Added subpanel....

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Open Neutral

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Given an existing 2AWG feed to a machine, with an unfused disconnect on the wall, and the machine with its own 100A breaker....

Is it allowable to put a subpanel at the disconnect, with a {subfeed} line/"main" breaker of ~30A, and several branch breakers?

The alternative I see is to replace the disconnect with a much larger subpanel, with its own 100A line breaker, 100A load breaker, and 30A load breaker.

Or pull separate feeds for the smaller circuit with its own subpanel.
 

iwire

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From a code standpoint the only issue is the load calculation.

What is the present load and how much load do you plan to add?
 

Open Neutral

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From a code standpoint the only issue is the load calculation.

What is the present load and how much load do you plan to add?

The present load is the machine tool which is nameplated at 24KVA.

The added loads would be:

a) lighting/DRO etc. for a few amps.
b) ~15-20A to machine adjacent; can't be used simultaneously if that even matters.
 

augie47

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State Electrical Inspector (Retired)
A couple of things come to mind:
1.What size overcurrent device is protecting the #2 on the line side.
(making sure its not a tap)
2. Are the lugs on the disconnect rated for the size and number of conductors,
3.Will the "tap" from the disconnect meet one of the 240.21 especially taking length and wire size into consideration.
 

Open Neutral

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A couple of things come to mind:
1.What size overcurrent device is protecting the #2 on the line side.
(making sure its not a tap)
If you mean the subpanel; I suppose a 30A breaker. The subpanel load is small vs. the machine.
2. Are the lugs on the disconnect rated for the size and number of conductors,
Good point; had not thought of that aspect. They'd need their own lugs....

3.Will the "tap" from the disconnect meet one of the 240.21 especially taking length and wire size into consideration.

Err... I think so. Reading http://ecmweb.com/code-basics/understanding-rules-feeder-taps we are talking 2-4 ft of metallic conduit from the disconnect to the subpanel.

The subpanel would have a line breaker of 30A. The stub cable could/would be #10, which is
"no less than 10% of the ampacity of the OCPD" [100A]

So the one issue I see is a 2nd set of lugs on the disconnect.
 

petersonra

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Northern illinois
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engineer
So the one issue I see is a 2nd set of lugs on the disconnect.

what kind of lugs does it have now? if they are mechanical lugs why not just replace them with a 2 hole lug?

I would also point out that some times disconnect switch line side lugs come with holes drilled in them so that smaller wires can be attached.
 

augie47

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State Electrical Inspector (Retired)
For clarity on:
1.What size overcurrent device is protecting the #2 on the line side.
(making sure its not a tap)


My question concerned what size overcurrent device was protecting the #2 at it's source.
If the #2 is fed from an overcurrent device appropriate for its amapcity (115 amps assuming no derating) then I see no problem from that standpoint.
If the #2 is in itself a "tap" from a larger conductor or larger OCP device you would be "tapping a tap" which is not allowed.
If the #2 is protected by the "next size up" rule or by a breaker exceeding it's ampacity such as in a motor feeder, then you would need to verify the actual load.
 

Open Neutral

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For clarity on:
1.What size overcurrent device is protecting the #2 on the line side.
(making sure its not a tap)


My question concerned what size overcurrent device was protecting the #2 at it's source.
If the #2 is fed from an overcurrent device appropriate for its amapcity (115 amps assuming no derating) then I see no problem from that standpoint.
If the #2 is in itself a "tap" from a larger conductor or larger OCP device you would be "tapping a tap" which is not allowed.
If the #2 is protected by the "next size up" rule or by a breaker exceeding it's ampacity such as in a motor feeder, then you would need to verify the actual load.

The #2 cable is fed from a 100A breaker in the main panel, <100 ft away.
Is that OK?

re: The lugs on the disconnect. I'm not on site but worst case we could find another disconnect that did have replaceable ones.
 

kwired

Electron manager
Location
NE Nebraska
You have stated this machine is 24KVA. You stated it is on a 100 amp overcurrent device. Unless I missed it, you did not state what the voltage is.

Kind of assuming though that it is 240 volts - 24,000/240=100 amps. Where is it that you see additional capacity which you have available unless you can assure that this load will not run at same time as the other loads?
 

Open Neutral

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You have stated this machine is 24KVA. You stated it is on a 100 amp overcurrent device. Unless I missed it, you did not state what the voltage is.

Kind of assuming though that it is 240 volts - 24,000/240=100 amps. Where is it that you see additional capacity which you have available unless you can assure that this load will not run at same time as the other loads?

We do assume that, for physical layout reasons, there won't be simultaneous use..unless one guy is using his left hand on one, right on the other......

