# Required wire size?

Hello,
Just starting to think about what size battery supply cables I need, and could use some experienced advice.

My system is 72 Volt with 10 hp continuous, 45 hp. peak. If my calculations are correct, at 45 hp=33.75 kw, it would be about 468 amps.

The charts show 4/0 wire limits at 380 amps, but that is supposed to be a very conservative rating. Assuming that I would only approach my max load occasionally, and for short periods, should 4/0 wire be adequate?

You should look at the name plate data for the current draw on your motor. Using 746W/hp will not give you the actual draw of your motor due to losses. According to the Canadian Electrical Code 4/0 is rated for 385A, or single copper conductor in free air (table 1). Table 2 is for not more than 3 copper conductors in raceway or cable which is what you will have and rates 4/0 for 235A. For 468A you would need 600MCM if you apply the 5% rule. But that is excessive since as you point out you will only hit peak for seconds.

I would determine my running load then calculate the wire size from that. Typically motors come specâ€™d with a continuous duty rating that is considerably lower than peak. I would base my wire size on that number. Also remember to use Table 2 not Table 1 or your wire will be smaller than you think it needs to be. That would suck when you pin it and your insulation melts and you get a line to line or line to chassis short and cook you controller, wire, motorâ€¦

[QUOTE=Mango;5518]You should look at the name plate data for the current draw on your motor. Using 746W/hp will not give you the actual draw of your motor due to losses. According to the Canadian Electrical Code 4/0 is rated for 385A, or single copper conductor in free air (table 1). Table 2 is for not more than 3 copper conductors in raceway or cable which is what you will have and rates 4/0 for 235A. For 468A you would need 600MCM if you apply the 5% rule. But that is excessive since as you point out you will only hit peak for seconds.

I would determine my running load then calculate the wire size from that. Typically motors come specâ€™d with a continuous duty rating that is considerably lower than peak. I would base my wire size on that number. Also remember to use Table 2 not Table 1 or your wire will be smaller than you think it needs to be. That would suck when you pin it and your insulation melts and you get a line to line or line to chassis short and cook you controller, wire, motorâ€¦[/QUOTE]

Excellent point on insulation melting. You have two concerns with wire and current. 1.Voltage drop from wire resistance 2. Heat from resistance causes insulation to melt. The higher temp the insulation is rated for, the smaller the gauge can be with the same current. I.E. 16gauge rubber is rated for 20 amps, so is 20 gauge THHN. I am using 2/0 cable for a simular current with the precaution of short cable runs, telflon insulation, separating leads with insulating standoffs.

Bill
You might consider welding cable. It is more flexible, much more heat resistant and wildly more expensive than THHN.
tommyt

Thanks guys,
I was planning to use stranded (welding lead) wire, but havenâ€™t found a chart that gives load rating for stranded. Iâ€™ll check with a welding supply house to see if they have a chart. It may also be a different load rating for DC vs. AC. Iâ€™d like to stick with my usually mode of operationâ€¦ overkill.

[QUOTE=namenerb;5531]Thanks guys,
I was planning to use stranded (welding lead) wire, â€¦ overkill.[/QUOTE]

use 2/0 stranded welding cables. Overkill, but avoids any resistance issues and fits nicely into standard terminal lugs.