Help sizing plug for overhead bus

So I recently started a new job in a new city, and while they're waiting for some big orders to hit they've asked me to overhaul the control panel for a mobile test skid out in the shop (they're adding some motors and a touchscreen).

Anyhow, while I was working on it, I noticed the plug hanging from the overhead bus was a 3p4w twist-lock rated for 125/250VAC. The problem here is the overhead bus is a 480Y/277V supply. I promptly locked out the CB feeding the bus.

So the setup is it's a 600A overhead bus protected by an 800A breaker. There are 4 drops: 2 of them are unused, 1 is cord connected to a machine, and the one I'm working on has a plug dangling from the end of an 8/4 cable and is protected by a 30A fused disconnect switch up top.

So, I'm trying to find a replacement for this and I'm having some issues. The new test skid panel will be 26 FLA with 15HP worth of motor load (10 + 2 + 3). Unfortunately, the motor loads can all run simultaneously. What I've found is that 30A plugs only have 5hp ratings attached to them. Hubbell has a 50A plug, but it isn't HP rated. The only options I've found so far are a 100A pin-and-sleeve plug and connector, totaling $700, or I can get a reverse service interlocking plug/receptacle set for $2500+.

Either of these will be tough sell for our shop manager, since they're a far cry from the $50 that our machinist bought the old [wrong] plug for at Lowe's.

So I guess my question is: Do I need a HP rated plug/receptacle for this application? Or will an amp rated plug suffice?

In case it matters, the skid panel in question has a UL98 fused disconnect switch.

As a background note, I'm used to dealing with UL508A, and while that has a little bit to say about plugs/cords used for loads or interconnects, it doesn't really say anything about the plugs used to feed the panel. NFPA79 doesn't say anything either that I could find.

So the only thing I can find that would save me so far is that I could mark the receptacle with, "For disconnecting use only, not for current rupturing" per UL508A 59.4. But in this case, does the plug need to be mounted through the enclosure wall, or can it be cord connected (on say a 3ft cord)? I can't really find anything in UL508A that deals with this situation.

As an aside, am I even allowed to put a 100A plug on a 30A circuit? I don't know any specific rule against it, but I know it's generally bad form to do that since someone might actually plug a 100A load in to it.
 

GoldDigger

Moderator
Staff member
You say that the motors can all run at the same time. That would imply that they might all start at the same time if the controls do not prohibit it. But it is not likely that they would all start when the plug is first connected.
On the other hand, there is nothing other than procedure and signage to keep someone from opening the circuit by unplugging the disconnect.
I think you are right to add up the HP of the individual motors to compare to the HP rating of the connector.
Do you think that it is likely that someone would unplug the skid while it is operating?
Do you think that labelling will significantly change that?

With respect to the bus and breaker, can you assure that the load connections to the bus cannot draw more than the bus rating?
If so you may have a tap rule like situation in which the oversized supply breaker could be safe. But with multiple loads, I do not see specific tap rules as applying, FWIW.

Tapatalk!
 

texie

Senior Member
So I recently started a new job in a new city, and while they're waiting for some big orders to hit they've asked me to overhaul the control panel for a mobile test skid out in the shop (they're adding some motors and a touchscreen).

Anyhow, while I was working on it, I noticed the plug hanging from the overhead bus was a 3p4w twist-lock rated for 125/250VAC. The problem here is the overhead bus is a 480Y/277V supply. I promptly locked out the CB feeding the bus.

So the setup is it's a 600A overhead bus protected by an 800A breaker. There are 4 drops: 2 of them are unused, 1 is cord connected to a machine, and the one I'm working on has a plug dangling from the end of an 8/4 cable and is protected by a 30A fused disconnect switch up top.

