Tap Conductors to the Control Circuit Transformer Primaries

[Dale Hayes]

NEC Article 240.2 defines the rules for tap conductors. I have a 500Amp overcurrent device that is tapped with 6ga 90C conductors (6ga is good for 65Amp). By using the 6ga conductor I have followed the 10% Amp and the 10Ft tap rules.

Q: If I tap from the 500Amp overcurrent device to a 480-120Vac 250VA Industrial Control Transformer used for my 120Vac control circuit, do the two wires suppling the primary side of the control transformer need to be sized at 6ga or can I tap the 500Amp overcurrent device with 12ga or 10ga 90C wire for my Industrial Control Transformer?

 

[RUWired]

The conductors need to be rated for 50 amps.(240.21(B)1(4). Taps suppying a transformer would need to be larger.

 

[petersonra]

The conductors need to be rated for 50 amps.(240.21(B)1(4). Taps suppying a transformer would need to be larger.

Why would you need larger tap conductors for a 250VA xfmr?

IMO, the inards of a control panel are not generally subject to the whims of the electrical code.

What size conductors does AB put on wires to the control circuit xfmr in a MCC bucket for a large starter? I know that is not the same thing, given as the thing is a listed assembly, but the principle is the same.

Just how do you plan to terminate #6 wires on a 250VA xfmr?

In any case, why not make the connection to the xfmr fuses downstream of the fuses on the #6 tap.

 

[RUWired]

Why would you need larger tap conductors for a 250VA xfmr?

IMO, the inards of a control panel are not generally subject to the whims of the electrical code.

What size conductors does AB put on wires to the control circuit xfmr in a MCC bucket for a large starter? I know that is not the same thing, given as the thing is a listed assembly, but the principle is the same.

Just how do you plan to terminate #6 wires on a 250VA xfmr?

In any case, why not make the connection to the xfmr fuses downstream of the fuses on the #6 tap.

Bob The OP has referenced 240.2 tap conductors. My reply was directed to his question based on the info provided. If the tap conductors for a transformer are related to a motor control circuit then 430.72(B) can be used for the conductors either inside or outside the enclosure of the tap.

The # 6 awg conductors would not be able to terminate to the transformer with the 10' tap rule i quoted, they would need an OCPD first.

The conductors would need to be larger for primary transformer tap conductors based on 240.21(B)3 not knowing the exact situation.

Rick

 

[Jraef]

If the control panel is going to be UL508 listed, UL requires only that the CPT primary conductors be required to handle the primary current and be protected by a BCP, i.e. the primary fuses (if selected correctly). The stipulation is AS LONG AS THEY DON'T LEAVE THE ENCLOSURE. That is why the CPT in a motor starter can have smaller wires.

The question is, if you are NOT building and listing the control panel to UL508, then it may be that the NEC rules are all you are left with.

 

[Smart $]

...

The question is, if you are NOT building and listing the control panel to UL508, then it may be that the NEC rules are all you are left with.

Perhaps, NFPA 79?

 

[Dale Hayes]

The 500Amp OCPD, the (2) tap conductors and the 250VA industrial control transformer are all in one conrol panel box. The (2) tap conductors originate at the load side of the 500Amp OCPD. The tap conductors do not leave the panel box. The (2) tap conductors land on the line side of (2) 1 1/2Amp Type CC KLDR rejection fuses. These (2) CC fuses are located in a fuseblock on top of the transformer.

It is my understanding that the transformer is protected by the 1 1/2Amp type CC fuses at the primaries.

Since the conductors are tapped from a 500Amp OCPD doesn't the NEC 240.21 1/10 tap rule apply? Shouldn't the tap conductors be based on the 500A / 10 = 50A ... 8ga Wire?

 

[Jraef]

Perhaps, NFPA 79?

Yep, that too, assuming it's a machine to which NFPA79 applies; "Industrial Machinery" seems to be more open to interpretation than I would think it should be. NFPA79 has some rules that I see violated quite often, but when I point it out, people (including a couple of AHJs) respond that it doesn't apply because whatever it is is not "industrial machinery", apparently as in metal working machine tools. I for one am confused by the prevalence of this attitude.

