sub panel feed

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Carlos J

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Hi guys, can't seem to get the right answer from anyone on weather is permitted to feed a sub panel from another sub panel, some people say yes is ok others say no. who is right? I looked for the answer in the 2011 code book but couldn't find it.
 
Carlos J said:
Hi guys, can't seem to get the right answer from anyone on weather is permitted to feed a sub panel from another sub panel, some people say yes is ok others say no. who is right? I looked for the answer in the 2011 code book but couldn't find it.
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The "no's" are espousing an electrical urban myth.

Welcome to the Forum. :)
 
To expand on my earlier answer of "It's done all the time", I'll add that one would want to be sure they're not overloading the existing main or "sub" panels.

I also looked at a job recently where the 20-30 year old "sub" panel was fed with a 3 wire SEU(no ground), grounds and neutrals not separated. I informed the customer that I would need to re-feed with a 4 wire.
 
Carlos J said:
Hi guys, can't seem to get the right answer from anyone on weather is permitted to feed a sub panel from another sub panel,
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Yes you can feed a sub panel from a sub panel and then you can feed another one from that one and another one from that one and another one from............................


Look at large facilities such as hospitals, there can be hundreds (maybe even thousands) of panelboards and they are not all fed from the MDP.

Roger
 
You didn't find it in the code, not because it wasn't there (it is, in a way :happyyes: ), and not because you were looking in the wrong places :happyno: , but rather because you were looking for the wrong thing :huh: . All you need, in order to confirm that the proposed installation is allowed, is in the 2011 NEC article 408.30, which says the panelboard has to be rated for the load, and 408.36, which says the panelboard has to be protected against overcurrent.

Welcome to the forum.
 
roger said:
Look at large facilities such as hospitals, there can be hundreds (maybe even thousands) of panelboards and they are not all fed from the MDP.
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Pretty well sums up what I was going to say.


As a general rule the capacity of each panel gets larger as you get closer to the source - but nothing says it has to either.
 
Yup. As long as your grounds remain isolated from neutral. Also consider your total voltage drop from from the main panel to the furthest branch outlet on the furthest sub panel. Should be within 5%.
 
DrSparks said:
Yup. As long as your grounds remain isolated from neutral. Also consider your total voltage drop from from the main panel to the furthest branch outlet on the furthest sub panel. Should be within 5%.
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Unless you have local amendments or other specifications that 5% is just a suggestion in an informational note and not an NEC requirement. Depending on the load supplied maybe it is a good idea, other times it may not matter all that much.
 
kwired said:
Unless you have local amendments or other specifications that 5% is just a suggestion in an informational note and not an NEC requirement. Depending on the load supplied maybe it is a good idea, other times it may not matter all that much.
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I agree. There's a huge difference if the end of the line is an LED can light or a 10kw electric furnace ;)
 
DrSparks said:
I agree. There's a huge difference if the end of the line is an LED can light or a 10kw electric furnace ;)
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Two huge differences in your example.

The 10 kW furnace may see much more voltage drop, but may not really be that noticeable if it has a 8% drop then what may be noticed if there is an 8% drop supplying the LED light.

Voltage drop to a motor - the motor will just draw more current - which of course will mean even more drop- but will still do the work as long as the drop isn't too much to create severe problems. More energy will be wasted, the motor will run hotter, but the user may not really notice any of this.
 
kwired said:
Voltage drop to a motor - the motor will just draw more current - which of course will mean even more drop- but will still do the work as long as the drop isn't too much to create severe problems. More energy will be wasted, the motor will run hotter, but the user may not really notice any of this.
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Even that is uncertain, if the motor is lightly loaded reducing voltage will reduce current.


There are energy saving devices that adjust the voltage supplied to loads like conveyors that spend a lot of time running under loaded.
 
iwire said:
Even that is uncertain, if the motor is lightly loaded reducing voltage will reduce current.


There are energy saving devices that adjust the voltage supplied to loads like conveyors that spend a lot of time running under loaded.
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More info please, not saying I totally disagree but kind of goes against what I have always known about motors, in particular AC induction motors.

You sure those energy saving devices aren't correcting power factor?

A lightly loaded motor already draws less current then if it were drawing full load rating, but also usually at a lower power factor then full load power factor. Changing voltage just changes current in opposite direction with power used by the motor remaining nearly the same, but other losses can go along with it
 
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