So, I'm trying to find a replacement for this and I'm having some issues. The new test skid panel will be 26 FLA with 15HP worth of motor load (10 + 2 + 3). Unfortunately, the motor loads can all run simultaneously. What I've found is that 30A plugs only have 5hp ratings attached to them. Hubbell has a 50A plug, but it isn't HP rated. The only options I've found so far are a 100A pin-and-sleeve plug and connector, totaling $700, or I can get a reverse service interlocking plug/receptacle set for $2500+.

Either of these will be tough sell for our shop manager, since they're a far cry from the $50 that our machinist bought the old [wrong] plug for at Lowe's.

So I guess my question is: Do I need a HP rated plug/receptacle for this application? Or will an amp rated plug suffice?

In case it matters, the skid panel in question has a UL98 fused disconnect switch.

As a background note, I'm used to dealing with UL508A, and while that has a little bit to say about plugs/cords used for loads or interconnects, it doesn't really say anything about the plugs used to feed the panel. NFPA79 doesn't say anything either that I could find.

So the only thing I can find that would save me so far is that I could mark the receptacle with, "For disconnecting use only, not for current rupturing" per UL508A 59.4. But in this case, does the plug need to be mounted through the enclosure wall, or can it be cord connected (on say a 3ft cord)? I can't really find anything in UL508A that deals with this situation.

As an aside, am I even allowed to put a 100A plug on a 30A circuit? I don't know any specific rule against it, but I know it's generally bad form to do that since someone might actually plug a 100A load in to it.
It would seem to me that you would need to go to something like a 60 amp pin and sleeve that would have a HP rating of 20 HP to be compliant.
 

GoldDigger

Moderator
Staff member
At least with a pin and sleeve construction people would be less likely to try to disconnect under load.

Tapatalk!
 
You say that the motors can all run at the same time. That would imply that they might all start at the same time if the controls do not prohibit it. But it is not likely that they would all start when the plug is first connected.
This is correct, or rather, they can start in quick succession. The motors are PLC controlled, but the touchscreen will have separate buttons for each. There is no "start all" button that will energize the contactors in the same scan cycle, but an operator could start all 3 as quick as they can tap the screen.

But to address your second part, there is no way for the motors to start on power-up (with the exception of welded contactors).

On the other hand, there is nothing other than procedure and signage to keep someone from opening the circuit by unplugging the disconnect.
This is also correct. I can say that it's not likely, since the skid has an E-Stop on it, but there's always the possibility.

I think you are right to add up the HP of the individual motors to compare to the HP rating of the connector.
Do you think that it is likely that someone would unplug the skid while it is operating?
Do you think that labelling will significantly change that?
We don't have any electricians in our shop, all the panel building and wiring is done in our main office. I'm the only person at this location with any electrical knowledge.

So I guess that's the long way of saying that no, I can't make any assurances on any of those things, and obviously no amount of labeling will help. While people here know the E-Stop is the first option, I cannot say with any certainty that someone won't unplug it while the motors are running.

It would seem to me that you would need to go to something like a 60 amp pin and sleeve that would have a HP rating of 20 HP to be compliant.
Do you have any part numbers by chance? The problem I have is I can't find anything that small with the HP rating I'm looking for. Hubbell's offerings for 60A are only rated for 10HP at this voltage. I'd gladly select a 60A plug if I could find one with a sufficient HP rating.
 

jim dungar

Moderator
Staff member
While the NEC is about minimum rules and not good design, it is also not about 'what if'.

Train your employees to use the plug only as a no-load break disconnect, then you will not need any HP rating at all.
 

tshea

Senior Member
Sounds like one of my customer's set up. What I did for them, to keep from starting fires, is to run a 4-3 with ground bus cable from a 100Amp bus switch to the skid disconnect.

Another customer of mine is a panel builder. They operate so far outside of the NEC my eyes fall out--#8 = 100+ Amps. All their projects are built under the eyes of EEs & PEs.

I tell them, my work falls under the NEC so I ahve to use wire that is sized in accordance with T316. They are OK with that.

In your case, the 120/240 plug and connector is a definite NO-NO! How long is the skid going to be in the shop? Is the power for test purposes?
 
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