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| Subject - NEC question | |
| veganfan |
In my studying of NEC I was checking out requirements for main disconnects for panelboards. MY understanding of 408.16(A) & 408.16(B) is that if panelboard has a neutral &/or 10% of maximum load is 30A or less a main is needed. If panelboard has no neutral & less than 10% max load is 30A or less than no main needed. Such as 480V 3 phase Y with no neutral panelboard feeding large motors &/or other panelboards then no main. A 277V of 480V y using neutral for lighting circuits would require a main. Is this correct? |
| JimmyDee | quote: I would say that it has to have the neutral and 10% of the branch circuits rated at less than 30 amps, if I reading it right. Jim |
| KSsparky | The way I see it, there is a problem with 408.16(B) anyway. "a power panelboard with supply conductors that include a neutral and having more than 10% of it's overcurrent devices protecting branch circuits rated 30 amperes or less..." A panel which conforms to that sentence is, by definition, a lighting and appliance panelboard. Is it not ? |
| cs409 | if you have 20 spaces(aka circuits). you take 20 x 10% = 2. so, when you install the 3rd single pole and u have the neutral u got to do the main breaker treat..... if it was a 12 space.... 12 x 10% =1.2 so on the 2nd single pole install with neutral, u main breader the panel. it isnt a neutral or 10%, its neutral "and" 10% as shown in Jims quote. |
| macmikeman | Remember the ocpd can be at any point on the feeder for this installation which translates to meaning at the spot where it gets fed from at switchgear or service will suffice. |
| cs409 | very true...... |
| veganfan | I thought the reason for 408.16(B) was to make difference from 408.16(A) in that one had to have ocpd on panleboard (main) where other can have ocpd from switch gear or equivalent?
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| Ryan_J | Almost every panel that is ever installed already meets this requirement. Think about it. The feeder circuit that is supplying the panel has a breaker...that protects the panel. Two exceptions to this would be services, which may have up to six and is specifically addressed in art 230 (I think 230.70), and also, panels supplied by a transformer secondary, which obviously would not have a breaker. This section doesn't mean that every panel that is a lighting and appliance branch circuit panel needs a main breaker...it just needs overcurrent protection. That protection is typically already there in a different panel. :) |
| cs409 | very true again,,,,,,,but if it didnt and the right conditions happen then 408.16(b) makes sure u do! Over current protection... |
| LIVEWIRE | I would like to hear more about "panels supplied by a transformer secondary, which obviously would not have a breaker". Isn't it true that if a panelboard, on the secondary of a transformer, is within sight and meets NEC distance requirements, the main in the panelboard also serves as the required single "secondary disconnect" for the transformer? |
| Ryan_J | Transformers do not require a means of disconnect. The rules for transformer secondaries can be found in 240.21(C). Compare 240.21(C)(2) to (C)(6) and you will see that transfomer secondaries don't always have to terminate main breaker, if kept under 10 feet and supplying a power panel (as oppossed to a lighting and appliance BC panel). After your head hurts from reading that, read 240.21(C)(1) and you will find that a typical transformer that we see installed will need a main breaker anyway, and that is to be sized as per table 450.3(B), secondary protection. :) |
| veganfan | Now I am getting more confused if 408.16(B) is not for main than what is the differance between 408.16(A) & 408.16(B)? |
| JimmyDee | Let me take a guess at it. You have a sq duct with 3, 500,000 mcm wire in it protected at 350 amps. This is power and no neutral is present. I have a 200 amp, 3 phase panel with a known connected fla motor load of 150 amps. I decide to tap off the 500k wire with 3/0 thwn wire and feed the panel which is within 5 feet of the sq duct. I have 4, 3 phase, 50 amp breakers feeding motors. No over-current protection is required for the panel. If, however, there is a neutral, and I decide to add a couple of 277 volt lighting branch circuits to the panel. I need over-current protection for the feeders. I think this is what it is saying. I welcome correction if necessary. Jim |
| cs409 | if you do the math and with the addition of the neutral, YES |
| Ryan_J | quote: (A) is for lighting and appliance branch circuit panbels, (B) is for power panels. |
| cs409 | its time for a beer |
| Ryan_J | quote: 
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