trip the breaker if it goes into a recharge cycle with near dead batteries is a good point.
However, many UPS systems charge at rates far below the load current they can produce, its why they can take hours to recharge but only produce the rated current for a short period of time.
I don't see how a cheater cord causes the warranty to be in jeopardy, but I think Derek's point that the thing may well trip the breaker if it goes into a recharge cycle with near dead batteries is a good point.
However, many UPS systems charge at rates far below the load current they can produce, its why they can take hours to recharge but only produce the rated current for a short period of time.
Re: oveloading a branch circuit
For a inexpensive stand-by unit this is probable true. A line interactive type, maybe-maybe not. Dual conversion, not.
[ October 14, 2004, 09:49 AM: Message edited by: dereckbc ]
As I said, it depends on the type of UPS being used, stand-by, line interactive, or dual conversion, and the design of the rectifiers.
For a inexpensive stand-by unit this is probable true. A line interactive type, maybe-maybe not. Dual conversion, not.
[ October 14, 2004, 09:49 AM: Message edited by: dereckbc ]
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