 

[tkb]

The tap conductors could be a small as required for the load and do not need to be 1/10 the ampacity since they do not leave the enclosure as per 240.21(B)(1)(2).

240.21 Location in Circuit

(1) Taps Not over 3 m (10 ft) Long. If the length of the tap conductors does not exceed 3 m (10 ft) and the tap conductors comply with all of the following:

(1) The ampacity of the tap conductors is a. Not less than the combined calculated loads on the circuits supplied by the tap conductors, and b. Not less than the rating of the device supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors.

(2) The tap conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.

(3) Except at the point of connection to the feeder, the tap conductors are enclosed in a raceway, which shall extend from the tap to the enclosure of an enclosed switchboard, panelboard, or control devices, or to the back of an open switchboard.

(4) For field installations, if the tap conductors leave the enclosure or vault in which the tap is made, the ampacity of the tap conductors is not less than one-tenth of the rating of the overcurrent device protecting the feeder conductors.

Informational Note: For overcurrent protection requirements for panelboards, see 408.36.

 

[Jraef]

The 500Amp OCPD, the (2) tap conductors and the 250VA industrial control transformer are all in one conrol panel box. The (2) tap conductors originate at the load side of the 500Amp OCPD. The tap conductors do not leave the panel box. The (2) tap conductors land on the line side of (2) 1 1/2Amp Type CC KLDR rejection fuses. These (2) CC fuses are located in a fuseblock on top of the transformer.

It is my understanding that the transformer is protected by the 1 1/2Amp type CC fuses at the primaries.

Since the conductors are tapped from a 500Amp OCPD doesn't the NEC 240.21 1/10 tap rule apply? Shouldn't the tap conductors be based on the 500A / 10 = 50A ... 8ga Wire?

By that logic, the primary wires of a 500VA CPT for a 600HP motor stater that has a 1200A circuit breaker need to be sized for 120A then? Come on...

As per UL508 rules, those 1-1/2A fuses are the Branch Circuit Protection for those conductors, even though they are down stream of the conductor tap source. The theory behind that is that there is a single load on those conductors and that load is protected by fuses, so the only risk of an unprotected fault is if those short little conductors get nicked and go to ground. I belive it's considered a low risk and the alternative, the oversized wire, is impractical in most cases. The same theory is used for Overload Relays in motor starters with MCPs if you think about it. The OLR is the thermal protection for the starter wires as well as the motor leads, yet they are down stream of the starter wires. The only risk is if one of those wires goes to ground INSIDE the motor starter itself; low risk.

A more real problem is the actual tapping itself. Very few lugs on circuit breakers are listed to have multiple conductors, and some of those that do will not allow disimilar sizes. Usually the tap-off is actually made on the motor starter where there is a tap off screw or something that facilitates that. I know that some breaker mfrs are now supplying small CPT tap off screws on their lugs, but not many of them yet.

 

[petersonra]

The question is, if you are NOT building and listing the control panel to UL508, then it may be that the NEC rules are all you are left with.

I don't see how you figure that. if the NEC rules apply, it does not matter if the thing is UL listed or not. You would have to apply the rules. You don't get to ignore applicable NEC rules just because something is listed.

IMO, if the NEC rules do not apply inside a listed piece of equipment, they don't apply inside the same piece of equipment that does not have the listing sticker on it.

Having said that, we build some UL listed stuff with this exact problem and to avoid having this argument in the field, we avoid the problem. It is a little wierd to see a #6 come to a fuse block from the distribution block and see #16 come out of the fuse block.

 

[Jraef]

I don't see how you figure that. if the NEC rules apply, it does not matter if the thing is UL listed or not. You would have to apply the rules. You don't get to ignore applicable NEC rules just because something is listed.

Sorry, but yes you do, to a point. If a panel is UL listed, an inspector does NOT have to go through the entire panel. There are differences in the rules for UL and the NEC, but if a panel is already listed, it is thereby acceptable per the NEC. If there is a mistake inside, that's a different matter. But I have been challenged by installing contractors on wire sizing inside of a control panel many many times based on the NEC, but the UL rules are different and prevail INSIDE the listed panel. I have never lost one of those challenges, AHJs always agree.

