wiring to brake resistors

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petersonra

petersonra

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engineer
A question has come up with an end user who is asking why is it that we can use 600V rated wire on brake resistors on 480V drives when the pulses are 680V?

He brought up 300.3 regarding different systems. I said the key word is "nominal" voltages in the code.

They want something a little better than my say so.

The same issue exists on VFDs as they are 680VDC pulses.

I seem to recall seeing something about this from UL a long while ago but cannot actually dig it up now. Anyone have any ideas?
 
600V is the RMS rating.
Some 'listed' DC installations like this are based on the Peak-Peak rating of the insulation.
 
Notice that the section quoted is from UL since NEC does not govern that wiring which is integral to the equipment rather than field constructed building wiring.

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Think about it this way:

RMS is just the convention we all use when discussing the voltage levels. Why is your DC bus voltage higher than the AC RMS voltage? Because the rectifier diodes only conduct at the PEAK of every sine wave, which is always higher than the RMS voltage (by 1.414). So if a cable insulation is rated for 600V RMS, it is ALREADY capable of seeing PEAK voltages of 848.4V.
 
Jraef said:
Think about it this way:

RMS is just the convention we all use when discussing the voltage levels. Why is your DC bus voltage higher than the AC RMS voltage? Because the rectifier diodes only conduct at the PEAK of every sine wave, which is always higher than the RMS voltage (by 1.414). So if a cable insulation is rated for 600V RMS, it is ALREADY capable of seeing PEAK voltages of 848.4V.
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That was my argument but the end user was unconvinced. He wants to use 1000V cable.
 
Jraef said:
Most likely, he wants him to use it, but he doesn't want to pay more for it. Right?
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We call those kinds of clients "ex-customers". We have a salesman who "fires" customers when they are more trouble than they're worth.
 
petersonra said:
That was my argument but the end user was unconvinced. He wants to use 1000V cable.
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If you can get hold of a copy of UL 83, the standard for thermoplastic insulated low voltage cables in which THHN falls, table 27.1 shows that the UL dielectric test is a minimum of 1500V peak and it goes up from there as the cable size increases. FWIW.
 
gadfly56 said:
We call those kinds of clients "ex-customers". We have a salesman who "fires" customers when they are more trouble than they're worth.
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these guys are good customers, but the particular person we are dealing with this time is a bit painful. he wants drawings for everything before even issuing a P.O.

we tell him normally we just give them a price that covers things laid out in the specs and the details come later. this guy wants all the details upfront.

it is an extremely expensive way to do quotes.

plus it drags things out a lot.

this 1000V wiring for brake resistors comes up now and then. It came up from two different customers this month.

the thing is that UL does not allow a UL508a panel to exceed 600V nominal anyway so it would be pointless to put 1000V wire in it.
 
petersonra said:
these guys are good customers, but the particular person we are dealing with this time is a bit painful. he wants drawings for everything before even issuing a P.O.
we tell him normally we just give them a price that covers things laid out in the specs and the details come later. this guy wants all the details upfront.
....
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And it makes it much easier for the customer to shop your price...you have done all of the up front design work and given it to them before you have a PO. What keeps the customer from taking that design work to another vendor and getting another price?
 
don_resqcapt19 said:
What keeps the customer from taking that design work to another vendor and getting another price?
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One place I worked took to putting copyright notices on every page of the design docs and a something about "this document is submitted solely in support of proposal ###. all designs and ideas expressed are sole property of OurFirm unless sold or licensed" (that's from memory).

Bob- Sounds like you need to either add a design fee or submit a quote for the design work itself with the drawings as deliverables.
 
gadfly56 said:
We call those kinds of clients "ex-customers". We have a salesman who "fires" customers when they are more trouble than they're worth.
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The most valuable skill for a businessman, right there. Not every customer makes a worthwhile customer. Business is a two way street.

If you've explained to him that it's an RMS voltage rating, and that understanding RMS ratings are basic electricity 101, and there isn't really a code provision that expressly states word for word "you shall be able to use peak voltage values so long as the RMS value is within the rating", and he is too stupid to get out of his own way, move on to a different project.
 
It is not my call. If it pans out into some work it will be profitable. If not, it won't. That is just the way it is.

This comes up periodically and the best answer I can come up with is UL508a. It is a legit argument that the brake resistor is part of the control panel most of the time, because it is.

In this case it is a little dicey given that the control panel and the resistor are separated by the length of the cable to the resistor. Does that make it not part of the control panel? I don't know.
 
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