Wire Sizing

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juanhorta

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Hi, I am new in this forum and I need to know how to calculate the size of a wire. this is the data I have.
Voltage: 400 DC
Current: 235 AMPS
Length of wire: 3 FT max.
What formula can I use that is approved by the NEC to calculate a wire size. I was told that 3/0 wire size will do it. Is this right?

Thanks
Juan H.
 
Re: Wire Sizing

it's possible that the size suggested would work for your application. but without a lot more information, it is near impossible to say for sure.

some of the things we would need to know

- type of load
- type of wire
- ambient temperature
- is wire to be installed in a raceway
- temp rating of termination points

[ September 01, 2005, 11:04 AM: Message edited by: petersonra ]
 
Re: Wire Sizing

Type of load: Going to a circuit breaker then to an inverter.
Type of wire: E-93681 (UL) MTW 600 V. VW-1, 3/0 AWG.
Ambient temp.: is about 117 F, but inside box where wire is located is 10 degrees higher.

is wire to be installed in a raceway: I do not know what this means, but is it not going through a conduit.
temp rating of termination points: about 127-130F.

Thanks


- type of load
- type of wire
- ambient temperature
- is wire to be installed in a raceway
- temp rating of termination points
 
Re: Wire Sizing

If you only need 3ft of wire, the application may not be subject to the NEC.

Is this wire internal to a piece of equipment or a custom panel?

We really do need more information. What is the wire being used for?
 
Re: Wire Sizing

The wire is for a costum panel and is connected to the input-lug of a circuit breaker coming from the output of a fuse. It is being used for free air and not raceway. there is no power being dissipated at this box where the wire is located. I was told I could use table 310-17 from NEC.
Is this right and what about the wire size?

Thanks
 
Re: Wire Sizing

If this is a factory manufactured panel, then it is not subject to NEC rules. It would need to follow UL508a guidelines and/or NFPA79 rules if part of an industrial machine.

<added>
MTW is not in table 310-17.

A FPN in 310-15 refers you to NFPA79 table 11.

A quick calculation shows that 13 watts is generated just in the 3 foot of wire if you do as proposed.

[ September 01, 2005, 11:54 AM: Message edited by: petersonra ]
 
Re: Wire Sizing

Hi, can you pleaase show me, how did you get to this 13 watt calculation? Please be detail with formulas and tables.
Thank you very much

Juan H.
 
Re: Wire Sizing

I know it was I^2 R, but where did you get the resistance from? How did you use the distance of the wire in this calculation? Did you use any of the tables. This had to be a very small resistance.
 
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