IMO, if the NEC rules do not apply inside a listed piece of equipment, they don't apply inside the same piece of equipment that does not have the listing sticker on it.

I'm not too sure about that either. If you are in a part of the country that does not require NRTL listing of control panels, I don't think an inspector is going to ignore things just because it is enclosed. Conversely if you are in a state that DOES require NRTL listing, an unlisted control panel could (would) be rejected regardless of passing NEC muster or not just because it s not NRTL listed.

Having said that, we build some UL listed stuff with this exact problem and to avoid having this argument in the field, we avoid the problem. It is a little wierd to see a #6 come to a fuse block from the distribution block and see #16 come out of the fuse block.

There is never a problem with over sizing conductors, so your method is never going to be wrong. I just don't accept it as necessary and have never done it, and never had a complaint about it either.

Open ANY manufactured starter or MCC bucket and the wire feeding the primary of the CPT is never over 12ga, usually 14ga (because that's the minimum per UL). That's because all UL says is that ungrounded conductors must have over current protection IN them. In the case of a tapped conductor situation, if the branch protective device is already the right size you don't need it again but if not, you do. So in the case of a starter for a larger motor, the BCP will end up too big and you must have new BCP devices IN the conductors; the primary CPT fuses satisfy that condition. It doesn't matter that the fuses are at the other end of the wire, as long as they are there. Again, AS LONG AS THE CONDUCTORS DON'T LEAVE THE BOX.

I think we may have to respectfully disagree on this one. There is nothing WRONG with over sizing the conductors, if you can, but I don't agree with the necessity.

 

[Dale Hayes]

By having the 500Amp OCPD, the (2) tap conductors, the (2) 1 1/2Amp CC transformer primary fuses and the transformer all in one conrol panel box and nothing leaves the box then the 1/10th tap rule doesn't necessarily apply.

So If I use 12Ga or 10Ga for my taps from the 500Amp OCPD to the (2) 1 1/2Amp CC fuses of the 250VA industrial control transformer I should be OK?

 

[tkb]

By having the 500Amp OCPD, the (2) tap conductors, the (2) 1 1/2Amp CC transformer primary fuses and the transformer all in one conrol panel box and nothing leaves the box then the 1/10th tap rule doesn't necessarily apply.

So If I use 12Ga or 10Ga for my taps from the 500Amp OCPD to the (2) 1 1/2Amp CC fuses of the 250VA industrial control transformer I should be OK?

That is the way I read it.
You could even use some #16 for you taps. IMO.

 

[Dale Hayes]

OK ... another Question. A 800Amp OCPD, (3) tap conductors, the (3) tap conductors originate at the load side of the 800Amp OCPD, the tap conductors do not leave the panel box and they are connected to a 65Amp fused device. Can the (3) tap conductors be 6GA 75C. (good for 65Amp) or must the taps be sized for 10% of the 800Amp OCPD using 4GA 75C (good for 85Amp) for the taps? Again, all components stay within the enclosure.

What if the conductors terminate on a 65Amp fused device outside the control box?

 

[Jraef]

OK ... another Question. A 800Amp OCPD, (3) tap conductors, the (3) tap conductors originate at the load side of the 800Amp OCPD, the tap conductors do not leave the panel box and they are connected to a 65Amp fused device. Can the (3) tap conductors be 6GA 75C. (good for 65Amp) or must the taps be sized for 10% of the 800Amp OCPD using 4GA 75C (good for 85Amp) for the taps? Again, all components stay within the enclosure.

I'm not checking your ampacity decision (no NEC here) but yes, assuming you meet all of the requirements in 210.21 posted earlier by tkb (8/5/11).

What if the conductors terminate on a 65Amp fused device outside the control box?

Then the 10% tap rule will apply.

 

[tkb]

Then the 10% tap rule will apply.

Depending on tap conductor length.