But I admit to head scratching on the machine's actual demand. 208-wye is what we have. I was worried about running it undervoltage... The Fine Manual alludes to 208 several places but is not direct/specific. [There was an optional 3ph. 36KW 600|480|240 in, 208v out transformer cabinet. THAT manual says "208V"]

Here's what we know..... https://picasaweb.google.com/100399...authkey=Gv1sRgCN-ks7jvic3x-wE&feat=directlink and I thought 74A would be 26.5KW..... And why would it have 100A as its own input breaker, vice ...??
 
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kwired

Electron manager
Location
NE Nebraska
We do assume that, for physical layout reasons, there won't be simultaneous use..unless one guy is using his left hand on one, right on the other......

But I admit to head scratching on the machine's actual demand. 208-wye is what we have. I was worried about running it undervoltage... The Fine Manual alludes to 208 several places but is not direct/specific. [There was an optional 3ph. 36KW 600|480|240 in, 208v out transformer cabinet. THAT manual says "208V"]

Here's what we know..... https://picasaweb.google.com/100399...authkey=Gv1sRgCN-ks7jvic3x-wE&feat=directlink and I thought 74A would be 26.5KW..... And why would it have 100A as its own input breaker, vice ...??
If those two nameplates are your two different machines that will not run at same time - go for it.

Three phase makes a difference in what I said in last post on available extra capacity - to begin with you have 1.73 times more capacity then I though you had, minus some for the fact it is only 208 instead of 240 volts.

100 amp input breaker?- maybe it needs that trip setting for motor starting? I doubt it needed 2 AWG copper supply conductors even if it needed a 100 amp breaker for motor starting. Without further information I would be considering 4AWG unless circuit length was a concern for supply conductors, I guess if this is a continuous load the 125% factor makes it need to be 90 amp circuit - some seem to think 90 amp devices don't exist and will automatically go with a 100.
 
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Open Neutral

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If those two nameplates are your two different machines
.....
Three phase..... minus some for the fact it is only 208 instead of 240 volts.

100 amp input breaker?- maybe it needs that trip setting for motor starting? I doubt it needed 2 AWG .....

The two nameplates are both on the big machine; one is on the outside, one on the inside of the same panel. It's a CNC lathe with a Gatling-gun like rotating tool holder; it has 5 "barrels" with different tools. Think it's called a "turning center". The other machine is far smaller; a 3 HP mill. The usage of either is occasional; this is in effect a prototype/experiment shop, not production environment.

It is ~85 ft from the main panel.

I, as well as an ex-plant electrician now EE friend Mike kibitzing from quite afar, are puzzled by the nameplates vs. 100A breaker vs. Fine Manual. But it's spec'd for 60 or 50 Hz. so that may be a factor.

To be fair, the #2 is AL not CU, a fact I left out initially.

Another question. I'm worried about the mechanics of getting 2 sets of lugs on the disconnect output. The output side shall be CU not AL. Is a taped split-bolt inside the cabinet kosher? Mike thinks so but he admits he's not read a code book in years. ("And to think I used to teach classes on it...")
 

Open Neutral

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100 amp input breaker?- maybe it needs that trip setting for motor starting? I doubt it needed 2 AWG copper supply conductors even if it needed a 100 amp breaker for motor starting. Without further information I would be considering 4AWG unless circuit length was a concern for supply conductors, I guess if this is a continuous load the 125% factor makes it need to be 90 amp circuit - some seem to think 90 amp devices don't exist and will automatically go with a 100.

OK, I'm also confused on something basic; why 310.15(B)(16) does not rule here. It says "3 current carrying conductors" and we'd have a neutral as well, but would I not need to make the main's breaker to be 90A {or even less..}? [Unless the alleged #2 AL is not; something I need to confirm...] And #4???
 

kwired

Electron manager
Location
NE Nebraska
OK, I'm also confused on something basic; why 310.15(B)(16) does not rule here. It says "3 current carrying conductors" and we'd have a neutral as well, but would I not need to make the main's breaker to be 90A {or even less..}? [Unless the alleged #2 AL is not; something I need to confirm...] And #4???
General purpose applications would require a 2 AWG aluminum to be protected by a 90 amp or less breaker. Motor applications could allow for even higher overcurrent device though because of the current drawn while the motor is starting. Your conductor is still protected from overload via the motor overload protection though, and the breaker is there for short circuits and ground fault protection.

That said you likely have a machine that is listed and may not quite follow rules the same as if you were building it to NEC rules, but still based on some of the data you submitted a difference between 90 and 100 amp breaker is nothing I would lose any sleep over - especially motors are a majority of the load.
 

kwired

Electron manager
Location
NE Nebraska
Thanks. Makes perfect sense to me.

Where in the Code is that?
Table 310.15(B)(16), in conjunction with 240.4, unless permitted otherwise in 240.4 A thru G.

G tells us several applications that have their own rules, and refers us to art 430 parts II, III, IV, V, VI, VII for motors.
 
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